Experimental study on NOx emissions of pulverized bituminous coal combustion preheated by a circulating fluidized bed

This study investigated nitrogen oxide (NO_x) emissions of pulverized coal combustion preheated by a circulating fluidized bed (CFB). During the test process, high-temperature fuel preheated in a CFB was burned in a down-fired combustor (DFC). The effect of air distribution on NO_x emissions was studied in the DFC, including three types of secondary air nozzle structures, five secondary air ratios, and three tertiary air position arrangements. Under stable conditions, the conversion ratio of fuel-nitrogen to N₂ in the CFB was 41.4%, which resulted in lower NO_x emissions in the platform. In this study, secondary air could be injected into the combustor at the top (annular) or through the side wall (circular) of the DFC, both with high combustion efficiency. This means that the secondary air is completely separated from the burner, and burner structure is greatly simplified. NO_x emissions from secondary air nozzle structures of center, annular, and circular ports were 565.66, 345.45, and 220.38 mg/Nm³ (@6% O₂) respectively. NO_x emissions initially decreased then increased with increases in secondary air ratio with the annular nozzle structure. NO_x emissions could be further inhibited by rationally arranging tertiary air positions.     

Numerical simulation on pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed

High temperature air combustion is a prospecting technology in energy saving and pollutants reduction. Numerical simulation on pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed was presented. The down-fired combustor, taken as the calculation domain, has the diameter of 220 mm and the height of 3000 mm. 2 cases with air staging combustion are simulated. Compared the simulation results with experimental data, there is a good agreement. It is found that the combustion model and NOx formation model are applicable to simulate the pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed. The results show that there is a uniform temperature profile along the axis of the down-fired combustor. The NOx emissions are lower than those of ordinary pulverized coal combustion, and the NOx emissions are 390 mg/m3 and 352 mg/m3 in Case 1 and Case 2, respectively. At the range of 300-600 mm below the nozzle, the NO concentration decreases, mainly resulting from some homogeneous reactions and heterogeneous reaction. NO concentration has a little increase at the position of 800 mm below the nozzle as the tertiary air supplied to the combustor at the position of 600 mm below the nozzle.     

The fate of char-N at pulverized coal conditions

The fate of char-N (nitrogen removed from the coal matrix during char oxidation) has been widely studied at fluidized bed conditions. This work extends the study of char-N to pulverized coal conditions. Coal chars from five parent coals were prepared and burned in a laboratory-scale pulverized coal combustor in experiments designed to identify the parameters controlling the fate of char-N. The chars were burned with natural gas (to simulate volatiles combustion) in both air and in a nitrogen-free oxidant composed of Ar, CO₂, and O₂. In some experiments, the char flames were doped with various levels of NO or NH₃ to simulate formation of NO_x from volatile-N (nitrogen removed during coal devolatilization). The conversion of char-N to NO_x in chars burned in the nitrogen-free oxidant was 50-60% for lignites and 40-50% for bituminous coals. In char flames doped with NO_x, the apparent conversion of char-N to NO_x (computed using the NO_x measurements made before and after the addition of char to the system) decreased significantly as the level of NO_x doping increased. With 900 ppm NO_x present before the addition of char, apparent conversion of char-N to NO_x was close to 0% for most chars. While there is no clear correlation between nitrogen content of the char and char-N to NO_x conversion at any level of NO_x in the flame, the degree of char burnout within a given family of chars does play a role. Increasing the concentration of O₂ in the system in both air and nitrogen-free oxidant experiments increased the conversion of char-N to NO_x. The effects of temperature on NO_x emissions were different at low (0 ppm) and high (900 ppm) levels of NO_x present in the flame before char addition.     

