Coal slime is a byproduct of the coal preparation process. Research on coal slime combustion is of primary importance for the utilization of coal slime and energy savings. To explore applications in circulating fluidized bed boilers, the combustion characteristics of single coal slime particles in air at different furnace temperatures were studied. The temperature changes in the center of the coal slime particles, and combustion images during the combustion process were obtained. The rapid pyrolysis of the coal slime resulted in the production of small molecules of volatile gas, tar, and other pyrolysis products. Next, the pyrolysis caused an unusual change in the surface structure of the particles. The temperature changes in the different types of coal slime varied; however, those in the same coal slimes with different particle sizes at the same furnace temperature were similar. With an increasing particle size, the diffusion of internal oxygen slowed, which resulted in slower combustion processes and increases in the ignition delay and burnout times. The exothermic performance index varied for the different coal slimes and decreased with an increase in the particle diameter, which was related to the contents of volatiles and ash in the coal slime.

•AFFC employs a main fuel distribution device and an adjustable fuel feeding method.•AFFC and its AFFM are used to respond to off-design T3 operations for a 30 kWe MGT.•A calculation method in terms of the AFFM characteristic number.•Effects of the AFFM on the AFFC combustion performance are analyzed under each T3.•Updated MGT equipped with AFFC and its AFFM only adds two control parameters.To respond to off-design operation of the micro-gas turbines, this research proposed an adjustable fuel feeding combustor (AFFC). The AFFC employed a main fuel distribution device and an active control method called the adjustable fuel feeding method (AFFM). Through the main fuel distribution device, the AFFC can switch various groups of working main fuel tubes (MFTs), ultimately achieving the AFFM. Each group has a different symmetrical distribution feature of working MFTs. Based on these features, the AFFM characteristic number (U) has a unique calculation value for the AFFM at each group, and particularly, it was entirely different from the values of U for the AFFM at the other groups. In this research, four different working MFT groups were presented, and the values of U were calculated to be 0.143, 0.333, 0.429 and 1 for the AFFM at each group, respectively. As U changes, the AFFC combustion performance was investigated numerically at various combustor inlet air temperatures (T3) of 600, 700, 800, 900 and 1000 K. Moreover, the CFD method applicability was verified by the experimental data. The results indicate that at different T3, NO emission has various trends with the rising U due to the coupling influences of the fuel flow and fuel jet velocity of each working MFT, while CO emission and combustion efficiency are only affected by the AFFM at the low T3 of 600 K. The outlet temperature distribution factor keeps growing, and the total pressure recover factor remains almost the same as U rises.Download high-res image (190KB)Download full-size image

Capturing SO2 using natural limestone in situ is an important way of desulfuration in a circulating fluidized bed (CFB) boiler. The limestone desulfuration mainly consists of two processes in the condition of standard pressure and air atmosphere: calcination and sulfuration. Water vapor in the flue gas influences the calcination, and the physical characteristics such as the pore structure of calcined CaO play an important role in the following sulfation. The impacts of water vapor during calcination may be reflected on the surface micromorphology and pore structure of calcined CaO. This work aims to understand the influence of water vapor on surface morphology and pore structure of calcined CaO. One kind of Chinese limestone was used to study the issues in a rotatable fluidized bed reactor. Scanning electron microscope (SEM), confocal scanning laser microscope (CSLM), mercury injection apparatus (MIP), and N2 adsorption instrument were employed to test the micromorphology and pore structure of calcined CaO. Results show that the existence of water vapor accelerates calcination and shortens the reaction time, but higher water vapor content results in slightly lower ultimate degree of conversion. Testing results of SEM and CSLM show that water vapor improves sintering and growth of grains of calcined CaO. The change of surface roughness also proves the above conclusion; besides, the results of MIP and N2 adsorption instrument show that there are pores with a size range of 15–80 nm inside calcined CaO without H2O(g), and after sintering under the condition of H2O(g), they will combine together to form relatively bigger pores with a size range of 40–100 nm. As a result, the average pore size increases and the specific surface area decreases but the specific pore volume is less influenced. The fractal dimensions of pore structure for calcined CaO under different concentrations of water vapor were calculated using the data of N2 adsorption. Results show that as the concentration of water vapor increases, the fractal dimension first decreases and then slightly increases but is still lower than that without H2O(g). It means that the pore structure of calcined CaO becomes simpler with the impact of water vapor, which is beneficial for the reaction between calcined CaO and gases such as SO2.

