For practical application in calcium looping, CaO-based sorbents need to be pelletized. However, the pelleting process, particularly the extrusion process, will lead to the loss of specific surface area, hence weakening CO2 sorption performance of sorbent pellets. In previous studies, only limited modification methods were developed for CO2 sorption performance enhancements of sorbent pellet, and the effects of modification are not satisfactory. In this work, the enhancement of CO2 sorption performance of extruded–spheronized pellets was achieved by incorporating the MgO support into CaO-based pellets. Three routines of pellet preparation were studied including soaking (routine I), mechanical mixing (routine II), and simultaneous hydration-incorporation (routine III). It was found that the sorbent pellets produced via routine III possess the best cyclic sorption performance because of the generation of additional pores within the pellets upon the release of CO2derived from the decomposition of magnesium acetate. In addition, the content of incorporated MgO significantly affects the cyclic CO2 capture performance of sorbent pellets. The pellets introduced to 25 wt % MgO display the highest conversion of 66.39%, which is approximately 1.52 times that of the pellets introduced to 5 wt % MgO. Moreover, the MgO-incorporated pellets prepared via extrusion–spheronization possess a high capability to resist attrition.

To examine the real effects of typical inorganics on the emission behavior of PM10 during coal combustion and explore the interactions between the typical vaporized and refractory inorganics, Na-, Si-, and (Na+Si)-loaded coals prepared from the ash-removed PDS bituminous coal and ZD subbituminous coal were combusted in a high temperature drop-tube furnace (DTF) at 1500 °C in air. The produced inorganic PM10 and PM2.5 were collected and characterized. The experimentally measured PM emission results from the combustion of the (Na+Si)-loaded coals were compared with the calculated results that are the sum of PM emitted from the combustion of the Na- and Si-loaded coals. The calculated yields of PM0.3 of two studied coals are higher than the experimental ones, while the calculated yields of PM0.3-2.5 are lower. The experimental yield of PM0.3-2.5 is mainly enhanced by the sodium condensing heterogeneously on fine quartz particles, while the sodium would contribute to PM0.3 homogeneously, leading to the higher calculated result derived from the Na-loaded coal. Na is prevalent in only submicron particles for both the Na- and the (Na+Si)-loaded coals. The homogeneous partitioning ratio of Na is unaffected by the loading of quartz, but the heterogeneous partitioning ratio is affected by the quartz particles on which vaporized Na can condense into particles larger than 0.2 μm. Si presents similar distribution characteristics of trimodal for both the Si- and the (Na+Si)-loaded coal. The partitioning of Si in PM0.3 appears to be affected by Na due to the higher partitioning ratio of the (Na+Si)-loaded coal. The apparent catalytic effects of Na species on the combustion reactivity should beneficiate the local reducing atmosphere, and thereby promote the reduction of SiO2 to SiO, which is easily vaporized to contribute to PM0.3 for the (Na+Si)-loaded coal.

•MgO-based sorbents were pelleted using an extrusion–spheronization method.•The physicochemical properties of the sorbents were changed during pelletization.•The pelletization process can improve the performance of NaNO3 doped sorbents.•After pelletization, the sorbents only doped with Na2CO3 would perform worse.The pelletization of MgO-based sorbents is necessary before implementing intermediate temperature CO2 capture for industrial applications. In the current work, MgO-based sorbent pellets were prepared using an extrusion–spheronization method, with the aim of achieving an effective granulation method for MgO sorbents. Another objective of this investigation is attempting to find out the differences between the sorbents in pelleted and powdered forms. The compression and friability tests have proved that the prepared pellets have outstanding mechanical properties, which are of significant importance for the fluidization in CFBC for the practical applications. N2 adsorption-desorption results have shown obvious variations in the surface areas and pore size distributions of the sorbents before and after granulation. These variations further result in different carbonation/regeneration rates and cyclic CO2 capture performance for the powdered and pelleted sorbents. All these differences in the physicochemical properties between the sorbents pellets and corresponding powders indicate that the results derived from the powdered sorbents in previous studies should be re-examined before the practical application for the realization of highly-efficient CO2 capture.

This study reports occurrence forms of key ash-forming elements in a defatted microalga, and for comparison, its corresponding raw microalga. Freeze-dried powders of a marine microalga (Nannochloropsis oceanica) were sieved to a size fraction of <75 μm and used as raw microalga. The raw microalga was then extracted with hexane to remove crude lipids and prepare a defatted microalga. The raw and defatted microalgae were subjected to chemical fractionation analysis, i.e., sequential leaching in H2O, 1.0 M ammonium acetate (NH4Ac), and 1.0 M hydrochloric (HCl) acid. The results demonstrate that, whereas the contents of Fe and Al in the raw and defatted microalgae are extremely low, those of other ash-forming elements follow a sequence of Cl > K > P > Mg > Na > Ca. Chemical fractionation results suggest that virtually all of the Na, K, and Cl in the raw and defatted microalgae are water-soluble. While majority of P in the two fuels are water-soluble and acid-soluble, most of Mg and Ca are leached in water and NH4Ac solution. As determined via chemical fractionation analysis, lipid extraction leads to the content of water-soluble Ca in the defatted microalga being ∼69.6% higher than that in the raw microalga counterpart, which is accompanied by a reduction in the amount of Ca leached in NH4Ac solution. Similar trend is also observed for Mg, but to a lesser extent.

The pulverized coal combustion in both air and oxy-fuel conditions generally experiences successive coal devolatilization and char combustion processes. The effects of these two processes have important influence on the emission characteristics of particulate matter. With some different characteristics of lignite from the higher rank coal, including the high contents of organically bound cations and the apparent char–CO2 gasification reaction upon oxy-fuel combustion, the effects of both coal devolatilization and char combustion processes on the emission characteristics of particulate matter during oxy-fuel combustion of lignite would accordingly be distinctive. A typical Chinese lignite was devolatilized in a N2 or CO2 atmosphere to generate N2–char and CO2–char, respectively. Raw coal and the two chars were burned in a drop-tube furnace under both air and oxy-fuel conditions. Afterward, the yields and inorganic compositions of segregated PM10 were carefully analyzed. The results have shown that the devolatilization in the N2 condition promotes the emission of sub-micrometer particles (PM0.5) and coarse particles (PM1–10) but the devolatilization process in CO2 inhibits their emission. N2–char and CO2–char present entirely different combustion behavior and, thus, reflect the discrepant emission characteristics of PM10. The rough higher yields of PM0.5 and PM1–10 from N2–char combustion compared to raw coal combustion may be driven by the higher particle combustion rate and/or temperature of N2–char. Furthermore, the quite severe gasification reaction of CO2 with char during char preparation results in an inhibited combustion rate of CO2–char, which can explain the case in which much less PM0.5 and PM1–10 are generated from CO2–char combustion than those from raw coal and N2–char combustion. The apparent effects reveal that more detailed work can be continued to further explore this research.

•Raw and modified montmorillonite show great ability to reduce PM0.2 formation.•Modification treatment promotes reaction between montmorillonite and alkali metal.•SiO bonds are easier to break at high temperature due to the acid modification.•New SiOAl bonds are formed after modified by polyhydroxy aluminum solution.Up to now, nearly all sorbents used to reduce the formation of particulate matter (PM) during coal combustion are raw minerals in nature. In this work, a new idea is proposed to improve the control ability by modifying the natural minerals. First, hydrochloric acid (HCl) and hydroxyl polymeric aluminum were selected to modify raw montmorillonite, respectively. Then combustion experiment of pulverized coal alone and that mixed with the raw/modified montmorillonite was performed under O2/N2 atmosphere in a lab-scale drop tube furnace (DTF). The produced PM was collected via a low pressure impactor system (LPI). The results show that the raw montmorillonite reduced the PM0.2 yield by 17.99%. Compared with raw montmorillonite, PM0.2 yield further decreased by 29.00% and 13.11% when acid-treated montmorillonite and aluminum-treated aluminum were added. Further characterization show that the formation of ultrafine PM was reduced mainly because of the chemical reaction between the sorbent and the vaporized alkali metal. More importantly, the modification treatment optimized the surface characteristics of montmorillonite. Moreover, modification treatment increased the number of free oxygen active sites in montmorillonite due to the breakage or formation of certain metallic bonds. These changes promoted the process of capturing alkali metal and thereby the modified montmorillonite showed better performance in reducing PM0.2 formation.