Experimental study on combustion characteristics and NO<Subscript>X</Subscript> emissions of pulverized anthracite preheated by circulating fluidized bed

A 30 kW bench-scale rig of pulverized anthracite combustion preheated by a circulating fluidized bed(CFB)was developed.The CFB riser has a diameter of 90 mm and a height of 1,500 mm.The down-fired combustion chamber(DFCC)has a diameter of 260 mm and a height of 3,000 mm.Combustion experiments were carried out using pulverized anthracite with 6.74%volatile content.This low volatile coal is difficult to ignite and burn out.Therefore,it requires longer burnout time and higher combustion temperature,which results in larger NOX emis-sions.In the current study,important factors that influence the combustion characteristics and NOX emissions were investigated such as excess air ratio,air ratio in the reducing zone,and fuel residence time in the reducing zone.Pulverized anthracite can be quickly preheated up to 800℃in CFB when the primary air is 24% of theoretical air for combustion,and the temperature profile is uniform in DFCC.The combustion efficiency is 94.2%,which is competitive with other anthracite combustion technologies.When the excess air ratio ranges from 1.26 to 1.67,the coal-N conversion ratio is less than 32%and the NOX emission concentration is less than 371 mg/m 3(@6%O2).When the air ratio in the reducing zone is 0.12,the NOX concentration is 221 mg/m 3(@6%O2),and the coal-N conversion ratio is 21%,which is much lower than that of other boilers.     

Effect of post-combustion air distribution on NOx original emission and combustion characteristics of 75 t/h coal slime circulating fluidized bed boiler

Coal slime has been discharged or piled up on the spot as solid waste for a long time, causing a lot of waste of resources and environmental pollution. A 75 t/h circulating fluidized bed (CFB) boiler with post-combustion technology was built for combustion of coal slime. To explore the post-combustion technology and the influences of post-combustion air speed and number of post-combustion air nozzles on combustion and original NO_x emission characteristics of Yankuang Coal Slime, the experiments were carried out on a 75 t/h circulating fluidized bed industrial boiler. The experimental results showed that post-combustion technology makes the NO_x original emission lower and can meet ultra-low emission requirement. Both the post-combustion air speed and the number of post-combustion air nozzles affected the post-combustion air flow. As the post-combustion air speed and the number of post-combustion air nozzles increase, NO_x original emissions decreased and CO emissions increased slightly.     

高压O2/CO2气氛下煤焦理论计算研究

研究表明在6MPa压力,富氧浓度为30%的条件下的增压富氧燃烧的经济性可以达到最佳。在此气氛下用一个简单的燃烧模型对碳球燃烧的情况进行理论计算,与空气气氛下的燃烧情况作对比,得到增压富氧燃烧在煤粉的燃烧时间和对烟气捕集回收二氧化碳上的优越性。从而为增压富氧燃烧的进一步发展提供理论基础。     

Generating efficiency and emissions of a spark-ignition gas engine generator fuelled with biogas–hydrogen blends

We investigated the generating efficiency and pollutant emissions of a four-stroke spark-ignition gas engine generator operating on biogas–hydrogen blends of varying excess air ratios and hydrogen concentrations. Experiments were carried out at a constant engine speed of 1200 rpm and a constant electric power output of 10 kW. The experimental results showed that the peak values of generating efficiency, maximum cylinder pressure, and NO_x emissions were elevated at an excess air ratio of around 1.2 as the hydrogen concentration was increased. CO₂ emissions decreased as the excess air ratio and hydrogen concentration increased, due to lean-burn conditions and hydrogen combustion. An efficiency per NO_x emissions ratio (EPN) was defined to consider the relationship between the generating efficiency and NO_x emissions. A maximum EPN value of 0.7502 was obtained with a hydrogen concentration of 15%, for an excess air ratio of 2.0. At this EPN value, the NO_x and CO₂ emissions were 39 ppm and 1678.32 g/kWh, respectively, and the generating efficiency was 29.26%. These results demonstrated that the addition of hydrogen to biogas enabled the effective generation of electricity using a gas engine generator through lean-burn combustion.     

Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, elevated levels of CO₂ and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal char on the char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal char, we computed the char consumption characteristics for a range of CO₂ and H₂O reaction rate coefficients for a 100 μm coal char particle reacting in environments of varying O₂, H₂O, and CO₂ concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the char particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of char oxidation and the radiant emission from burning char particles. However, the overall effect of the combined steam and CO₂ gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on char combustion in oxygen-enriched environments, due to the higher char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O₂ concentration) and as the particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.     