Limestone has been widely used to remove SO2 from circulating fluidized bed (CFB) boilers. As the main component of limestone, CaCO3 first undergoes calcination to CaO, which is then sulfated by SO2 to form CaSO4 in the furnace. Water vapor accounts for a high concentration in coal-fired CFB furnaces, which could influence CaO sulfurization. This work aims to understand this issue. Sulfurization experiments were conducted with the CaO calcined from a natural limestone in a laboratory-scale fluidized bed reactor. Scanning electron microscopy, a mercury injection apparatus along with a N2 adsorption instrument, and X-ray diffraction were employed to test the micromorphology, pore structure, and crystal structure of the sulfurization products, respectively. The results show that CaO sulfurization was improved by water vapor. In addition, an optimum amount of water vapor exists to achieve maximum sulfurization. The results of micromorphology results indicate that sintering, fusion, and growth of grains of sulfurization products were improved by water vapor. The testing results of pore structure show that the pores whose size ranges from 2 to 100 nm will partially or completely change to be larger than 100 nm in the presence of water vapor. The mechanisms can be uncovered in terms of the crystal structure: the diffusion of solid-state ions for both reactant CaO and product CaSO4 is enhanced by water vapor, and the former belongs to surface diffusion without influencing the crystal structure, while the latter is more inclined to volume diffusion because crystal defects are formed due to water vapor.

•A novel temperature-controlled probe was designed to collect ash deposits.•The highest deposition propensity appeared under 21% O2/79% CO2 atmosphere.•PM10 concentration decreased in the order of 30% O2/70% CO2 > 21% O2/79% CO2 > air.A bituminous coal was burned in a bench-scale fluidized bed to investigate the associated fly ash depositions under both oxy-fuel and air combustion conditions. The results showed that the highest deposition propensity appeared under 21% O2/79% CO2 atmosphere, and only a small difference was observed between 30% O2/70% CO2 and air atmosphere. The chemical compositions and micro-morphologies of ash deposits and fly ash were analyzed by inductively coupled plasma-atomic emission spectrometry and by scanning electron microscopy, while particulate matter with an aerodynamic diameter less than 10 μm (PM10) was measured by electrical low pressure impactor. The results showed that there were no obvious differences in the chemical compositions of fly ash and ash deposits between combustion conditions, except for the slight variations in K, Na and S contents. The concentration of PM10 decreased in the order of 30% O2/70% CO2 > 21% O2/79% CO2 > air.

Fluidization–suspension combustion technology is an effective method to utilize coal water slurry (CWS) as a fuel in industrial boilers. The evolution of surface morphology and pore structure of CWS spheres under fluidization–suspension combustion is studied. A bench-scale fluidized bed was used for combustion of CWS spheres with a bed temperature of 850 °C, fluidization number of 4, and bed height of 90 mm. After 15, 30, and 45 s of combustion, the samples were removed from the bed for scanning electron microscope (SEM) and N2 adsorption tests. The combustion mechanism of CWS spheres in the fluidization–suspension combustion state is discussed. The results show that after 15 s, the CWS spheres burst due to volatile release, and some particles fragmented to produce a large number of pores. Thus, the specific surface area and volume of pores increased rapidly. After 30 s, combustion occurred mainly at the exterior surface of CWS spheres and appeared as layer by layer inward combustion. This was confirmed by the fact that the specific surface area and volume did not change. After 45 s, as combustion proceeded, the flame front entered the interior surface through pores, and burnt the interior framework to make the pores collapse. Thus, the specific surface area and volume of pores decreased rapidly. In the whole combustion process, the fractal dimension first increased and then decreased, which demonstrates that the pore structure had experienced a process that went from complicated to simple.