•Efficient CaO pellets were synthesized with one step via gel-casting technique.•Microcrystalline cellulose is effective to improve the sorbent structure.•Synthesized CaO pellets possess good chemicophysical performance for CO2 capture.•Further efforts are needed to simplify the manufacturing process of this technique.The traditional development of CaO-based sorbents to be used in calcium looping process (CLP) consists of two steps: producing powders with improved cyclic performance by sorbent modification, followed by the subsequent granulation. However, this traditional routine is complicated and energy-consuming. Moreover, the improved performance would be weakened again during the granulation process. Hence, the implement of the modification and granulation within one step to produce CaO-based sorbents is of significant importance for CLP. This work, for the first time, proposes a simple and facile gel-casting technique to achieve the one-step synthesis of highly-efficient CaO-based sorbents pellets. The pellets modified with microcrystalline cellulose (CaO-MC) achieved considerable CO2 capture capacity of ~ 0.48 g CO2/g sorbent at the 25th cycle under the mild calcination condition and retained relatively excellent performance of 0.41 g/g over 25 moderate cyclic tests, higher than those of the unmodified pellets (CaO-pellet). The decomposition of microcrystalline cellulose in CaO-MC does not change the cubic crystal structure of CaO, but brings more pores that contribute a lot to its superior CO2 capture performance. Moreover, all synthesized pellets possess good attrition resistance, indicating the applicability of the prepared pellets to the application in CLP.Download high-res image (227KB)Download full-size image

The high levels of CO2 in oxy-fuel combustion are expected to have important effects on the transformation of pyrite, a major contributor to ash deposition. As a successive work to the previous study that explored the chemical role of CO2 in pyrite decomposition, this paper is purposely designed to investigate the effects of CO2 on the oxidation of pyrrhotite generated from pyrite decomposition. Pyrrhotite oxidation in pure CO2 was respectively investigated at 900, 950 and 1000 °C on a well-designed thermo-gravimetric reactor (TGR). The time-resolved data of sample weight loss and gas evolution were collected online by a data collection module and a Horiba PG-350 gas analyzer, respectively. The solid products were characterized by X-ray diffraction (XRD). The results demonstrate that the influence of CO2 on the oxidation of pyrrhotite is chemical in nature. The transformation of pyrrhotite in CO2 consists of three stages, i.e., the fast weight loss stage, the slow weight loss stage and the slow weight gain stage. The mechanisms involving CO2 at these three distinct stages are quite different. At the first stage, ferrous sulfide is formed by the decomposition reaction of pyrrhotite with CO2. At the second stage, the oxidation reactions of ferrous sulfide with CO2 are responsible for the formation of magnetite or hematite. Hematite may also be formed through further oxidation of magnetite by CO2. The third stage is dominated by the oxidation of magnetite to form hematite with CO as the only gas product.

The loss-in-capacity and attrition of CaO-based sorbents are the two major barriers to CO2 capture in calcium looping process (CLP). In this work, an extrusion–spheronization method was adopted to prepare pellets with strong anti-attrition ability, and novel biomass templates of raw and defatted microalgae were used to enhance the CO2 capture performance of CaO-based pellets with low microalgae doping ratios (0.5–2 wt.%). Experiments without and with excessive (5–20 wt.%) microalgae doped pellets were also produced for comparisons. The results show that a small amount (0.5–2 wt.%) of microalgae was able to significantly enhance the CO2 capture capacity of the pellets. On the contrary, when excessive microalgae were doped, the interaction between CaO and the produced tar and coke from the microalgae during the initial calcination stage leads to the deactivation of the CaO-based pellets; consequently, an inferior CO2 capture capacity is observed in the initial cycle. Comprehensively considering the overall CO2 capture capacity and the cyclic stability, the addition of 2 wt.% microalgae was regarded as the optimal doping ratio. Moreover, the pellets doped with 2 wt.% of defatted microalgae (lower economic value after being defatted) display even better cyclic CO2 capture capacity. Such a low doping ratio is very promising for the practical application of the CaO-based pellets in the CLP.

•Numerous oxygen species active for Hg0 oxidation are obtained on catalyst surface.•The adsorbed active surface oxygen species was significant for the Hg0 oxidation.•Ce0.47Zr0.23Mn0.3O2 performs well in Hg0 oxidation at low temperature.•SO2 promoted the Hg0 oxidation over Ce0.47Zr0.23Mn0.3O2 in the presence of O2.To develop an efficient catalyst used for elemental mercury (Hg0) oxidation at low temperature in coal-fired power plant, Mn doped CeO2-ZrO2 catalysts were synthesized and studied concerning their performance on Hg0 oxidation. Under the atmosphere of simulated coal-fired flue gas, Ce0.47Zr0.23Mn0.3O2 (CZM0.3) exhibited the best activity at 150 °C. It was found that Hg0 oxidation performance over CZM0.3 was determined by the flue gas compositions. Gaseous O2 was important for the oxidation process, because it regenerated the lattice oxygen and replenished the chemisorbed oxygen, which boosted Hg0 oxidation. HCl and NO could improve the Hg0 oxidation efficiency slightly, respectively. In N2 + HCl + O2 atmosphere, the Hg0 oxidation efficiency observed was significantly higher than those in N2 + O2 or N2 + HCl atmosphere. Compared with the effects of HCl on the Hg0 oxidation activity, the similar results were obtained for effects of NO on the performance Hg0 oxidation. NO could facilitate the Hg0 oxidation activity, especially in the presence of O2. In N2 + SO2 atmosphere, SO2 inhibited Hg0 oxidation. In N2 + O2 + SO2 atmosphere, SO2 promoted Hg0 oxidation probably due to the generation of SO3. Water vapor inhibited Hg0 oxidation, because it diminished the HCl and Hg0 adsorption. The characterization of the CZM0.3 indicated that the higher activity of the catalyst was most likely attributed to the presence of more oxygen vacancies, enhanced Mn4 +/(Mn4 ++ Mn3 +) ratio and more surface adsorbed oxygen on the catalyst surface.

Enlarging the capacity of utility boiler is recognized as a good way to improve the electricity generating efficiency. An increasing number of 1000 MW ultrasupercritical (USC) utility boilers are installed in China. This contribution reports the results of systematic field measurements on PM2.5 [particulate matter (PM) with aerodynamic diameters < 2.5 μm] emitted from a 1000 MW USC utility boiler equipped with a selective catalytic reduction (SCR) denitrification (DeNOx) unit, two electrostatic precipitators (ESPs), and a limestone-gypsum wet flue gas desulfurization (WFGD) system. The PM samples were collected using a Dekati low pressure impactor (DLPI) and/or a Dekati gravimetric impactor (DGI) at multiple sampling sites. The results demonstrate that the particle size distributions (PSDs) of the PM at both inlet and outlet of the SCR unit exhibit a bimodal distribution, with a fine mode at < 0.3 μm and a coarse mode at > 0.3 μm. Passing the PM-containing flue gas through the SCR leads to the PSDs of the fine mode particles being shifted to larger size, possibly due to the formation of ammonium sulfate and/or ammonium bisulfate as well as the reduction of flue gas temperature. The removal efficiencies of the SCR for PM1 and PM2.5 are 14.3–33.6% and 13.3–30.5%, respectively, depending on the boiler load. The ESPs substantially reduce the mass concentrations of PM1 and PM2.5 from 84.6–107 mg/Nm3 and 417–440 mg/Nm3 to 0.298–1.22 mg/Nm3 and 0.812–4.41 mg/Nm3, respectively, with overall removal efficiencies of 98.7–99.7% for PM1 and 99.0–99.8% for PM2.5. The WFGD process leads to the disappearance of the fine mode PM and an overall removal efficiencies of up to 28.7% for PM1 and 39.6% for PM2.5. Moreover, promoting the installation of 1000 MW USC utility boilers is likely to simultaneously achieve the reduction in the PM2.5 emission besides the improvement of electricity generation efficiency, particularly when advanced dust removal devices are employed.

