Combustion and air emissions from co‐firing a wood biomass,a Canadian peat and a Canadian lignite coal in a bubbling fluidised bed combustor

The effects of particle size, fuel blending ratio, moisture content and excess air ratio on combustion efficiency and air emissions (CO₂, CO, SO₂ and NO_x) from the co‐combustion of white pine or peat with a Canadian lignite coal, were examined in a pilot‐scale bubbling fluidised bed combustor. Pelletising was important for the efficient combustion of wood due to its high volatile content. Co‐firing lignite and pine pellets gave a proportional reduction in SO₂ and NO_x emissions with blending ratio, while co‐firing of peat and lignite resulted in increased SO₂ emissions, but decreased NO_x emissions. Moisture promotes combustion but with increased CO emissions, and results in increased NO_x emissions, and decreased SO₂ emissions. High excess air decreased CO, but moderately increased SO₂ and NO_x emissions. © 2011 Canadian Society for Chemical Engineering     

Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a circulating fluidized bed combustor

Emissions of NO_x and N₂O were measured during mono-combustion of dried sewage sludge and co-combustion with coal in a bench-scale circulating fluidized bed combustor (CFBC). The results were compared with previous results obtained using a bubbling fluidized bed combustor (BFBC). The increase in NO_x with sludge ash accumulation in the combustor was less for the CFBC than the BFBC, partly because of the higher attrition rate of sludge ash in CFBC resulting from the higher gas velocity. The influence of sludge ash on the formation of NO_x in CFBC was less than that in BFBC during sludge combustion. The effects of fuel type on NO_x and N₂O emissions were also evaluated.     

Coal combustion characteristics in a pressurized fluidized bed

The characteristics of emission and heat transfer coefficient in a pressurized fluidized bed combustor are investigated. The pressure of the combustor is fixed at 6 atm. and the combustion temperatures are set to 850, 900, and 950 °C. The gas velocities are 0.9, 1.1, and 1.3 m/s and the excess air ratios are 5, 10, and 20%. The desulfurization experiment is performed with limestone and dolomite and Ca/S mole ratios are 1,2, and 4. The coal used in the experiment is Cumnock coal from Australia. All experiments are executed at 2 m bed height. In this study, the combustion efficiency is higher than 99.8% through the experiments. The heat transfer coefficient affected by gas velocity, bed temperature and coal feed rate is between 550-800 W/m² °C, which is higher than those of AFBC and CFBC. CO concentration with increasing freeboard temperature decreases from 100 ppm to 20 ppm. NO_x concentration in flue gas is in the range of 5-130 ppm and increases with increasing excess air ratio. N₂O concentration in flue gas decreases from 90 to 10 ppm when the bed temperature increases from 850 to 950 °C.     

Co-combustion of coal and biomass in a circulating fluidized bed combustor

In this research, co-combustion of coal and rice husk was studied in a circulating fluidized bed combustor (CFBC). The effects of mixed fuel ratios, primary air and secondary air flow rates on temperature and gas concentration profiles along riser (0.1 m inside diameter and 3.0 m height) were studied. The average particle size of coal from Maetah used in this work was 1,128 mm and bed material was sand. The range of primary air flow rates was 480–920 l/min corresponding to U _g of 1.0–2.0 m/s for coal feed rate at 5.8 kg/h. The recirculation rate through L-valve was 100 kg/hr. It was found that the temperatures along the riser were rather steady at about 800–1,000 degrees Celsius. The introduction of secondary air improved combustion and temperature gradient at the bottom of the riser, particularly at a primary air flow rate below 1.5 m/s. Blending of coal with biomass, rice husk, did improve the combustion efficiency of coal itself even at low concentration of rice husk of 3.5 wt%. In addition, the presence of rice husk in the feed stocks reduced the emission of both NO _x and SO₂.     