In-furnace desulfurization has been widely used in circulating fluidized bed boilers. SO2 is removed by reacting with limestone during the process of sulfation after calcination in air combustion. Although CaCO3 is the main component of limestone, there are also other impurities such as CaMg(CO3)2 and SiO2 which can influence the desulfurization. The porous CaO produced by calcination plays an important role in sulfation, and water vapor in the furnace influences the calcination. This work aims to understand the impacts of impurities and water vapor on limestone calcination. Two kinds of China limestone were used to investigate the issues in a rotatable fluidized bed reactor. Mercury injection apparatus (MIP), scanning electron microscope–energy dispersive spectrometer (SEM-EDS), and X-ray diffraction (XRD) techniques were employed to analyze the pore structure, micromorphology, and crystal structure of the CaO calcined, respectively. The results show that the water vapor improves the calcination rate and shortens the reaction time, and those influences are stronger for higher impurity limestone possibly because of more defects in the crystal structure. Water vapor can directly influence the chemical reaction of calcination without affecting the diffusion property of CO2. Higher water vapor content results in slightly lower ultimate degree of conversion of limestone, but for different kinds of limestone the difference is not obvious. The results of SEM and MIP also mean that the existence of water vapor improves sintering and growth of grains. The results of XRD give further evidence to the previous conclusion. These tests and analysis give rise to the mechanisms behind the impacts of water vapor on limestone calcination: the binding ability of H2O to active site O* in Ca–O is stronger than that of CO2. H2O tends to replace CO2 on the active site to increase the release of CO2 in calcination. Water vapor also accelerates sintering, most possibly in the initial stage when the sintering neck is formed. There exist two possibilities: H2O molecules are absorbed on the active site of Ca–O* to promote the formation of the sintering neck of CaO by interaction between H2O molecules (such as hydrogen bond). Water vapor can also act as a solvent to improve the solid state diffusion from surface to sintering neck which also benefits the fusion and growth of minicrystals.

The technology of oxy-fuel combustion in a circulating fluidized bed boiler is one of the advanced technologies for carbon capture and storage; however, operating problems related to ash deposition are worth investigating. When limestone is added as a sorbent during oxy-fuel combustion, excess CaO particles in the fly ash deposit on heating surfaces and react with high concentrations of CO2 in the flue gas. These carbonation reactions lead to structural change and formation of bonded deposits. Therefore, deposition experiments with limestone were carried out in a bench-scale fluidized bed under oxy-fuel combustion conditions to evaluate deposition propensity and compositions of ash deposits formed under different experimental conditions. Effects of the molar ratio Ca/S, probe surface temperature, and combustion atmosphere on deposition behavior were evaluated. Experimental results showed that during the initial stage of deposition, the degree of carbonation increases, with increasing molar ratios of Ca/S; although this is not significant for the total deposition process. Deposits were loose for 1 h, following deposition. Raising the surface temperature of the probe reduced the deposition rate of the fly ash, since this was strongly affected by thermophoresis. Except for elements K, Na, and S, there were no significant changes for other chemical components in the ash deposit under the varied conditions. There were clear differences in the deposition rates of fly ash for oxy-fuel and air combustion cases, which were probably caused by differences in the ash formation mechanism for the case of high O2 concentration, and limestone addition.

Ash deposition on heat-exchanger surfaces in boiler systems can cause numerous problems, including slagging, fouling, and corrosion. These deleterious processes can be compounded if the boiler combustion process is changed from air to oxy-fuel. In this paper, fly ash deposition characteristics under both air and oxy-fuel combustion conditions were investigated using a bench-scale fluidized-bed combustor (FBC) based on measurements of ash deposition rates via a temperature-controlled probe. Three different combustion atmospheres were studied, and results demonstrated that, under similar combustion temperature profiles and equivalent fluidization velocities, the deposition rate increased when transitioning from combustion atmospheres consisting of 21% O2/79% CO2 to air to 30% O2/70% CO2. To determine the primary factors associated with the observed variations in deposition rates, the chemical compositions and micromorphologies of ash and fly ash deposits were analyzed by inductively coupled plasma–atomic emission spectrometry (ICP–AES) and scanning electron microscopy (SEM). Particulate matter with aerodynamic diameters less than 10 μm (PM10) was measured by an electrical low-pressure impactor (ELPI), and the particle size distributions (PSDs) and carbon contents of the collected filter ash were also ascertained. The results indicate that the higher deposition propensity associated with a 30% O2/70% CO2 atmosphere can be largely attributed to a wider PSD rather than any changes in the chemical compositions of the fly ash or deposited ash, in which there are no obvious differences between air and oxy-fuel combustion. In addition, the slightly higher concentration of fine particles produced under this atmosphere also promotes the deposition of fly ash.