EGR与进气富氧对直喷柴油机NO和碳烟排放的影响

使用增压中冷直喷柴油机,采用进气富氧与高比率EGR相结合的技术,实现富氧燃烧条件下的低NO-碳烟排放.单独使用富氧燃烧,NO的排放将随氧体积分数的上升而增加.单独使用高EGR,碳烟(Smoke)的排放会随EGR率的增加而增加.将富氧进气与高比率EGR的结合,可以通过富氧的强氧化性降低Smoke排放,通过大比率EGR来控制燃烧温度,抑制NO的过度增长.试验结果表明:1,600,r/min(经济转速)下,EGR率为35%～45%,进气氧体积分数为21%～23%;2,200,r/min(最高转矩)下,EGR率为20%～50%,进气氧体积分数为22%～24%;在上述范围内的EGR与O2搭配,可以实现低于原机的NO-Smoke排放.综合考察发动机在各种掺比下的功率、油耗,探索出适合发动机各个工况的富氧及EGR组合区域,在该区域内发动机的功率、油耗和排放水平都能得到兼顾.     

Effects of operating conditions and fuel properties on emission performance and combustion efficiency of a swirling fluidized-bed combustor fired with a biomass fuel

This work reports an experimental study on firing 80 kg/h rice husk in a swirling fluidized-bed combustor (SFBC) using an annular air distributor as the swirl generator. Two NO_x emission control techniques were investigated in this work: (1) air staging of the combustion process, and (2) firing rice husk as moisturized fuel. In the first test series for the air-staged combustion, CO, NO and C_xH_y emissions and combustion efficiency were determined for burning “as-received” rice husk at fixed excess air of 40%, while secondary-to-primary air ratio (SA/PA) was ranged from 0.26 to 0.75. The effects of SA/PA on CO and NO emissions from the combustor were found to be quite weak, whereas C_xH_y emissions exhibited an apparent influence of air staging. In the second test series, rice husks with the fuel-moisture content of 8.4% to 35% were fired at excess air varied from 20% to 80%, while the flow rate of secondary air was fixed. Radial and axial temperature and gas concentration (O₂, CO, NO) profiles in the reactor, as well as CO and NO emissions, are discussed for the selected operating conditions. The temperature and gas concentration profiles for variable fuel quality exhibited significant effects of both fuel-moisture and excess air. As revealed by experimental results, the emission of NO from this SFBC can be substantially reduced through moisturizing rice husk, while CO is effectively mitigated by injection of secondary air into the bed splash zone, resulting in a rather low emission of CO and high (over 99%) combustion efficiency of the combustor for the ranges of operating conditions and fuel properties.     

Co-combustion of refuse derived fuel with Korean anthracite in a commercial circulating fluidized bed boiler

The effect of co-combustion of Refuse Derived Fuel (RDF) with Korean anthracite on the combustion and environmental performance was observed in the Tonghae commercial Circulating Fluidized Bed (CFB) boiler. High contents of oxygen and CaO in RDF reduced the amount of air required for combustion and the limestone flow rate for SO₂ capture in the CFB boiler, respectively. The temperature in the furnace exit increased slightly due to re-combustion of volatiles which resulted in limiting the co-combustion ratio of RDF for the CFB boiler to operate stably. With the increasing co-combustion ratio of RDF, the output voltages of electrostatic precipitator (EP), which consists of 2-channels and 5-stages collecting plates, decreased linearly. Eventually, stability of the EP could not be maintained above 5% of the RDF co-combustion ratio. The emissions of NO_x, HCl and dioxin during co-combustion did not change appreciably as compared to the case when Korean anthracite was burnt alone, which were also low enough to meet Korean emissions limits. On the other hand, chlorine contents in ashes emitted from the CFB boiler increased gradually with the increasing co-combustion ratio, which implied that most of chlorines were fixed by limestone.     