Coal combustion characteristics in a circulating fast fluidized bed

The effect of coal size (0.73–1.03 mm), excess air ratio (1.0–1.4), operating bed temperature (750–900‡C), coal feeding rate (1–3 kg/h), and coal recycle rate (20–40 kg/h) on combustion efficiency, temperature profiles along the bed height and flue gas composition have been determined in a bubbling and circulating fluidized bed combustor (7.8 cm-ID x 2.6 m-high). Combustion efficiency increases with increasing excess air ratio and operating bed temperature and it decreases with increasing particle size in the bubbling and circulating fluidzing beds. In general, temperature profiles and combustion efficiency are more uniform and higher in a circulating bed than those in bubbling bed. Combustion efficiency also increases with increasing recycle rate of unburned coal in the circulating bed. The ratio of CO/CO₂ of flue gas decreases with increasing bed temperature and excess air ratio, whereas the ratio of O₂(CO + CO₂) decreases with bed temperature in both bubbling and circulating fluidized beds.     

Effect of Operating Conditions on NOx and CO Emissions in a Pilot‐Scale Vortexing Fluidized‐Bed Combustor with Flue Gas Recirculation

In order to study the effect of flue gas recirculation (FGR) by primary air mixed with FGR or nitrogen on NO_x emissions in a fixed total primary air flow rate, the bed temperature, stoichiometric oxygen ratio in the combustion chamber, and excess oxygen ratio were investigated in a pilot‐scale vortexing fluidized‐bed combustor (VFBC). As a result, the NO_x emissions could effectively be decreased with FGR in the VFBC. The lower NO_x emissions were attributed to the fact that the FGR allows NO_x to enter the reactor again with the chance of being reduced and that the FGR contains more CO₂ to suppress NO_x formation or promote NO_x reduction through its conversion to CO by reaction with char.     

Circulating fluidized bed combustion of a high-sulphur eastern canadian coal

Combustion tests were carried out with Minto coal in combination with three different limestones in the University of British Columbia (UBC) pilot scale (152 mm square x 7.3 m tall) circulating fluidized bed combustion (CFBC) unit. Operating conditions were chosen to be typical of those employed in large-scale CFBC power boilers. Recycling of fine particles captured by the secondary cyclone was found to be of considerable importance in increasing sulphur capture, enhancing combustion efficiency and reducing the amount of calcium sulphide in the solids residues. NO_x emissions increased as the Ca:S ratio increased. Local gas concentrations inside the reactor were strongly influenced by the core-annulus solids distribution patterns which characterize circulating fluidized beds.     

Olive cake combustion in a circulating fluidized bed

In this study, a circulating fluidized bed of 125 mm diameter and 1800 mm height was used to find the combustion characteristics of olive cake (OC) produced in Turkey. A lignite coal that is most widely used in Turkey was also burned in the same combustor. The combustion experiments were carried out with various excess air ratios. The excess air ratio, λ, has been changed between 1.1 and 2.16. Temperature distribution along the bed was measured with thermocouples. On-line concentrations of O₂, SO₂, CO₂, CO, NO_x and total hydrocarbons were measured in the flue gas. Combustion efficiencies of OC and lignite coal are calculated, and the optimum conditions for operating parameters are discussed. The combustion efficiency of OC changes between 82.25 and 98.66% depending on the excess air ratio. There is a sharp decrease observed in the combustion losses due to hydrocarbons and CO as the excess air ratio increases. The minimum emissions are observed at λ=1.35. Combustion losses due to unburned carbon in the bed material do not exceed 1.4 wt% for OC and 1.85 wt% for coal. The combustion efficiency for coal changes between 82.25 and 98.66% for various excess air ratios used in the study. The ash analysis for OC is carried out to find the suitability of OC ash to be used as fertilizer. The ash does not contain any hazardous metal.     

NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> emission from incineration of organic liquid waste in a circulating fluidized bed

An incineration test of a toxic chemical organic waste liquid was conducted on a circulating fluidized bed (CFB) incinerator. The flue gas was measured online with the advanced SAE-19 flue gas analyzer. The effects of several factors, in terms of flow rate of waste liquid, ratio of waste liquid injected into dense bed of the CFB, excess air coefficient, the secondary air fraction and bed temperature on NO _x emissions, were verified. The experimental results show that NO emissions in flue gas increase with increase in the flow rate of the waste liquid injected into the bed or the excess air coefficient or the bed temperature and those decrease with increase in the ratio of waste liquid injected into the dense bed of the CFB or the secondary air fraction. During the test runs, NO _x concentration in flue gas met the national regulation on NO _x emissions due to suppressive effect of low temperature and staged combustion in CFB on NO _x formation. This paper was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Investigation of co-combustion of coal and olive cake in a bubbling fluidized bed with secondary air injection

In this study, a bubbling fluidized bed of 102 mm inside diameter and 900 mm height was used to burn olive cake and coal mixtures. Tunçbilek lignite coal was used together with olive cake for the co-combustion tests. Combustion performances and emission characteristics of olive cake and coal mixtures were investigated. Various co-combustion tests of coal with olive cake were conducted with mixing ratios of 25%, 50%, and 75% of olive cake by weight in the mixture. Operational parameters (excess air ratio, secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The results were compared with that of the combustion of olive cake and coal. Flue gas concentrations of O₂, CO, SO₂, NO_x, and total hydrocarbons (C_mH_n) were measured during combustion tests.For the setup used in this study, the optimum operating conditions with respect to NO_x and SO₂ emissions were found to be 1.35 for excess air ratio, and 30 L/min for secondary air flowrate for the combustion of 75 wt% olive cake and 25 wt% coal mixture. The highest combustion efficiency of 99.8% was obtained with an excess air ratio of 1.7, secondary air flow rate of 40 L/min for the combustion of 25 wt% olive cake and 75 wt% coal mixture.     

石化污泥与煤流化床混烧污染物排放特性

在一座热态循环流化床燃烧试验装置上对石化污泥与煤进行混烧试验,通过对焚烧过程中烟气成分进行分析,着重考察了质量掺混比、二次风率、过剩空气系数和床温对SO₂、NO_x和多环芳烃排放浓度的影响。试验结果表明,随着质量掺混比的增大,SO₂和NO_x的排放浓度下降,而多环芳烃排放浓度呈上升趋势。随着二次风率的增加,SO₂的排放浓度上升,而NO_x的排放浓度呈下降趋势。随着过剩空气系数的增加,SO₂的排放浓度下降,NO_x的排放浓度呈上升态势。随着床温或者过剩空气系数的增加,烟气中多环芳烃排放浓度均呈先下降后上升趋势。综合考虑稳定燃烧和降低污染物排放等因素,得出一系列最佳燃烧参数。燃烧温度应该控制在850～860℃,过剩空气系数应该控制在1.35左右,二次风率应该控制在40%左右,质量掺混比应该控制在30%左右。在本次试验范围内,各工况SO₂、NO_x和多环芳烃的排放浓度均满足国家排放标准。     

Fluidized bed combustion of pre-dried Thai bagasse

Sugar cane bagasse is one of the most viable biomass fuels in Thailand. However, because of high moisture content in the fuel (of 45–55%), combustion of “as-received” bagasse is rather ineffective and unstable process.In this experimental work, the bagasse was preliminarily dried before conducting combustion tests on a fluidized bed combustor with a conical bed. Silica sand was used as the inert bed material for ensuring sustainable fuel ignition and combustion in this reactor. Effects of operating variables (fuel feed rate and excess air) as well as static bed height on the axial temperature and gas concentration profiles (for O₂, CO₂, CO and NO_x) in the combustor were investigated. For estimating the environmental impact by this bagasse-fuelled system, CO and NO_x emissions from the combustor were quantified. The combustion efficiency was found to be in the range of 96 to 99.7% for firing the pre-dried sugar cane bagasse in wide ranges of the operating variables.     

Pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed

A new technique of achieving high temperature air was adopted by combustion in high excess air ratio in a circulating fluidized bed (CFB). Experiments on pulverized coal combustion in high temperature air from the CFB were made in a down-fired combustor with the diameter of 220 mm and the height of 3000 mm. High temperature air with lower oxygen concentrations can be achieved steadily and continuously by combustion in the circulating fluidized bed. Pulverized coal combustion in high temperature air shows a uniform temperature profile along the axis of the down-fired combustor and the combustion efficiency is 99.8%. The NO_x emission is 390 mg/m³, 13% lower than the regulation for thermal power plants in China. The HCN and NH₃ emissions, as well as N₂O, are about zero in the exhaust.     

