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As the world’s largest energy consumer (i.e., 24% of the world) and one of the largest biomass resource reserve countries (i.e., total reserves of 73.42 × 108 t a–1), for China, using biomass to pr...

A good understanding of the catalytic mechanism of alkali and alkaline earth metal species during pyrolysis is required for the development of biomass utilization, to take advantage of its special thermochemical properties. The mechanism of the volatile–biochar interaction, especially between H radical/CO2/H2O and K/Ca in biochar, was systematically investigated by combining experimental analyses and density functional theory calculation. The results indicate that, at 500–900 °C, the yield of biochar from rice husk pyrolysis is basically constant, which mainly shows the mutual conversion between gas and liquid products. With the increase of loading alkali and alkaline earth metal species (AAEMs), the tar yield gradually decreased, and with it reaching a certain content, a significant catalytic effect of AAEMs can be observed. K is more beneficial to maintain the stability of the biochar structure than Ca. The intensities of CO and C–O–C functional groups would appear with a certain increase with the increasing concentration of bond-linking AAEMs. At 700–900 °C, the precipitation ratio of K is almost twice that of Ca and even more. The presence of AAEMs can largely inhibit the formation of three-ring tar components, and more converted to one-ring tar components. The H radicals in the volatile can be bonded to C in the C–O–K structure, thereby causing the K element to decouple with the functional group, and tend to be adsorbed on the carbon matrix. For the Ca-loaded structure, it is manifested in strong adsorption of H radicals by Ca, resulting in the weakening of C–O bonds and the looseness of the biochar structure. The interaction between K/Ca in biochar and H2O/CO2 is reflected in the mutual attraction of AAEMs and the O atom in the H2O/CO2 molecule.

The heterogeneous NO reduction characteristics with biomass char and coal char blends in a microfluidized bed reaction analyzer (MFBRA) and its kinetic analysis were investigated. Sawdust char had ...

To investigate pyrolysis characteristics of Shenhua bituminous coal under pressurized conditions, the effects of pressure (0-1.2 MPa) on the physicochemical structure and combustion reactivity of pyrolysis char samples prepared at 1073 and 1273 K were studied in a pressurized drop tube furnace (PDTF). The low temperature nitrogen adsorption test was used to characterize the physical structure of char samples. The results showed that, the increase of pyrolysis pressure is conducive to formation of mesopores, and the specific surface area is larger at 1273K. Raman and XPS analysis were used to characterize the chemical structure of char samples. The results showed that, the increase of pressure can promote the transformation of smaller aromatic structure into larger aromatic structure, and the graphitization degree are more obvious at 1273K. With the increase of pressure, the proportion of C=C/C-C bond in char structure increases and the proportion of C-H bond decreases. It indicates that the unst...

The characteristics of gas–liquid–solid products during corn straw–H2O gasification were studied by the one-stage fluidized-bed/fixed-bed quartz reactor. The results indicate that the transformation between gas and solid products is mainly presented with the increase of feeding time (0–40 min). The aromaticity of biochar decreases slightly in the feeding period of 10–20 min but gradually increases in 20–40 min. The oxygen-containing functional groups in biochar increase gradually with the increase of the feeding time. The K content in biochar increases by 27% in 10–30 min, while that of Ca increases by 36% in 10–20 min. When the carbon conversion rate is less than 8%, the reactivity of biochar produced by 30 min of feeding is highest. The contents of phenol and 2,3-dihydrobenzofuran of tar increase by 25 and 34%, respectively in 20–40 min. The char–tar–H2O interaction in biomass gasification is a two-stage reaction. The first stage is dominated by the volatile–char interactions, while the second stage is ...

The reaction characteristics of different mixing ratios of coal/char to NO reduction were investigated under the conditions of relatively low O2 concentration at 1073–1203 K. The inlet NO concentra...

To further advance pulverized coal (PC) combustion theory and enable the rational development of horizontal bias combustion technology, combustion experiments were conducted in a 250 kW pilot-scale bias combustion simulator; multiple research means of combustion temperatures, flame spectra, burnout rates of residual solids, and NOx formation were used. A blend of sub-bituminous coal from Indonesia and bituminous coal from Australia was tested. The effects of bias concentration ratio (BCR) on the ignition characteristics of parallel bias PC jets in a reducing atmosphere were investigated. The results indicate that with increasing BCR for parallel bias PC jets, the standoff distance gradually decreased; the peaks of subsequent combustion temperature and visible-light intensity gradually increased; the continuous flame regions became advanced and concentrated; the flame stability gradually increased; the burnout rate gradually increased; the NOx formation gradually decreased, and the ignition characteristics...