Incorporation of CaO in inert solid support has been identified as an effective approach to improve the cyclic CO2 capture performance for CaO-based sorbents. In this work, Yb2O3-supported CaO-based sorbents were fabricated via the wet-mixing technique. Elemental dispersion observed by FSEM-EDS showed that the ultrafine active species of CaO particles was finely separated by Yb2O3. Different amounts of Yb2O3 were introduced and 10−15 wt% was found to be the optimal content range for the support to function as the metal skeleton. In comparison with pure CaO, the improved CO2 capture performance was observed for CaYb10 over prolonged carbonation-calcination cycles under a severe test condition. This improvement could be ascribed to the enriched macropores within 50–100 nm, which was identified by the N2 adsorption–desorption analysis. In addition, the microstructure of the synthetic sorbent (analyzed through TEM images) indicated that CaO particles of ∼160 nm were formed with the Yb2O3 nanocrystallines (∼15–25 nm) adhered to the surface functioning as physical barriers and as a result, the sintering was effectively retarded. Generally, the enhanced cyclic CO2 capture performance and the promoted sintering-resistant property of Yb2O3-supported CaO-based sorbents made Yb2O3 a promising inert solid support.

The rapid decrease of CO2 capture capacity is one of the most challenging problems hindering the use of naturally occurring limestone in the calcium looping process. In this work, the mechanical modification method (dry planetary ball milling) was used to improve the cyclic CO2 capture performance of naturally occurring limestone. Low-cost Bayer aluminum hydroxide sourced from the industrial-scale production of alumina from bauxite ore was used as the precursor of the inert support to enhance the CO2 sorption stability of the ball-milled sorbents. It was found that the CO2 uptake of the milled sorbents could be further improved by increasing the ball-milling time because this generated more amounts of fine particles. Moreover, the pellets produced from ball-milled limestone powder possessed a relatively high CO2 capture capacity of 0.252 g/g in the 25th cycle, which is nearly 1.3 times the capture capacity of naturally occurring limestone powder. This indicates that the combination of mechanical modification and pelletization is an effective approach to produce highly efficient CO2 capture pellets from naturally occurring limestone.

The sacrificial biomass templating technique was used to enhance the sorption performance of CaO-based pellets that were prepared via an extrusion-spheronization method. Five types of biomass materials were used as the templates: microcrystalline cellulose, corn starch, rice husk, sesbania powder, and lycopodium powder. It is found that the addition of biomass templates is effective to improve the cyclic CO2 sorption capacity of the CaO-based pellets. However, two opposite enhancement tendencies of CO2 uptake were observed with the increment of biomass addition. For microcrystalline cellulose, corn starch, and rice husk, more addition amounts would result in better improvement of CO2 sorption performance of the CaO-based pellets. It is attributed to the generated porous microstructure and large amounts of small grains. However, for sesbania powder and lycopodium powder, a decreasing enhancement tendency of the CO2 sorption performance was found with the increasing addition amount. It is probably due to the accelerated sintering of the sorbent because of the presence of excessive amounts of alkali metal elements. Moreover, all biomass-templated CaO-based pellets possess a high anti-attrition capacity.

This study reports the impacts of two commercial wet electrostatic precipitators (WESPs) on the emission of fine particulate matter (PM2.5) and SO3. Field measurements were carried out at two 300 MW coal-fired power station units equipped with WESPs between limestone-gypsum wet flue gas desulfurization (WFGD) unit and the stack. PM samples were collected at the inlet and outlet of the WESPs as well as the inlet and outlet of the dry ESP, and SO3 was collected at the inlet and outlet of the WESP. The results show that 99.21% of the PM2.5 emitted from the boiler is removed in the dry ESP. The WFGD has a removal efficiency of 24.19% for PM2.5 but produces new gypsum particles and then increases the emission of PM1 (by ∼24%). PM at the WESP inlet is centered at 1 μm, and after passing through the WESP, concentrations of PM0.3, PM1, and PM2.5 are reduced by 73.72–93.75%, 83.33–94.41%, and 79.91–90.23%, respectively. SO3 emission is also reduced by the WESP due to the capture of H2SO4 via electrostatic force and the absorption of SO3 by the basic sprayed liquid, with a removal efficiency of 52.03–59.09%. However, new PM larger than 2 μm is generated from the entrainment of circulating liquid droplets in the WESP, which partially offsets the capture of PM2.5. As the boiler load decreases from 100% to 70%, the removal efficiency of PM0.3 increases a little, whereas the removal efficiencies of PM1 and PM2.5 decrease from 89.34% and 89.85% to 83.33% and 79.71% due to the decreased removal efficiency for PM larger than 2 μm. WESP contributes less to the PM2.5 removal compared to the dry ESP (0.54% vs 99.27%) while it effectively reduces fine PM emission.

This contribution reports direct comparison of the particulate matter (PM) removal efficiencies of electrostatic precipitators (ESPs) and fabric filters (FFs) installed in two 300 MW coal-fired power station units that are equipped with identical boilers and DeNOx units. Field PM measurements were carried out at the horizontal ducts at the inlets of the DeNOx units, as well as the inlets and outlets of the two dust collectors when the two boilers burned the same coal under identical combustion conditions. The PM and total fly ash were collected via a low-pressure impactor (LPI) and/or a smoke analyzer. The collected samples were then subjected to analysis for chemical composition and morphology using an X-ray fluorescence (XRF) probe and a field emission scanning microscope with an energy-dispersive X-ray analyzer (FESEM-EDX). The results show that both the ESPs and the FFs can effectively capture PM, with a PM2.5 collection efficiency of 99.63% and 99.95%, respectively. For the PM with similar properties, the collection efficiencies in the FFs are only marginally higher than those in the ESPs. The most notable discrepancies between the fractional PM removal efficiencies of ESPs and FFs are observed for PM in the size range 0.3–2 μm. The PM removal performance of both the ESPs and the FFs is related to particle size. At 0.3–2 μm, the performance of the ESPs is more sensitive to particle size than the FFs. The PM collection efficiency of the FFs is inversely related to particle size, and the capture of PM around 0.4 μm is slightly weak, most likely due to the transition of capture mechanisms from impaction to Brownian diffusion.

This study reports the formation and emission characteristics of particulate matter (PM) with aerodynamic diameters less than 2.5 μm (PM2.5) from two coal-fired circulating fluidized bed (CFB) boilers (B1 and B2) respectively equipped with electrostatic precipitator (ESP) and hybrid electrostatic filter precipitator (EFP). PM and total fly ash samples were collected at the inlets and outlets of the ESP and the EFP via a low-pressure impactor, a gravimetric impactor, and/or a smoke analyzer, and they are further characterized with X-ray fluorescence microscopy as well as field emission scanning electron microscopy with energy disperse X-ray analysis. Results show that PM2.5 generated from the two CFB boilers is of unimodal mass size distribution, with the only size peak in the coarse mode. Different from PM formed in pulverized coal-fired boilers, coarse mode PM generated in the CFB boilers is primarily irregular in shape, and some small particles are observed to adhere on large ones due to the lower combustion temperature and the more vigorous collision and abrasion in CFB combustion. More PM2.5 is produced from CFB combustion as a result of enhanced fragmentation of minerals in coal and sorbent limestone compared with the conventional pulverized coal combustion. The yield of ultrafine PM is inhibited in CFB combustion due to the lower combustion temperature and fixation of volatile species by sorbent limestone, which lead to the absence of an ultrafine modal peak in PM2.5. Downstream from the boilers, most PM is removed in dust collectors, with PM2.5 removal efficiencies of 98.90% and 99.96% for the ESP and the EFP, respectively. And a significant difference appears in the size range 0.1–2 μm, where the PM removal efficiency of the ESP is ∼1.5% lower than that of the EFP.