Combustion of olive cake and coal in a bubbling fluidized bed with secondary air injection

Combustion performances and emission characteristics of olive cake and coal are investigated in a bubbling fluidized bed. Flue gas concentrations of O₂, CO, SO₂, NO_x, and total hydrocarbons (C_mH_n) were measured during combustion experiments. Operational parameters (excess air ratio (λ), secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The temperature profiles measured along the combustor column was found higher in the freeboard for olive cake than coal due to combustion of hydrocarbons mostly in the freeboard. Combustion efficiencies in the range of 83.6–90.1% were obtained for olive cake with λ of 1.12–2.30. For the setup used in this study, the optimum operating conditions with respect to NO_x and SO₂ emissions were found as 1.2 for λ, and 50 L/min for secondary air flowrate for the combustion of olive cake.     

Effects of design features on combustion efficiency and emission performance of a biomass-fuelled fluidized-bed combustor

This paper presents an analysis of some measures leading to intensification of the combustion process in a biomass-fuelled fluidized-bed combustor with a cone-shape bed (or ‘conical FBC’). Two combustors firing rice husks with elevated fuel-ash content were the focus of this study. Compared to the pilot 350-kW_th conical FBC exhibiting combustion efficiency of up to 96%, the newly constructed 400-kW_th combustor included geometrical and design modifications aimed at improving the combustion efficiency and emission performance of the reactor. Differences between the air distributors and Δp–u diagrams (accounting for the total pressure drop across the air distributor and gas–solid fluidized bed) for the two reactors are discussed. Axial temperature and gas concentration (O₂, CO and NO_x) profiles in the combustors were compared for similar operating conditions (excess air and heat release rate per unit cross-sectional area). At excess air of 40–60%, the bed temperature in the advanced conical FBC was substantially, by about 180 °C, higher than that in the pilot combustor, mainly, due to better fuel–air mixing and higher residence time of reactants. The formation and decomposition of CO and NO in the bed region as well as in the freeboard of these two combustors showed quite different trends under similar operating conditions. At excess air of 40–60%, the CO emission from the advanced conical FBC was found to be much (7–8 times) lower than that from the pilot combustor, while the NO_x emissions were represented by almost the same values. High (over 99%) combustion efficiency was achieved when firing rice husk in the advanced 400 kW_th conical FBC for the range of excess air.     

Modelling of CO2, CO, SO2, O2 and NOx emissions from the oxy-fuel combustion in a circulating fluidized bed

The paper presents a model of coal combustion in air and oxygen-enriched CFB environment. A computer program to calculate the CO₂, CO, SO₂, NO_x and O₂ emissions from the combustion of solid fuels in a circulating fluidized bed boiler was created. The validity of this program was verified by measurements on a 0.1MW_th OxyFuel-CFB Test Rig.The calculations have been carried out for air and so-called oxy-fuel conditions, i.e. when combustion runs in a gas mixture based on O₂ and N₂, with various fractions of oxygen.The comparison between measured and predicted by model CO, SO₂, NO_x and O₂ emissions is shown in this paper. The results of the calculation showed, that the kinetic equations of some reaction have to be modified. Authors propose to use the reaction surface area instead of the specific internal surface area of char in rate constant formulas as the combustion nature changes from internal-kinetic to external-diffusion controlling regime.     

