Methods for measuring the concentrations of SO2, and of gaseous reduced sulphur compounds in the combustion chamber of a circulating fluidized bed boiler

The present work was aimed at developing and improving methods for measurement of gaseous sulphur compounds in the combustion chamber of a fluidized bed boiler (FBB). The sampling of SO₂ was improved by removing NH₃ and H₂O with a sorbent immediately after the probe. The concentration of reduced sulphur species was determined by means of two conventional SO₂ analyzers and an intermediate converter, where the reduced species are oxidized to SO₂. Gas phase sulphides were also sampled with a gas quenching probe by means of a basic solution which was subsequently analysed by wet chemistry. The methods were tested during coal combustion in a 12 MW circulating FBB without limestone for two cases of air‐staging.     

Characterization of the reactivity of limestones with SO2 in a fluidized bed reactor

The reaction between SO₂ and calcined limestone particles has been studied in a fluidized bed combustor. Measurements of sorbent reactivity with SO₂ were made for small batches of limestone injected into the combustor. Simultaneous continuous combustion of bituminous coal provided conditions like those of a boiler for study of the sulphation reaction. A semi-empirical rate model of the CaO-SO₂ reaction has been developed. External mass transfer of SO₂, diffusion within the particles and chemical reaction are taken into account. The limestone reactivity with SO₂ is characterized by two parameters which are dependent on the temperature and sorbent particle size. A model for predicting the limestone requirements in a fluidized bed boiler has been developed. Parameters from the batch experiments are included. The predictions for sulfur retention agree with the experimental results. In addition, effects of operating conditions (gas velocity, recycle, limestone particle size) on the retention of SO₂ were simulated using the model.     

Influence of limestone addition on the behaviour of NO and N2O during fluidised bed coal combustion

Atmospheric bubbling fluidised bed coal combustion (ABFBCC) of a bituminous coal and an anthracite with particle diameters in the range 500–4000 μm was investigated in a pilot-plant facility, with and without limestone addition. The experiments were conducted at steady-state conditions using three excess air levels (10, 25 and 50%) and bed temperatures in the range 750–900 °C. Combustion air was staged, with primary air accounting for 100, 80 and 60% of total combustion air.During limestone addition, in general, the NO emission decreases with the decrease in excess air and the increase in air staging, for both coals (as also observed without limestone). The bed temperature does not influence the NO emission significantly (as also observed without limestone); however, it was observed that during bituminous coal combustion there is a slight trend for a decrease on the NO emission with temperature increase in the range 825–900 °C, whereas for anthracite coal the trend is the opposite. On the other hand, the N₂O emission increases with: the decrease in excess air, the increase in air staging (as opposed to what was observed without limestone), and the decrease in bed temperature (as also observed without limestone).Taking the coal combustion without limestone as reference, it was observed that the effect of limestone addition on the NO and N₂O emission depends on the first stage stoichiometry: (1) under first stage fuel lean conditions the NO emission increases, while that of N₂O decreases, (2) under first stage fuel rich conditions (for example, high air staging) the opposite trend is observed.     

Sulfur fate during bituminous coal combustion in an oxy-fired circulating fluidized bed combustor

To clarify the sulfur transformation behavior during oxy-fired circulating fluidized bed (CFB) combustion, experiments on SO₂ emission characteristics were carried out in a 50 kWth CFB combustor. Results show that SO₂ emission is quite dependent on the bed temperature in different atmospheres without limestone injection. With Ca/S=2.5, SO₂ emission in 21%O₂/79%CO₂ atmosphere is smaller than that in air atmosphere, but SO₂ emission decreases with the increase of O₂ concentration. The calcium forms in the ash prove the combination of calcination/carbonation and direct sulfation mechanism of limestone under oxy-combustion conditions. And the desulfurization efficiency of limestone (as deducting the self-retention efficiency from the total sulfur removal efficiency) increases from 40% to 52% as the O₂ concentration increases from 21% to 40%.     