In this work, a novel joint processing route that integrates the disposal of phosphogypsum waste with CO2 emissions reduction in a cement plant was proposed. The route mainly includes three parts: direct aqueous carbonation of phosphogypsum, use of the obtained carbonation product for CO2 capture in the calcium looping process (CLP), and manufacture of cement clinker using the spent CaO-based sorbent. The direct use of the CO2 derived from cement plant flue gas (20 vol % CO2) is able to convert 94.5% of CaSO4 in the phosphogypsum into CaCO3. However, a long time of 90 min is required for the completion of the conversion. Therefore, we proposed to introduce a part of the highly concentrated CO2 gas stream separated from the CLP and, hence, to increase the overall CO2 concentration of the carbonation gas stream. It was found that only 45 min is needed to achieve a comparable carbonation level when the gas stream containing 45 (or 60) vol % CO2 was used. Moreover, the solid carbonation residues derived from phosphogypsum carbonation possess relatively good cyclic CO2 capture performance, which is superior to the CaCO3 reagent. It indicates that the joint processing route proposed here is feasible, which can not only recycle the phosphogypsum waste but also reduce the CO2 emissions in the cement plant.

•A screening of 12 inert supports for CaO-based sorbents was conducted.•For the first time, 2 inert supports were used to improve the CaO performance.•BET surface area and melting point were found to affect the sorbent performance.A screening of inert solid supports as metal skeletons of CaO-based sorbents was conducted in this work, aiming to provide the basis and guideline for the selection of inert solid refractories to produce high temperature CO2 capture sorbent. We studied 12 different refractories including Al-, Ti-, Mn-, Mg-, Y-, Si-, La-, Zr-, Ce-, Nd-, Pr- and Yb-based supports, among which Yb and Pr are newly introduced ones. The sorbents were synthesized using the same wet-mixing method, by which the inert supports could be well dispersed among the ultra-fine active specie of CaO/CaCO3 particles to fully play the role of frameworks for sintering resistance. The sorbents stabilized by these inert supports were also cyclically tested under the same conditions. Y- and Al-based supports were found to exhibit much superior cyclic performance than the other supports. Mn-, Mg-, La-, Yb- and Nd-based supports are also good candidates, whereas the other ones containing Ti-, Ce-, Zr-, Si- and Pr-based supports show less effectiveness. The melting point of the inert support and surface area of the synthetic sorbents are found to be the key factors to affect the sorbent performance.

Particulate matter (PM) sampling was performed on an industrial boiler (3 MW) that burned various blends of high-Cl coal and low-Cl coal. Blends with high-Cl coal mass fractions of 30%, 50% and 70% were fired during the sampling. The results showed that both the ultrafine PM yield and its Cl content increased when blends with a higher fraction of high-Cl coal were burned. To clarify the effects of increased Cl content on the formation of ultrafine PM during the combustion of coal, combustion of every single coal with several concentrations of extra added HCl was further conducted in a well-controlled drop tube furnace. Based on the field and laboratory study, it was concluded that increasing the concentration of HCl in the combustion atmosphere alone would not significantly promote the yield of ultrafine PM from the combustion of coal. Further characterization revealed that during coal combustion, Cl in coal migrated into ultrafine PM as chlorides, and the presence of alkali and alkaline earth metals was required for the conversion of HCl to chlorides for cations. The combined effects of Cl and Na in coal blends resulted in the increase of ultrafine PM formation and its contents of Cl and Na. Moreover, partitioning of S into the ultrafine PM was reduced due to the competition of cations with the increased content of Cl in the blends.

•The chars from PVC pyrolysis in a wire-mesh reactor were characterized.•Initiation of dehydrochlorination can start at a temperature as low as 200 °C.•Cyclization/aromatization takes place at the early stage of dehydrochlorination.•Hydrocarbon starts to release at the late stage of dehydrochlorination.•The weight loss of PVC pyrolysis is mainly due to dehydrochlorination at <450 °C.The study aims to understand the fundamental mechanism of the pyrolysis of polyvinylchloride (PVC) by investigating the chars produced in a wire-mesh reactor, where the interactions of evolving volatiles and pyrolysing PVC particles as well as the secondary reactions of the volatiles are minimized. The initiation of PVC pyrolysis can start at a temperature as low as 200 °C on the surface of PVC particles via dehydrochlorination, as confirmed by the surface color change and the X-ray photoelectron spectroscopy (XPS) results. However, significant dehydrochlorination reaction mainly starts at ∼300 °C, leading to the formation of conjugated polyene sequences. The results also suggest that the cyclization/aromatization reaction may take place at the early stage of the dehydrochlorination process, as the hydrocarbon release already starts (i.e., at ∼350 °C with a Cl loss of ∼80%) before the termination of the dehydrochlorination process. The initial released hydrocarbons have an H/C atomic ratio close to 1, more likely via intramolecular cyclization/aromatization reaction. However, the contribution of the hydrocarbon release to weight loss is small at low temperatures (<450 °C), and the majority of the weight loss is caused by the dehydrochlorination.

•A comprehensive comparison of modified sorbents by 8 different acids was reported.•For the first time, 3 organic acids were used to modify the CaO sorbent.•Improved structure was achieved through the acidification-decomposition process.•Superior performance of the modified sorbent was proved under oxy–fuel calcination.The acidification-decomposition process was used to enhance the performance of the natural limestone. A series of potential organic acids was employed to modify limestone to produce CaO-based sorbents for high temperature CO2 capture. The organic acids studied include formic acid, acetic acid, propionic acid, citric acid monohydrate, oxalic acid dihydrate, lactic acid, l-(−)-malic acid, and l-(+)-tartaric acid. Particularly, the latter three are novel acids used for the first time to modify limestone to obtain the CaO-based sorbents. The cyclic carbonation–calcination performance of the sorbents was investigated. All the sorbents, except the one modified by formic acid, exhibit the improved performance for capturing CO2. In particular, the novel sorbent modified by l-(+)-tartaric acid (S–T) shows the best cyclic performance with a high carbonation conversion of ∼36% at the 26th cycle, almost twice of that of unmodified sorbent. The superior performance of this sorbent was also found when tested under the simulated oxy–fuel atmosphere (realistic calcination atmosphere for post-combustion CO2 capture). It is believed that the improved structure by organic acid, which is in favor of mitigating the sintering and loss of specific surface area (SSA), has contributed to the superior performance of the sorbent.

•Polystyrene was pyrolysed in a wire-mesh reactor with minimized secondary reactions.•Selectivity of primary product follows the trend of monomer > trimer > dimer.•High temperature promotes the formation of monomer at the expense of trimer.•Selectivities of monomer, dimer and trimer show negligible changes with conversion.This paper reports the formation of styrene monomer, dimer and trimer in the primary volatiles during fast pyrolysis of polystyrene in a wire-mesh reactor where secondary reactions of primary pyrolysis volatiles are minimized. Styrene monomer is the most abundant product in the primary volatiles from polystyrene pyrolysis, with a selectivity of 48–69 wt% depending on pyrolysis temperature, while dimer and trimer only have selectivities of 8–10 and 9–30 wt%, respectively. High temperature promotes the formation of styrene monomer and suppresses the formation of styrene trimer, but with little effect on the formation of styrene dimer. It is also found that the selectivities of styrene monomer, dimer and trimer show negligible changes with increasing the conversion level at the same pyrolysis temperature. The results suggest neither 1,3-hydrogen transfer nor intermolecular benzyl radical addition is the dominant formation mechanism for styrene dimer. The styrene dimer is more likely produced via the 1,7- and 7,3-hydrogen transfer mechanism.

•Low temperature economizer promoted Hg removal performance by ESP in the test.•Unburned carbon adsorbed more Hg at lower temperatures caused by the LTE.•Temperature is the most sensitive factor on Hg0 oxidation by SCR catalysts.The impacts of a low temperature economizer (LTE) on mercury removal across an electrostatic precipitator and influence of load variation on mercury conversion over selective catalytic reduction (SCR) catalysts were determined at two coal-fired boilers. When the LTE was on, the total and elemental mercury removal efficiency increased by 42.87% and 18.85%, respectively, due to the improvement of adsorption and oxidation capacity of the fly ash at lower temperature. Mercury speciation at the inlet and outlet of the SCR system were analyzed, and the impacts of load variation and catalyst aging on Hg0 conversion were discussed. The variable loads resulted in simultaneous changes of the gas hourly space velocity, the ambient temperature, and the oxygen content. The results showed the load ratio was significant for Hg0 conversion by the SCR catalysts and load reduction benefitted Hg0 conversion. When the load ratios were 100%, 75% and 60%, the Hg0 conversion were 61.78%, 65.71% and 72.12%, respectively. Moreover, Hg0 conversion was more significantly affected by the catalyst aging than NOx reduction. Among the three factors, the most important one is the flue gas temperature based on the grey relational analysis.