In this study, the effect of particle size on the release of NO during the char combustion under particle packed-layer conditions was investigated at combustion temperatures of 900 °C~1400 °C. The results show that after pyrolysis, the small size char favored more pores and specific surface area. No significant variations of the intrinsic reactivity of O2 and NO with char particle size were measured. By increasing char particle size, more NO conversion from nitrogen in char (char-N) was found regardless of the combustion temperature and bulk oxygen concentration. This can be explained as follows. By increasing char particle size, less O2 penetrated into the pores and more NO formed because of a less accessible pore surface area and a decreased NO reduction time. The effects of reaction temperature and bulk oxygen concentration were also discussed. A quantified description using a first-order reaction model shows that the char particle size will finally influence the accessible pore surface area and depth ...

To further understand the bias combustion behavior of new coal used for horizontal bias combustion and obtain its characteristic parameters, combustion experiments were conducted in a 250-kW pilot-scale bias combustion simulator; multiple research measurements of flame spectra, combustion temperatures, and burnout rates of residual solids were used. Bituminous coal from Australia, sub-bituminous coal from Indonesia, and a blend of these coals were tested. The effects of the raw-coal equivalent moisture (RCEM), pulverized-coal fineness (PCF), and coal type on the ignition characteristics of parallel-bias pulverized-coal jets in a reducing atmosphere were investigated. The results indicate that, with decreasing RCEM and PCF, the standoff distance gradually decreased, the flame stability gradually increased, the burnout rate gradually increased, and the ignition characteristics gradually improved. The RCEM had negative effects on ignition in the early stage but positive effects during subsequent combustion. ...

The effect that ambient oxygen concentration has on NO that is released during coal char combustion was studied under particle packed-layer conditions at combustion temperatures of 700–1100 °C. The results show that the addition of quartz sand in the packed-layer mixture can effectively minimize the interaction of the char particles by inhibiting the secondary reaction of char and NO. During the char combustion tests, char-N/NO conversion decreased with an increasing ambient oxygen concentration at low temperatures (700–900 °C). At this temperature range, with coal char generally burning in zones I or II, more oxygen penetrated into the pores and less NO formed because of a more accessible pore surface area and an increased NO reduction time; a weak increasing trend was observed at a high temperature (∼1100 °C). At this temperature and above, more char-N/NO conversion was observed because less O2 diffused into the char particles, and NO was reduced less during its formation close to the char particle exte...

Experiments were conducted under a hot condition of bituminous coal ignition in a 250 kW pilot-scale bias combustion simulator. The effects of the primary air velocity (PAV) on the ignition characteristics of bias pulverized coal jets in a reducing atmosphere were investigated to develop a better new burner for experimental bituminous coal. Multiple means of measurement and sampling were used for the axial and radial temperatures, flame spectrum, flue gas components, and residual solid inside the furnace. The standoff distance changed nonmonotonically between axial distances of 90 and 330 mm with increasing PAV and was shortest for a PAV of 16 m/s. The radiation heat transfer from the hot environment had more effect on the ignition than the convection heat transfer from high-temperature flue gas in the initial stage, while the convection heat transfer from the high-temperature flue gas played a greater role in the subsequent combustion of the char. At PAVs of 13 and 16 m/s, there was volatile and char hom...

In this study, pulverized coal bias ignition experiments were conducted in a 250 kW pilot-scale bias combustion simulator to investigate the effects of the combustion conditions on the particulate matter (PM) formation characteristics in a reducing atmosphere; the amount of PM10 was determined using an electrical low-pressure impactor. The particle size distributions of PM10 from pulverized coal bias ignition under various combustion conditions in a reducing atmosphere differ from those in the tail flue gas of a coal-fired boiler. Mainly PM0.38 is produced when the pulverized coal concentration (PCC) is 0.53, the lowest amounts of PM0.38 and PM0.38–10 are produced when the PCC is 0.4, and mainly PM0.38–10 is produced when the PCC is 0.33; the PCC significantly affects formation of PM0.5–0.794. As the bias concentration ratio (BCR) of the pulverized coal increases, the pulverized coal jets are prone to releasing viscous minerals, which easily coalesce into supercoarse particles of size greater than 10 μm; ...

An in situ decoupling analytical method was proposed and a multistage in situ reaction analyzer based on a micro fluidized bed (MFB-MIRA) was developed for in situ and on-line analysis of multistage gas–solid reaction processes. The responses of the pipe system to various gas concentration waves were analyzed. The in situ decoupled combustion properties of three coals were investigated using a two-stage reaction procedure involving devolatilization in Ar and combustion in O2/Ar. The Peclet number of the pipe flow was as large as 87, indicating approximate plug flow in the MFB-MIRA. A concentration correction model was built to simulate the real gas release process in the micro fluidized-bed reactor. The measurement distortions in the char combustion concentration curves were found negligible, which proved the reliability of using the measured results directly to derive the carbon conversion processes. The MFB-MIRA gives a good mass balance. The combustion reaction orders of the three in situ chars in O2 w...