Effect of co-combustion of chicken litter and coal on emissions in a laboratory-scale fluidized bed combustor

Co-combustion of chicken litter (CL) with coal was performed in a laboratory-scale fluidized bed combustor to investigate the effect of CL combustion on pollutant emissions. The emissions of major gaseous pollutants including CO, SO₂, H₂S and NO and temperature distribution along the combustor were measured during the tests. Effects of CL fraction and secondary air on combustion characteristics were studied. The experimental results show that CL introduction increases CO emissions and reduces the levels of SO₂. The ratio of H₂S/SO₂ increases with increasing fraction of CL. NO emissions either increase or decrease depending on the percentage of CL in the mixed fuels. The temperature in the freeboard region increases with increasing the fraction of CL while the reverse is true for the bed temperature.     

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.     

Effects of operational parameters on emission performance and combustion efficiency in small-scale CFBCs

A well-designed CFBC can burn coal with high efficiency and within acceptable levels of gaseous emissions. In this theoretical study effects of operational parameters on combustion efficiency and the pollutants emitted have been estimated using a developed dynamic 2D (two-dimensional) model for CFBCs. Model simulations have been carried out to examine the effect of different operational parameters such as excess air and gas inlet pressure and coal particle size on bed temperature, the overall CO, NO_x and SO₂ emissions and combustion efficiency from a small-scale CFBC. It has been observed that increasing excess air ratio causes fluidized bed temperature decrease and CO emission increase. Coal particle size has more significant effect on CO emissions than the gas inlet pressure at the entrance to fluidized bed. Increasing excess air ratio leads to decreasing SO₂ and NO_x emissions. The gas inlet pressure at the entrance to fluidized bed has a more significant effect on NO_x emission than the coal particle size. Increasing excess air causes decreasing combustion efficiency. The gas inlet pressure has more pronounced effect on combustion efficiency than the coal particle size, particularly at higher excess air ratios. The developed model is also validated in terms of combustion efficiency with experimental literature data obtained from 300 kW laboratory scale test unit. The present theoretical study also confirms that CFB combustion allows clean and efficient combustion of coal.     

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.     

Peach and apricot stone combustion in a bubbling fluidized bed

In this study, a bubbling fluidized bed combustor (BFBC) of 102 mm inside diameter and 900 mm height was used to investigate the combustion characteristics of peach and apricot stones produced as a waste from the fruit juice industry. A lignite coal was also burned in the same combustor. The combustion characteristics of the wastes were compared with that of a lignite coal that is most widely used in Turkey. On-line concentrations of O₂, CO, CO₂, SO₂, NO_X and total hydrocarbons (C_mH_n) were measured in the flue gas during combustion experiments. By changing the operating parameters (excess air ratio, fluidization velocity, and fuel feed rate), the variation of emissions of various pollutants was studied. Temperature distribution along the bed was measured with thermocouples.During the combustion tests, it was observed that the volatile matter from peach and apricot stones quickly volatilizes and mostly burn in the freeboard. The temperature profiles along the bed and the freeboard also confirmed this phenomenon. It was found that as the volatile matter of fruit stones increases, the combustion takes place more in the freeboard region.The results of this study have shown that the combustion efficiencies ranged between 98.8% and 99.1% for coal, 96.0% and 97.5% for peach stone and 93.4% and 96.3% for apricot stones. The coal has zero CO emission, but biomass fuels have very high CO emission which indicates that a secondary air addition is required for the system. SO₂ emission of the coal is around 2400–2800 mg/Nm³, whereas the biomass fuels have zero SO₂ emission. NO_X emissions are all below the limits set by the Turkish Air Quality Control Regulation of 1986 (TAQCR) for all tests. As the results of combustion of two biomass fuels are compared with each other, peach stones gave lower CO and NO_X emissions but the SO₂ emissions are a little higher than for apricot stones. These results suggest that peach and apricot stones are potential fuels that can be utilized for clean energy production in small-scale fruit juice industries by using BFBC.     