•V2O5–MoO3/TiO2 shows perfect Hg0 oxidation ability in actual flue gas.•Little Hg0 is oxidized by the actual running SCR deNOx system.•Hg0 oxidized by V2O5–MoO3/TiO2 and actual SCR catalyst are both inhibited by NH3.•V2O5–MoO3/TiO2 exhibits stronger NH3 resistance than actual SCR catalyst.To investigate the mercury oxidation ability of V2O5–MoO3/TiO2 in actual flue gas, a test was performed in a coal-fired power plant. The mercury oxidation ability of the operating selective catalytic reduction (SCR) deNOx system was also detected. The mercury conversion of V2O5–MoO3/TiO2 exceeded 90% in the actual flue gas, and the commercial SCR system almost lost the ability. This is mainly because the actual SCR catalyst has been running for 35,000 h and the test of V2O5–MoO3/TiO2 was in absence of NH3. To clarify the assumption, a laboratory experiment was carried out. V2O5–MoO3/TiO2 and the fresh commercial SCR catalyst were tested in the simulated flue gases in presence and absence of NH3. The results indicate that the mercury conversion of V2O5–MoO3/TiO2 is higher than that of the fresh commercial SCR catalyst even in the presence of NH3. The mercury oxidation ability of the fresh commercial SCR catalyst is stronger than that of the actual service ones. However, it is found that NH3 would inhibit mercury oxidation over V2O5–MoO3/TiO2 in the experiments. The main reason is that NH3 could compete with HCl for the same active sites on the catalyst surface.

The pelletization of CaO-based sorbents is necessary for its practical application in the calcium looping process. In this work, three groups of composite pellets with different structures (non-shell pellets, core-in-shell pellets with inert shells, and structurally improved core-in-shell pellets with semi-reactive shells) were prepared from limestone powder and calcium aluminate cement. For the core-in-shell pellets, 2 wt % rice husks were added to the shells to enable the formation of relatively porous and strong shells. Both the CO2 uptake and mechanical strength of the cement-bound pellets were investigated to find the promising structure for the pelletization of the CaO-based sorbent. Moreover, wet curing was used for the first time, and prolonging the curing time could be effective to enhance the mechanical strength of the pellets. It was found that the core-in-shell pellets with semi-reactive shells via adding a moderate amount of limestone to the outer shell was able to largely improve the overall CO2 uptake capacity and, meanwhile, maintain the relatively good mechanical property. Particularly, when the limestone content of the core was fixed at 80 wt %, the pellets containing 60 wt % limestone in the shell exhibited a high total CO2 uptake capacity of 2.97 g/g during 17 cycles, a value more than twice that of the pellets that did not have limestone in the shells. As a result of limestone addition, the average crushing force of the cured pellets decreased by only 11.8%. Comprehensively, considering the CO2 uptake and mechanical strength, the core-in-shell pellets consisting of highly reactive cores and semi-reactive shells were the most promising to be used in the calcium looping process.

•H2 yield was promoted by adding K2CO3 or CH3COOK but inhibited by adding KCl.•K2CO3 showed different catalytic activity from CH3COOK from 600 °C to 700 °C.•Carbon conversion in SEG process was enhanced by adding K2CO3 or CH3COOK.•The results can be used to select a proper catalyst in SEG process.Sorption enhanced gasification (SEG) of biomass with steam was investigated in a fixed-bed reactor to elucidate the effects of temperature, catalyst type and loading on hydrogen production. K2CO3, CH3COOK and KCl were chosen as potassium catalyst precursors to improve carbon conversion efficiency in gasification process. It was indicated that from 600 °C to 700 °C, the addition of K2CO3 or CH3COOK catalyzed the gasification for hydrogen production, and hydrogen yield and carbon conversion increased with increasing catalyst loadings of K2CO3 or CH3COOK. However, the hydrogen yield and carbon conversion decreased as the amount of KCl was increased due to inhibition of KCl on gasification. The maximum carbon conversion efficiency (88.0%) was obtained at 700 °C corresponding to hydrogen yield of 73.0 vol.% when K2CO3 of 20 wt.% K loading was used. In particular, discrepant catalytic performance was observed between K2CO3 and CH3COOK at different temperatures and the corresponding mechanism was also discussed.

Chlorella vulgaris and Dunaliella salina are two kinds of microalgae, which are widely distributed in China. Thermal decomposition of low-lipid C. vulgaris and D. salina were performed using thermogravimetric analysis. The effect of heating rates on pyrolytic characteristics was investigated, and thermal decomposition kinetics was determined as well. Furthermore, pyrolysis experiments were carried out on a fixed-bed reactor. The gas, char, and tar yields were analyzed, and the mass balance was from 88.4 to 96.8%. C. vulgaris had higher H2 yields and lower CH4 yields than D. salina during pyrolysis. The theoretical calorific value of the pyrolytic gas of D. salina was higher than that of C. vulgaris because D. salina had a higher amount of high heating value components, such as C2H6, C2H4, and C2H2. The biochar from microalgae had a smaller Brunauer–Emmett–Teller surface area than the char from pyrolysis of lignocellulosic biomass. Highest yields of pyrolytic oil were 49.2 and 55.4% (water-free basis) for C. vulgaris and D. salina at 500 °C, respectively. The characteristics of bio-oil from microalgae pyrolysis, including water content, density, acidity, and heating value, were investigated as well as the chemical composition at different pyrolysis temperatures. The microalgae pyrolytic oil was found to have significant levels of alkanes, alkenes, alkines, and esters and be particularly high in nitrogenous compounds. In comparison to the bio-oils from common lignocellulosic biomass, the microalgae oil had lower oxygen and water contents, a lower total acid number, and a higher heating value.

The devolatilization process has important influence on the formation of PM10 (particulate matter with an aerodynamic diameter of ≤10.0 μm) in oxy-fuel combustion of pulverized coal but has been explored little. A bituminous coal was devolatilized in either CO2 or N2 at 1573 K on a drop-tube furnace (DTF) to produce CO2-char and N2-char. Coal and its char samples were burned at 1573 K and in 29 vol % O2/71 vol % CO2. PM10 was collected and segregated into 13 size fractions, which were subjected to subsequent analysis. The results show that the particle mass size distributions of PM10 from coal and chars have similar peak and trough sizes, suggesting that the devolatilization process has insignificant influence on the major pathways of PM10 formation. Three particle modes can be identified, i.e., ultrafine mode (<0.5 μm, PM0.5), central mode (0.5–2.5 μm, PM0.5–2.5), and coarse mode (2.5–10 μm, PM2.5–10). Coal combustion produces more PM0.5 and PM0.5–2.5 than char combustion, suggesting that the devolatilization process has important influence on the production of PM0.5 and PM0.5–2.5. In contrast, the PM2.5–10 yield is insignificantly affected by the devolatilization process under the investigated conditions. In addition, the combustion of CO2-char generates more PM0.5 and PM0.5–2.5 than that of N2-char, indicating that the devolatilization in CO2 favors the formation of PM0.5 and PM0.5–2.5.