Assessing combustion and emission performance of direct use of SVO in a diesel engine by oxygen enrichment of intake air method

This work investigated the effect of the oxygen enrichment in the intake air of diesel engines on the combustion and emissions performance using rape seed oil (RSO) as a fuel. The purpose of the paper is to investigate the potential of oxygen enrichment in the intake air method to restrain the deterioration of particulate emissions of the RSO due to its high viscosity so as to explore the possibility of direct use of SVO (straight vegetable oil) in diesel engines, which can reduce CO₂ emissions and save cost. The combustion parameters such as ignition delay, heat release rate, in-cylinder peak temperature and pressure were determined. Engine out particulate and gaseous emissions of the RSO were measured at oxygen concentrations from 21% (by volume) (no enrichment) to 24% (by volume) and compared to diesel results. The enrichment of the intake air with oxygen decreased the ignition delay and premixed combustion duration, and increased the in-cylinder peak pressure and temperature. The particulate, CO and hydrocarbon emissions were significantly reduced while the NOx emissions increased as the oxygen enrichment rate increased. 22% oxygen enrichment rate was suggested to achieve lower than diesel particulate emissions with the lowest NOx penalty. Increased NOx could be controlled by other methods. The results show that the oxygen enrichment in intake air method enabled direct combustion of SVO in diesel engines with reduced particulate, hydrocarbon and CO emissions.     

Combustion kinetics of coal chars in oxygen-enriched environments

Oxygen-enhanced and oxygen-fired pulverized coal combustion is actively being investigated to achieve emission reductions and reductions in flue gas cleanup costs, as well as for coal-bed methane and enhanced oil recovery applications. To fully understand the results of pilot scale tests and to accurately predict scale-up performance through CFD modeling, accurate rate expressions are needed to describe coal char combustion under these unconventional combustion conditions. In the work reported here, the combustion rates of two pulverized coal chars have been measured in both conventional and oxygen-enriched atmospheres. A combustion-driven entrained flow reactor equipped with an optical particle-sizing pyrometry diagnostic and a rapid-quench sampling probe has been used for this investigation. Highvale subbituminous coal and a high-volatile eastern United States bituminous coal have been investigated, over oxygen concentrations ranging from 6 to 36 mol% and gas temperatures of 1320-1800 K. The results from these experiments demonstrate that pulverized coal char particles burn under increasing kinetic control in elevated oxygen environments, despite their higher burning rates in these environments. Empirical fits to the data have been successfully performed over the entire range of oxygen concentrations using a single-film oxidation model. Both a simple nth-order Arrhenius expression and an nth-order Langmuir-Hinshelwood kinetic equation provide good fits to the data. Local fits of the nth-order Arrhenius expression to the oxygen-enriched and oxygen-depleted data produce lower residuals in comparison to fits of the entire dataset. These fits demonstrate that the apparent reaction order varies from 0.1 under near-diffusion-limit oxygen-depleted conditions to 0.5 under oxygen-enriched conditions. Burnout predictions show good agreement with measurements. Predicted char particle temperatures tend to be low for combustion in oxygen-depleted environments.     

NOx emissions and thermal efficiencies of small scale biomass-fuelled combustion plant with reference to process industries in a developing country

Solid biomass materials are an important industrial fuel in many developing countries and also show good potential for usage in Europe within a future mix of renewable energy resources. The sustainable use of wood fuels for combustion relies on operation of plant with acceptable thermal efficiency. There is a clear link between plant efficiency and environmental impacts due to air pollution and deforestation. To supplement a somewhat sparse literature on thermal efficiencies and nitrogen oxide emissions from biomass-fuelled plants in developing countries, this paper presents results for tests carried out on 14 combustion units obtained during field trials in Sri Lanka. The plants tested comprised steam boilers and process air heaters. Biomass fuels included: rubber-wood, fuelwood from natural forests; coconut shells; rice husks; and sugar cane bagasse. Average NO_x (NO and NO₂) emissions for the plants were found to be 47 gNO₂ GJ⁻¹ with 18% conversion of fuel nitrogen. The former value is the range of NO_x emission values quoted for combustion of coal in grate-fired systems; some oil-fired systems and systems operating on natural gas, but is less than the emission levels for the combustion of pulverized fuel and heavy fuel oil. This value is significantly within current European standards for NO_x emission from large combustion plants. Average thermal efficiency of the plants was found to be 50%. Observations made on operational practices demonstrated that there is considerable scope for the improvement of this thermal efficiency value by plant supervisor training, drying of fuelwood and the use of simple instruments for monitoring plant performance.     