Combustion characteristics of a two-stage swirl-flow fluidized bed combustor

In order to investigate the combustion characteristics of a two-stage swirl-flow fluidized bed combustor, combustion experiments of low-grade anthracite coal were performed. Experimental parameters were the fluidizing air velocity, coal feed rates, bed temperature, stoichiometric air ratio, swirl nozzle diameter and rotational diameter. The experimental results showed that, due to the swirl flow, the elutriation rates of fines were lower than those of the single-stage fluidized bed combustor. The combustible contents of the ash in the outflow streams were also reduced. Therefore, the combustion efficiency of the two-stage swirl-flow fluidized bed combustor was 20% greater than that of the single-stage fluidized bed combustor under the same operating conditions.     

NO emission on pulverized coal combustion in high temperature air from circulating fluidized bed – An experimental study

High temperature air was adopted by combustion in high excess air ratio in a circulating fluidized bed. Experiments on pulverized coal combustion in high temperature air from the circulating fluidized bed were carried out in a down-fired combustor with the diameter of 220 mm and the height of 3000 mm. The NO emission decreases with increasing the residence time of pulverized coal in the reducing zone, and the NO emission increases with excess air ratio, furnace temperature, coal mean size and oxygen concentration in high temperature air. The results also revealed that the co-existing of air-staging combustion with high temperature air is very effective to reduce nitrogen oxide emission for pulverized coal combustion in the down-fired combustor.     

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.     

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     

Effects of fuel type and operating conditions on SO2 emission in fluidized bed combustion

The SO₂ emission from six fluidized bed combustors was examined. Approximately 71.5% of the fuel sulphur was found to be emitted as SO₂ in sorbent-free tests. General observations on the effects of Ca/S molar ratio, limestone size and recycle systems are presented. The effects of limestone type and superficial velocity were found to be insignificant, as was the effect of bed temperature in the range 800-950°C. In tests with fine solid and liquid fuels, circulating FBC's were found to provide significantly better sulphur capture than bubbling FBC's.     

Rice husk co-firing with coal in a short-combustion-chamber fluidized-bed combustor (SFBC)

This study encompassed the characteristics and performance of co-firing rice husk, a by-product of rice-milling process, with coal in a short-combustion-chamber fluidized-bed combustor (SFBC). Bed phenomena investigated in a cold-flow model combustor showed that with the different mixes of materials, the anticipated offshoot of combustion, the minimum fluidizing velocity (U_mf) was 0.4-0.8 m/s. In concord with axial temperature profiles, axial gas concentration profiles implied that a recirculating ring was able to circumscribe CO within the short-main chamber. The formation, decomposition, and eventual maturity of NO_x characterized the NO_x evolution, inferred from concentration profiles. The impacts of fluidizing velocity and blending ratio on gas emissions and combustion efficiency (E_c) are described. The fluidizing velocity had consequential effect on gas emissions, except NO_x. Surprisingly, NO_x did not hinge much on increased N-content of the mixtures with coal. As expected, increased SO₂ was relevant to increased coal mass. Increased fluidizing velocity adversely affected E_c while increased coal fraction enhanced E_c, mostly >97%.     

循环流化床燃煤过程NOx和N2O产生-控制研究进展

循环流化床(CFB)燃烧技术因其燃料适应范围广、污染物排放低等优点,近几十年得到广泛应用. 随着当前环保要求的日益提高,CFB燃煤过程N2O排放浓度较高成为其应用的瓶颈问题. 因此系统总结CFB燃煤过程中NOx和N2O排放的研究现状对开发新型CFB燃煤技术具有重要意义. 本工作首先讨论了CFB燃煤过程中NOx和N2O的均相和异相反应机理,然后应用这些机理分析了床温、过剩空气系数、分级燃烧,以及煤种对CFB燃煤过程NOx和N2O排放的影响. 在此基础上,对常见的抑制NOx和N2O排放的工艺从机理角度进行了归纳总结. 最后,对2种本工作认为有应用前景的CFB燃煤减排NOx和N2O新技术?反向分级燃烧技术及CFB解耦燃烧技术进行了简要论述.     