Coal fly ash is a potential candidate for CO2 mineral sequestration. If calcium is extracted selectively from coal fly ash prior to carbonation (namely indirect carbonation), a high-purity and marketable precipitated calcium carbonate (PCC) can be obtained. In the extraction process, recyclable ammonium salt (i.e., NH4Cl/NH4NO3/CH3COONH4) solution was used as a calcium extraction agent in this study. The influence of time, temperature, agent concentration, and solid-to-liquid ratio on calcium extraction efficiency was explored. NH4Cl/NH4NO3/CH3COONH4 are confirmed to be effective calcium extraction agents for the high-calcium coal fly ash investigated, and about 35–40% of the calcium is extracted into the solution within an hour. The calcium extraction performance is best for CH4COONH4, followed by NH4NO3 and NH4Cl. Increasing temperature from 25 to 90 °C and agent concentration from 0.5 to 3 mol/L only subtly increases calcium extraction efficiency for NH4Cl and NH4NO3, while the positive effect of increasing temperature and agent concentration is more obvious for CH3COONH4. In the carbonation process, carbonation efficiency, namely conversion of Ca2+ into precipitated calcium carbonate(PCC), is only 41–47% when the leachate is carbonated by CO2. A newly proposed method of substituting CO2 with NH4HCO3 as the source of CO32– yields much higher carbonation efficiency (90–93%). Furthermore, the carbonation reaction rate is also largely improved when carbonating the leachate by NH4HCO3. In addition to these benefits, CO2 capture and storage can be simultaneously realized on-site if integrating the leachate carbonation process with an ammonia–water CO2 capture process using NH4HCO3 as a connector. In this way, the costs associated with CO2 compression and transportation can be eliminated. PCC with a purity up to 97–98% is obtained, which meets the purity requirement (≥97%) of industrially used PCC. It is estimated based on the experimental results that 0.17 tons of PCC can be produced from 1 ton of coal fly ash by this method, bounding 0.075 tons of CO2 at the same time, and 0.036 tons more CO2 can be avoided if the obtained PCC is substituted for the PCC manufactured by the conventional energy-intensive method.

This paper investigates the leaching behavior of trace elements in a typical ash dump in the Guizhou province of western China. The ash samples obtained from a power plant were sieved into three sizes: <45, 45−71, and >71 μm. Column leaching tests were performed for the size-classified fly ash samples. The HCl solution of pH 4.5 was used to simulate local precipitation. The flow rate was chosen as 1 mL/min. Leaching time intervals were selected in a range from 15 min to 40 days. The results show that the surrounding soil is strongly affected by the ash dump, and the concentrations of some harmful elements in soil are much higher than the background values. The maximal single element pollution index of the soil sample is 134.81 for Cd. In comparison to the concentrations of trace elements in the upstream water, Cr, Cd, Pb, Zn, and Co are greatly enriched in the downstream water. The pollution of the underground water is mitigated by the use of leakproof film. The column leaching test results show that the concentrations of Pb and Cd fluctuate in short-time column leaching. The concentrations of Cd are kept stable after 16 days in three size-classified fly ash samples. However, the concentrations of Pb increase from 1 day to 40 days in the fly ash with a small particle size but are kept stable after 16 days for large particle size fly ash. In comparison to Cd and Pb, the concentrations of Zn in short-time column leaching increase significantly with relatively small fluctuations.

The present paper was addressed toward the impacts of indigenous and added minerals on the organic and inorganic sulfur transformations during CO2-pyrolysis in a fixed-bed reactor at a temperature range of 400−800 °C. A Chinese bituminous coal was separated into three density fractions using the float-sink method: light (<1.4 g/cm3), medium (1.4−2.0 g/cm3), and heavy (>2.0 g/cm3). The sulfur retention of the three coal fractions was characterized in detail to study the influence of indigenous minerals on organic and inorganic sulfur transformation. Moreover, the role of added minerals was investigated by adding NaCl and kaolinite. The results indicated that indigenous minerals promoted the sulfur retention in coal char, especially in C>2.0, but had little effect on the decomposition of inorganic sulfur. Meanwhile, it was found that the added minerals also increased the sulfur retention. NaCl had a remarkable effect on sulfur retention at a low temperature range but appeared to have little capacity of sulfur retention due to the volatilization of itself at high temperatures above 700 °C. As for kaolinite, it was observed that there was an indistinct ability to retain sulfur during CO2-pyrolysis in our study. In comparison with added minerals, indigenous minerals had more significant effects on sulfur transformation because they were inherently embedded in the organic matrix of coal, while added minerals were physically dispersed within coal.

Fine potassium-enriched particulates generated from coal combustion are harmful to the environment and human health. To study the formation and control mechanisms of fine potassium-enriched particulates during coal combustion, raw lignite, potassium-rich coal obtained from raw lignite, and potassium-rich coal mixed with kaolin were combusted in an electrically heated drop-tube furnace. The effects of the combustion temperature and atmosphere were considered. The particulate matter generated is subject to size distribution, concentration, elemental composition and morphology analysis. Thermodynamic calculations were also performed to simulate the vaporization behavior of potassium compounds in coal under various conditions. As a result, when the combustion temperature was increased from 900 to 1300 °C under O2/N2 conditions, the transformation of potassium from potassium-rich coal to PM1 decreased, because the formation mechanism of PM1 from volatile potassium had changed. When the combustion temperature was 900 or 1100 °C, K2SO4 was the main potassium compound vaporized and condensed to form PM1, with an average size of 0.5 μm. When the combustion temperature increased to 1300 °C, in addition to a small amount of K2SO4, gaseous KOH becomes the main potassium compound vaporized, most of which may react with aluminosilicates to form coarse particulates. The remaining vaporized K2SO4 and other sulfates/oxides condensed to form PM1, with an average size of 0.2−0.3 μm. As the combustion temperature was increased, volatile potassium transferred to coarse particulates in increasing amounts. Kaolin could effectively capture the vapor of potassium compounds during the potassium-rich coal combustion. When the potassium-rich coal mixed with kaolin was combusted at 900, 1100, and 1300 °C, the maximum capture efficiency was obtained at 1100 °C, as the result of combined effects of chemical adsorption increasing and physical activity decreasing with an increasing combustion temperature.

Nanoparticles are thought to induce more severe health impacts than larger particles. The nanoparticles from coal-fired boilers are classified into three size fractions with a 13-stage low pressure impactor. Their physicochemical properties are characterized by the high-resolution field emission scanning electron microscope and X-ray fluorescence spectrometer (XRF). The results show that coal-derived nanoparticles mainly consist of individual primary particles of 20–150 nm and their aggregates. Inorganic nanoparticles primarily contain ash-forming elements and their aggregates have a dense structure. Organic nanoparticles are dominated by the element carbon and their aggregates have a loose structure. Nanoparticles from the same boiler have a similar composition and are primarily composed of sulfur, refractory elements and alkali/alkaline elements. Some transition and heavy metals are also detected. For different boilers, greater differences are observed in the production of the nanoparticles and their composition, possibly due to the use of low-NOx burners. Coal-derived nanoparticles have a small size, large specific surface area and complicated chemical composition, and thus are potentially more harmful to human health.

Measurements of the characteristics of particulate matter (PM) were performed at the inlet and outlet of the electrostatic precipitators (ESP) of four boilers in two full-scale pulverized coal power plants. PM was collected with a 13-stages low-pressure-impactor (LPI) having aerodynamic cut-off diameter ranging from 10.0 to 0.03 μm for a size-segregated collection. The properties of PM including its concentration, mass size distribution, emission characteristics, percent penetration of PM through ESP and elemental composition were investigated. The experimental results indicate that, in all the cases the mass size distribution of PM10 had typical bimodal. PM1 contained up to 1.15 wt% of the total particle (TP) generated in the boilers. PM2.5 contained about 2 wt%–7 wt% of the TP and PM10 contained about 4 wt%–19 wt% of the TP. When additive limestone used for desulphurization as sorbent besides PM generated from coal combustion, there was new PM generated from limestone. Penetration as a function of particle diameter had a clear peak in particle size ranging from 0.2 to 0.6 μm. Particles in the submicrometer size range were much more difficult to be collected with ESP than larger particles. Distributions of individual elements within PM10 were different.