In-furnace desulfurization technology is usually used in circulating fluidized bed (CFB) boilers, and natural limestone is the most common calcium-based sorbent for sulfur removal. Limestone desulfurization could be influenced by water vapor in the flue gas and also impurities such as SiO2, Al2O3, and Fe2O3 contained in the natural limestone. The limestone desulfurization process can be divided into two steps: calcination to produce CaO and sulfurization of CaO formed. This paper studies the impacts of water vapor and impurities on the limestone calcination and sulfurization of CaO calcined in a microfluidized bed reactor analyzer (MFBRA). The apparent activation energy under different operating conditions was first calculated, and then the influence of water vapor and impurities on the activation energy was preliminarily discussed. The results show that reaction temperature, water vapor, and impurities all have influences on the indirect desulfurization of limestone under the condition of standard pressu...

Five coal chars were prepared in a flat flame flow reactor (FFR) which can simulate the temperature and gas composition of a real pulverized coal combustion environment. Reactivity of these chars with NO in the temperature range of 1273-1573 K has been characterized by experiments in a high-temperature drop tube furnace (DTF). Two normalized parameters ((X) over bar and m(c)) are proposed to analyze the effect of inherent metal content on char reactivity. The inherent metal catalysts in chars, such as magnesium, potassium, sodium, calcium, and iron, can significantly increase reactivity of char by reducing the activation energy. The reactivity of NO-char reaction increases with m value monotonously and linearly in a certain range. Experimental results showed that the catalytic activity of magnesium, potassium, sodium, calcium, and iron at the temperatures of 1273-1573 K were in decreasing sequence. In this study, the magnesium, which was rarely studied in other literature, appears to have a great catalytic effect on the reduction of NO by chars.

Experiments were conducted in a drop-tube furnace (DTF) to investigate NO emission characteristics of low-rank bituminous coal burning in the primary combustion zone. The combustion tests were performed under high-stoichiometric-ratio (SR) combustion conditions (SR ≥ 1) and low-SR combustion conditions (SR < 1). Along the length of the furnace, concentrations of flue gas species, such as NO, HCN, NH3, CH4, CO, CO2, and O2, were carefully measured during pyrolysis and different SR combustion conditions. The results for high-SR combustion conditions show that high concentrations of O2 favor the conversion of volatile N and char N into NO, with extensive NO formation taking place in the initial stages of combustion. Reducing species, such as CH4, C2H6, and NH3, in volatile matter were mostly oxidized and generated little effect on NO elimination. Results from low-SR combustion conditions show that there is insufficient O2 for oxidization of combustibles, allowing more hydrocarbons to react with NO. For the c...

Oxy-fuel combustion is a promising carbon capture technology in which both the conversion of fuel-N to NO and the reduction of recycled NO contribute to lowering of final NO exhausted from the coal combustion system. Combustion characteristics for both air and oxy-fuel conditions were numerically investigated in a pilot scale test facility for an Australian sub-bituminous coal. On the basis of De Soete’s mechanism, the additional reactions for formation of fuel-NO and reduction of recycled NO were added into computational fluid dynamics (CFD) codes, using user-defined functions (UDFs). The NOx predictions in air and oxy-fuel combustion were compared to experimental data. The NO emission in oxy-fuel condition is predicted to be significantly lower than that in air combustion, even without recycled NO. The effect of the nitrogen partitioning ratio between volatile and char on the NOx emission was also investigated.

Five Chinese coal chars were prepared in a flat flame flow reactor that can simulate the conditions of real pulverized coal combustion. The kinetics of the NO−char reaction at 1273, 1373, 1473, and 1573 K was studied in a drop tube furnace. A new model of the high-temperature NO−char reaction that took into account the pore diffusion and thermal annealing of char was proposed. The NO−char reaction was predicted by a combination of the model and computational fluid dynamics. Through the comparison of experimental and predicted results, the kinetic parameters for all chars were determined. The predicted results agree well with experimental results at 1273, 1373, and 1473 K, but the model largely underestimates the experimental data at 1573 K. This behavior is explained by a new mechanism of the high-temperature NO−char reaction. In addition, the model successfully normalizes the reactivity of chars with different ranks at high temperature, which is considered to be valuable in predicting the NO reduction on...

Four coal chars were prepared in a flat flame flow reactor (FFR), which can simulate the temperature and gas composition of a real pulverized coal combustion environment. The pore structure of chars was measured by mercury porosimetry and nitrogen adsorption, and the Hg and Brunauer−Emmett−Teller (BET) surface areas were obtained. The kinetics of NO−char was studied in a drop-tube furnace (DTF) and thermogravimetric analyzer (TGA). In the TGA experiments, the random pore model (RPM) was applied to describe the NO−char reactions and obtain the intrinsic kinetics. By presenting the data of DTF and TGA experiments on the same Arrhenius plot, it can be concluded that TGA is an available tool to study the kinetics of a high-temperature NO−char reaction. With respect to the DTF experiments, in comparison to the BET surface area, the Hg surface area is a better basis for normalizing the reactivity of different coal chars because of less scatter in the measured values, better agreement with TGA experimental data,...

Experiments with a small-scale furnace for a down-fired pulverized-coal 300-MWe utility boiler were carried out on a single-phase test facility to investigate the influence of different secondary a...