Comparisons of pulverized coal combustion in air and in mixtures of O2/CO2

Pulverized coal combustion in air and the mixtures of O₂/CO₂ has been experimentally investigated in a 20 kW down-fired combustor (190 mm id×3 m). Detailed comparisons of gas temperature profiles, gas composition profiles, char burnouts, conversions of coal–N to NO_x and coal–S to SO₂ and CO emissions have been made between coal combustion in air and coal combustion in various O₂/CO₂ mixtures. The effectiveness of air/oxidant staging on reducing NO_x emissions has also been investigated for coal combustion in air and O₂/CO₂ mixtures. The results show that simply replacing the N₂ in the combustion air with CO₂ will result in a significant decrease of combustion gas temperatures. However, coal combustion in 30% O₂/70% CO₂ can produce matching gas temperature profiles to those of coal combustion in air while having a lower coal–N to NO_x conversion, a better char burnout and a lower CO emission. The results also confirm that air/oxidant staging is very effective in reducing NO_x emissions for coal combustion in both air and a 30% O₂/70% CO₂ mixture. SO₂ emissions are proved to be almost independent of the combustion media investigated.     

Analysis of combustion efficiency in CFB coal combustors

Fluidized bed technology is well known for its high combustion efficiency and is widely used in coal combustion. In this study, the combustor efficiency has been defined and investigated for CFB coal combustor based on the losses using a dynamic 2D model. The model is shown to agree well with the published data. The effect of operating parameters such as excess air ratio, bed operational velocity, coal particle diameter and combustor load and the effect of design variables such as bed height and bed diameter on the mean bed temperature, the overall CO emission and the combustion efficiency are analyzed for the small-scale of CFBC in the presently developed model. As a result of this analysis, it is observed that the combustion efficiency decreases with increasing excess air value. The combustion efficiency increases with the bed operational velocity. Increasing coal particle size results in higher combustion efficiency values. The coal feed rate has negative effect on the combustion efficiency. The combustor efficiency considerably increases with increasing combustor height and diameter if other parameters are kept unchanged.     

DETERMINATION OF DILUTE SLURRY DENSITIES IN A VERTICAL ANNULUS USING ISOKINETIC SAMPLING

The effects of coal properties on N₂O and NO_x formation from circulating fluidized bed combustion of coal was examined through burning nine typical coals and a coal shale, widely used in China over a wide range of coal ranks, in a bench-scale circulating fluidized bed. It was found that N₂O and NO_x formation had similar dependence on coal rank. Fixed carbon content and nitrogen content were the most important coal properties to influence N₂O and NO_x emissions from circulating fluidized bed combustion of coal. A coal with high fixed carbon content had high conversion ratio of fuel-N into N₂O and NO_x. The conversion ratio of fuel-N into N₂O or NO_x increased with nitrogen content of coal, whereas it decreased with O/N ratio. No significant correlation between conversion ratio of fuel-N into N₂O or NO_x and C/N ratio was identified. To clarify the coal property effect, investigation of a wide range of coal rank, is important.     

The application of FLOX/COSTAIR technologies to reduce NOx emissions from coal/biomass fired power plant: A technical assessment based on computational simulation

Nitrogen oxides (NO_x) is one of the harmful emissions from power plants. Efforts are made to reduce NO_x emissions by researchers and engineers all the times. NO_x emissions are from three resources during the combustion: prompt NO, fuel NO and thermal NO. The last one - thermal NO, which is described by ‘Zeldovich-mechanism’, is the main source for NO_x emissions. The thermal NO emission mainly results from the high combustion temperature in the combustion process. In order to control the NO formation, the control of peak combustion temperature is the key factor, as well as the oxygen concentration in the combustion areas. Flameless oxidation (FLOX) and continuous staged air combustion (COSTAIR) are two relatively new technologies to control the combustion temperature and the reaction rate and consequently to control the NO_x emissions.In this study both FLOX and COSTAIR technologies are assessed based on a 12 MW_e, coal-fired, circulating fluidised bed combustion (CFBC) power plant by using ECLIPSE simulation software, together with a circulating fluidised bed gasification (CFBG) plus normal burner plant. Two different fuels - coal and biomass (straw) are used for the simulation. The technical results from the study show that the application of FLOX technology to the plant may reduce NO_x emissions by 90% and the application of COSTAIR technology can reduce NO_x emissions by 80-85% from the power plant. The emissions from the straw-fuelled plants are all lower than that of coal-fuelled ones although with less plant efficiencies.