The present study is a further effort to extend our knowledge of the included and excluded mineral characteristics responsible for the formation of particulate matter (PM). A Chinese bituminous coal was first separated into three density fractions using the float-sink method: heavy (>2.0 g/cm3), medium (1.4−2.0 g/cm3) and light (<1.4 g/cm3). Then, combustion and pyrolysis of coal with different density fractions were carried out in a laboratory-scale drop tube furnace to understand the formation mechanism of inhalable particulate matter, less than 10 μm (PM10) and less than 1 μm (PM1). PM10 was collected with a 13-stage low pressure impactor (LPI) having aerodynamic cutoff diameters ranging from 10.0 to 0.03 μm for a size-segregated collection. The experimental results indicated that the light fraction of the coal produced 44 wt % of total PM10 and 45 wt % of total PM1. The medium fraction of the coal contributed 52 wt % of total PM10 and 49 wt % of total PM1. The heavy fraction contributed 4 wt % of total PM10 and 6 wt % of total PM1. The light fraction and the medium fraction of the coal contained mostly included mineral and the heavy fraction contained largely excluded minerals. The PM10 and PM1 contents formed by the excluded minerals were very low compared to those formed primarily from included minerals. The proportion of the minerals in the light density fraction converted into PM1 and PM10 was the highest, with their weight percentages being 9.59% and 43.49%, respectively. There were three reasons for this. One of reasons was the mineral particle size. The median mineral size in the light density fraction coal was smallest. However, the median size of each coal fraction was almost the same. Another reason was mineral transformation during combustion. The light fraction and the medium fraction of the coal contained mostly included minerals, and the heavy fraction contained largely excluded minerals. The transformations of included and excluded minerals were largely different and played a different role during coal combustion. The last reason was char fragmentation. Char formed by the light coal fraction was easier to fragment and subsequently formed more fine ash particles. This was because the swelling ratio, BET surface area, and total pore volume of char decreased with increasing parent coal density.

Homogeneous mercury speciation in combustion-generated flue gases was modeled by a detailed kinetic model. This kinetic model includes the oxidation and chlorination of key flue-gas components, as well as six mercury reactions involving HgO with new reaction rate constants calculated neither from experimental data nor by estimated, which was commonly used by other investigators before, but directly from transition state theory (TST). The microcosmic kinetic mechanisms of reactions between mercury and oxidizing species were investigated by ab initio calculations of quantum chemistry. The geometry optimizations of reactants, transition states, intermediates and products were made by the quantum chemistry MP2 method at SDD basis function level. Among those reactions involving HgO, the progress of reaction HgO + HCl → HgCl + OH is HgO + HCl → TS1(HgClOH) → M(HgClOH) → TS2(HgClOH) → HgCl + OH, in which the controlling step is HgO + HCl → TS1(HgClOH) → M(HgClOH). The progress of reaction HgO + HOCl → HgCl + HO2 is HgO + HOCl → M(HgClOOH) → TS(HgClOOH) → HgCl + HO2, in which the controlling step is M(HgClOOH) → TS(HgClOOH) → HgCl + HO2. Four other reactions are one-step, with no intermediates formed. The performance of the model was assessed through comparisons with experimental data conducted by three different groups. The comparison shows that model calculations were in agreement with only one set of all the three groups experimental data. The deviation occurs due to the absence of accurate rate constants of existing mechanism, the adding of reactions involving HgO, as well as the exclusion of heterogeneous Hg oxidation mechanism. Analyses by quantum chemistry and sensitivity simulations illustrated that the pathway Hg + ClO = HgO + Cl is more significant than some of the key reactions in the kinetic mechanism proposed in the literature, which indicates the necessity of including reactions involving HgO in the mercury kinetic mechanism. Studies on the effects of oxygen show that O2 weakly promotes homogeneous Hg oxidation, especially under the condition of low Cl2 concentration.Six mercury reactions involving HgO with new reaction rate constants were directly calculated by transition state theory (TST). Homogeneous mercury speciation in combustion-generated flue gases was modeled by a detailed kinetic model. Simulation results on the effects of oxygen show that O2 weakly promotes homogeneous Hg oxidation, especially under the condition of low Cl2 concentration.

Combustion of pulverized coal was studied in a drop tube furnace to understand coal mineral properties with the emission of particulate matters (PM). Experimental conditions were selected as follows: coal particle size was smaller than 63 μm; reaction temperature was 1 100°C, 1 250°C and 1 400°C respectively; oxygen content was 20% and 50% respectively. PM was collected with a 13-stagelow pressure impactor (LPI) having an aerodynamic cut-off diameter ranging from 10.0 μm to 0.03 μm for a size-segregated collection. Such properties as concentration, particle size distribution and elemental composition of PM were investigated. The experimental results indicate that the emitted PM has a bimodal distribution having two peaks around 4.0 μm and 0.1 μm. Increasing temperature leads to the formation of more PM; varied oxygen content leads to much change of emitted PM. PM was also subjected to XRF analysis to quantify the elemental composition. The results show that PM of 0.1 μm is rich in sulfates. Meanwhile, SiO2 and Al2O3 are prevalent in PM of 4.0 μm, which means that the last peak around 4.0 μm is mainly aluminosilicate salts.

•The sodium aluminosilicate plays an important role in PM2.5 reduction by kaolin.•The capability of kaolin to reduce PM2.5 depends on the sodium occurrence in coals.•The effect of kaolin on PM2.5 reduction becomes weaker during O2/CO2 combustion.•Particle collision may be taken into consideration for PM2.5 reduction.Little work has been performed on the importance of sodium and its occurrence in coal to PM2.5 (particles less than 2.5 μm in aerodynamic diameter) reduction by kaolin during O2/N2 combustion and O2/CO2 combustion at high temperatures. In this study, the combustion experiment of a treated low-sodium coal with sodium aluminosilicate additive was conducted in a lab-scale drop tube furnace (DTF) at 1500 °C to reveal the contribution of mineral melting and coalescence to PM2.5 reduction. Meanwhile, two typical Na-loaded coals (in which the sodium was loaded in the form of NaCl and sodium carboxylate, respectively) with kaolin added were also burnt under O2/N2 and O2/CO2 atmospheres to investigate the effect of interaction between kaolin and different chemical form sodium on PM2.5 reduction. The results show that sodium aluminosilicate is able to promote the migration of PM0.5–2.5 (particles in aerodynamic diameter of 0.5–2.5 μm) to form coarse particles. Due to the stronger reactivity of sodium carboxylate reacting with kaolin than that of NaCl, PM0.2–0.5 (particles in aerodynamic diameter of 0.2–0.5 μm) decreases more significantly in the combustion when adding kaolin into the NaAc-loaded coal than into NaCl-loaded coal. In addition, the PM0.2–0.5 reduction in O2/CO2 combustion is lower than that in O2/N2 combustion owing to the less vaporization of metals and the slower diffusion rate of vapors in the O2/CO2 atmosphere in comparison to those in the O2/N2 atmosphere. The mineral coalescence varied in interactions of kaolin with NaAc and NaCl. Besides, the PM0.5–2.5 emission differed as a result of differences in coal characteristic and the atmosphere, and this would cause the difference of collision frequency between particles and additive. With the joint actions of mineral coalescence and particle collision, the NaAc-loaded coal has a higher PM0.5–2.5 reduction by kaolin than NaCl-loaded coal, especially under the O2/N2 combustion. An expression describing the relationship of PM0.5–2.5 reduction, mineral coalescence and particle collision was fitted and it is found that the mineral coalescence has a stronger influence than particle collision on PM0.5–2.5 reduction by kaolin.

The high levels of CO2 and H2O in oxy-fuel/oxy-steam combustion are expected to have important effects on the oxidation of ferrous sulfide, which is critical to the understanding of ash deposition. In this work, the kinetics of ferrous sulfide oxidation in CO2, H2O, or the mixtures of CO2 and H2O are explored on a purposely-designed thermo-gravimetric reactor (TGR). Based on the data obtained from TGR, XRD and SEM, the roles of CO2 and H2O in the oxidation of ferrous sulfide are elucidated. It is found that pyrrhotite (FeS1+x) transformation in CO2, H2O, or their mixtures is dominated by the oxidation of ferrous sulfide (FeS). The adsorption and desorption elemental reactions of CO2 or H2O on ferrous sulfide are essentially the same. For all cases, the three-dimensional diffusion model can be used to account for the oxidation of ferrous sulfide. The reaction rate constants for the oxidation of ferrous sulfide increase with increasing the partial pressure of the oxidant. The dependence of the reaction rate constant on the partial pressure of CO2 or H2O can be well explained by the Langmuir–Hinshelwood rate forms. In the mixtures of CO2 and H2O, the active sites for the FeS–CO2 reaction and the FeS–H2O reaction are partially independent and partially shared.