A simplified model of SO2 capture by limestone in 71 MWe pressurized fluidized bed combustor

A mathematical model of SO₂ capture by uncalcined limestone particles with solid attrition under pressurized fluidized bed combustion conditions was developed based on the shrinking unreacted-core model. Since the thickness of the product layer is sufficiently much smaller than the particle size, a flat surface model was employed. The difference in SO₂ capture behavior between continuous solid attrition and intermittent attrition was investigated. The reaction rate for intermittent solid attrition was found to be lower than that for continuous attrition mode under low SO₂ concentration conditions. A simple mathematical expression to calculate reaction rate of SO₂ capture per unit external surface area of limestone is proposed.The present simplified mathematical model of SO₂ capture by single limestone particle under periodical attrition conditions was applied to the analysis of a large-scale pressurized fluidized bed combustor. By giving the period of attrition as a parameter, the experimental results agreed well with the model results. From the vertical concentration profile of SO₂ concentration, the emission of SO₂ was found to be governed by the balance between SO₂ formation rate from char and SO₂ capture by limestone at the upper surface of the dense bed. A simplified expression to estimate SO₂ emission from pressurized fluidized bed combustors was proposed.     

Removal of SO2 in Semi‐Dry Flue Gas Desulfurization Process with a Powder‐Particle Spouted Bed

Semi‐dry flue gas desulfurization was investigated with several kinds of SO₂ sorbents, such as slaked lime, limestone, Mg(OH)₂ and concrete pile sludge, in a powder‐particle spouted bed. Slurry droplets including sorbent fine particles were fed to a spouted bed of coarse inert particles spouted with hot gas containing SO₂. SO₂ removal efficiency was strongly affected by the approach to saturation temperature, Ca/S molar ratio and particle size of sorbent. Slaked lime showed the highest desulfurization efficiency. In this process, despite very short gas residence time, more than 90% SO₂ removal was easily achieved by choosing appropriate conditions.     

Investigation of sulphation behavior of two fly ash samples produced from combustion of different fuels in a 165 MWe CFB boiler

Emission of sulphur dioxide (SO₂) from combustion of fossil fuel is an important environmental issue. Circulating fluidized bed combustion (CFBC) technology can use limestone sorbent to achieve in situ SO₂ emissions control. This paper presents the chemical and physical analysis results of two fly ash samples derived from a 165 MWe CFBC boiler burning two different fuels with addition of limestone, as they pertain to sulphation behavior. One of the samples in this study was produced from combustion of a bituminous coal with high iron content, the other from firing of blended coal and petroleum coke fuel. The physical examination was conducted by scanning electron microscope (SEM) coupled with an energy dispersive X-ray (EDX) system for analysis of the surface structure or morphology of the sample, as well as the calcium and sulphur distribution. Some large particles derived from high-iron-content fuel were covered by dense iron shells; however, in general such a dense rim was found to not significantly impede the overall desulphurization performance in FBC in terms of the limestone utilization. The large particles (~ 100 μm in diameter) in both samples typically consisted of a CaSO₄ shell and an almost pure CaO core; however, numerous small particles of diameters of 10-20 μm consisted predominantly of CaO without sulphate shells. In particular, the emphasis of this investigation has been focused on the remaining capacity of the fly ash for reaction with sulphur dioxide and to clarify the effects of iron, both samples have been doped with additional iron content, and their sulphation behavior examined, and while both experienced a small reduction in sulphation capacity, the fly ash with the initial low iron content experienced the lowest reduction of sulphation capacity after doping, which is not supportive of the idea that iron has an important effect on sorbent capacity.     

Calcium sulphide formation in fluidized bed boilers

Analyses of samples of bed ash from a stationary fluidized bed boiler show the presence of calcium sulphide. In some samples, half of the total sulphur was present as sulphide. The samples containing CaS were obtained under unstaged conditions and with a high excess air ratio, 1.3 to 1.4. The samples were taken after a stop in the limestone addition, i.e. at high SO₂ emissions of about 1000 mL/m³ (ppm). No CaS was found during limestone addition when the SO₂ emission was 300–400 mL/m³. This indicates that formation of large amounts of CaS may be initiated as the SO₂ concentration exceeds some critical level.