Odorous sulfur-containing gases released during sewage sludge processing cause significant environmental pollution. This study investigates the effect of alkali addition on sulfur transformation as well as the release of the sulfur-containing gases during sewage sludge pyrolysis at 150−450 °C. The presence of alkali can promote the transformation of unstable organic aliphatic and aromatic sulfurs into more stable sulfoxides and sulphonic acid at low temperatures (i.e., 250 °C). Alkalis also fix the inorganic sulfide and sulphate in char. Therefore, the release of the sulfur-containing gases can be greatly reduced with alkali addition. Stronger alkalis show more significant effect on the reduction of sulfur-containing gases, likely because of more OH radicals generated for the transformation of organic sulfurs.

This contribution reports the real effects of coal particle combustion on the emission behavior of particulate matter (PM) with aerodynamic diameters of <10 µm (PM10). To do this, two coals, i.e., the ZD subbituminous coal and the PDS bituminous coal, were acid-washed to remove most inorganic compositions. The acid-washed coals were then loaded with identical contents of organically-bound Na and excluded quartz to produce the Na + Si loaded coals, which were then combusted via a high temperature drop tube furnace (DTF) at 1500 °C in air to collect the inorganic PM10 for subsequent analyses. Burning the Na + Si loaded coals with simplified and identical mineral compositions enabled us to purposely study the real effects of coal particle combustion on PM10 emission. The results demonstrate the trimodal distributions of PM10 produced from the Na + Si loaded coals containing such simple inorganic compositions. The combustion of PDS Na + Si loaded coal seems to form more PM than that of ZD Na + Si loaded coal. In contrast to the combustion of the ZD subbituminous coal, more Na and SiO are vaporized to form more PM0.5 due to the higher peak burning temperature of the PDS bituminous coal during the stage of volatile matter combustion, meanwhile, more coalescence of quartz occurs due to its melting point being decreased by the heterogeneous condensation of Na, causing more PM2.5–10 being generated from the combustion of the PDS bituminous coal. When the PDS Si-only loaded coal is burned, the coalescence of quartz particles should be suppressed due to the lack of condensed Na. In addition, its lower char burning temperature can also suppress the coalescence of fine quartz particles, forming less PM2.5–10 compared with the combustion of the ZD Si-only loaded coal. The results suggest that both the stages of volatile matter combustion and char combustion of coal particles can affect the emission of PM10.

The influence of KCl and CaCl2 on the primary reactions of cellulose pyrolysis is studied using a wire-mesh reactor from 250 °C to 600 °C, focusing on the reaction intermediates. A pre- column derivatization with benzoyl chloride prior to HPLC analysis is applied for the quantification of anhydro-sugars (levoglucosan, cellobiosan, maltosan) from pyrolysis. At low temperatures, the additions of inorganics salts, especially CaCl2, weakens hydrogen bonds, resulting in high yields of levoglucosan and cellobiosan from the cleavage of glycosidic bonds rather than from dehydration reactions. At elevated temperatures, dehydration reactions in the sugar units are mainly responsible for the destruction of sugar rings followed by the scission of pyran rings, leading to the weight-loss of CaCl2-loaded cellulose in the form of low molecular weight organic species. Meanwhile, accumulated unsaturated structures suppress the cleavage of glycosidic bonds, leading to the formation of char. However, KCl appears to catalyze the cleavage of glycosidic bonds or the scission of pyran rings directly, which perhaps occurs through a homolytic mechanism, leading to low molecular weight species. Furthermore, maltosan is shown to be a secondary product and is catalyzed by KCl and CaCl2 indirectly through the repolymerization of levoglucosan in the solid phase. A modified mechanism is also proposed regarding cellulose pyrolysis and the primary catalysis of KCl and CaCl2.

This study reports the release of five sulfur-containing odorants (H2S, SO2, COS, CS2 and CH3SH) during the pyrolysis of sewage sludge. Pyrolysis experiments using both raw sewage sludge and seven sludge samples loaded with various organic sulfur compounds were performed to determine the effects of the type of organic sulfur on the release of sulfur-containing gases. It was observed that the formation of H2S, the predominant odorant, significantly increases at temperatures above 150 °C. Aliphatic and aromatic sulfur compounds were found to be the two main organic sulfur sources for the release of sulfur-containing gases. The release of sulfur-containing gases from aliphatic sulfur greatly increases at 250 °C, whereas significant sulfur-containing gases are released from aromatic sulfur at relatively high temperatures of 350–450 °C. It was also observed that the presence of sulphonic acid and thiophene in sludge does not significantly affect the release of sulfur-containing gases.

The particulate matter and trace elements from a 660 MW coal-fired power plant boiler which equipped with a novel electrostatic precipitator were sampled and analyzed. To promote the thermal efficiency of power plants, a low temperature economizer was installed at the inlet of electrostatic precipitator to collect the heat generated from flue gas. The low temperature economizer can reduce flue gas temperature, and then affect the operation of electrostatic precipitator. Therefore, this experiment was carried out to investigate the collection characteristics of this novel electrostatic precipitator on particulate matter. In addition, the distribution of trace elements in solid combustion residues was also studied. The results indicate that the low temperature economizer can markedly decrease the amount of particulate matter at the outlet of electrostatic precipitator. The collection efficiency of electrostatic precipitator on particulate matter is significantly improved by the low temperature economizer, whereby the collection efficiencies of PM2.5 and PM1.0 can reach 99.7% and 99.2%, respectively. Most of the trace elements remain in the fly ash collected by the electrostatic precipitator, and less than 10% remain in the bottom ash, but very rare emit from the electrostatic precipitator. The low temperature economizer not only reduces the emission of particulate matter, but also diminishes the emissions of trace elements in flue gas. The enrichment characteristics of trace elements in submicron particles were also studied.

•A novel compound demister with some advantages was proposed.•The performance was evaluated by experimental and numerical methods.•At low gas velocities or for small droplets, the compound demister is superior to the wave-plate demister.•At high gas velocities, the compound demister shows higher resistance to droplet re-entrainment.A novel compound demister that combines an upstream tube bank and downstream wave plates was proposed in this work for application in multistage flash (MSF) desalination process. Its performance was evaluated by experimental and numerical methods. Compared with the individual tube-bank and wave-plate demisters, the compound demister is found to have the highest separation efficiency (> 95%) with much less fluctuation for a wide range of gas velocities. At low gas velocities (≤ 4 m/s) or for removing small droplets (< 20 μm), the separation efficiency of the compound demister is much higher than that of the wave-plate demister mainly because of the large separation capability of the tube bank. At high gas velocities (> 4 m/s), the compound demister shows higher resistance to droplet re-entrainment that occurs at inlet gas velocity of approximately 7 m/s compared with the tube-bank demister. This is due to the compensation from the wave plates in the compound demister that separate secondary droplets generated by tubes. The compound demister possesses higher dry pressure drops than either the tube-bank or wave-plate demister, but is acceptable for industrial application. All these advantages make the compound demister a promising candidate for droplet removal in the desalination process.

Ultrafine particulate matter (PM) is an important part of PM2.5, which is enriched with hazardous components and more harmful; meanwhile it cannot be effectively removed by the common-used dust collectors. In-furnace sorbent injection is an emerging technology to reduce the emission of PM in the coal combustion and the sorbent is a key factor determining its feasibility. In this study, to seek new PM sorbents, eight minerals were first separately added into a pulverized coal and burned in a drop-tube furnace (DTF) at 1773 K. The derived PM was collected via a Dekati Low pressure impactor (DLPI) sampling system and impacts of each mineral on the PM emission were evaluated. Then, the tested minerals were burned with pure sodium acetate (NaAc) under the same conditions to determine their Na fixation abilities. Finally, the PM reduction mechanism of the sorbent was discussed based on the particle size distribution, mass yield, composition and micromorphology of PM and the chemical and physical properties of the sorbent. A novel Ti-based PM reduction sorbent was screened out, which exhibited an ultrafine PM (PM0.2) reduction efficiency of ∼39% under the experimental conditions at an addition ratio of 5% (wt.%, coal basis). The Ti-based sorbent reacted with Na-contained vapour and formed sodium titanates. Fixation of the Na-contained vapour by the sorbent was considered to be the primary PM capture mechanism. What’s more, the high sintering temperature of the Ti-based sorbent facilitates its PM reduction performance. Under the high temperature combustion conditions, the Ti-based sorbent exhibited a good performance in capturing ultrafine PM and Na-contained vapour, indicating its potential of being a high temperature sorbent.