The potential for low NOx from a precessing jet burner of coal

The effectiveness of the Precessing Jet nozzle at yielding low NO_x levels from burning coal was examined at the pilot scale. Thus, a coal burner of nominal thermal load of 138 kW was sampled two dimensionally, and subsequently modeled. The one-dimensional, steady-state, semiempirical mathematical model considered the release and combustion of volatiles, and subsequent oxidation of char. A comprehensive reaction scheme was formulated to account for the oxidation of the resulting CO, the formation of NO from various sources (volatile, char, preheated air) and the oxidation of H₂S to SO₂. The agreement between the experimental and predicted profiles of coal burnout, [O₂], [NO] and gas temperature was not good near the burner; however, the agreement improved in the postflame region. The model was also used to simulate the center line characteristics of the same flame, but with the secondary air preheated to 500°C. This flame was then scaled for constant velocity and constant residence time to 20 MW. It was deduced that constant residence-time scaling predicts ignition and combustion of the coal at the same axial location as the baseline flame. However, constant velocity scaling shifts combustion closer to the burner. Constant-velocity scaling was found to be more suitable for the theoretical scaling of pulverized coal flames. It was not possible to comment on the potential of the burner for low NO_x in cement kilns, because the measured and computed gas temperatures were low. However, the model predicted low concentrations of fuel NO_x.     

Nitrogen transfer of fuel-N in chemical looping combustion

Chemical-looping combustion (CLC) is a novel technique used for CO₂ separation that has been investigated for gaseous fuel and solid fuel. The nitrogen transfer of fuel-N in the coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier under a continuous operation in a 1 kW_th interconnected fluidized bed prototype. The effects of the fuel reactor temperature, coal type and operation conditions on the release of gaseous products of nitrogen species in the air reactor and the fuel reactor are carried out. Results show that the nitrogen transfer direction of fuel-N is toward N₂ formation in the fuel reactor independent of fuel type. In the fuel reactor N₂ is the sole product of nitrogen transfer of fuel-N. The concentration of N₂ in the fuel reactor exit gas increases with the fuel reactor temperature. The NO_x precursor of HCN can be oxidized by the oxygen carrier to form NO or N₂ in the fuel reactor. However, in the fuel reactor NO from coal devolatilization and HCN oxidization by oxygen carrier is completely reduced to N₂. The other NO_x precursor of NH₃ is completely converted to N₂ due to oxidization by NiO and the catalytic effect of Ni on the decomposition of NH₃. After coal devolatilization, char-N conversion in the fuel reactor is toward N₂ formation according to the investigation of solid–solid reaction between char and oxygen carrier. The amount of residual char has a potential to cause formation of nitrogen contaminants in the air reactor. In the air reactor, NO is the only nitrogen contaminant, and there is no NO₂ formation. The high fuel reactor temperature results in little residual char coming into the air reactor. The proportion of char-N converted to NO in the air reactor increases from 16.98% to 18.85% when the fuel reactor temperature changes from 850 to 950 °C. For the fuels containing more volatile matter, the possibility of NO formation in the air reactor is smaller than the fuels containing less volatile matter. For the fuels containing less volatile matter, char gasification rate is still a significant factor both for the carbon capture efficiency and NO formation.     

Modeling of NOx emissions from a W-shaped boiler furnace under different operating conditions

A numerical model for gas-particle flow dynamics has been combined with an NO_x chemistry post-processor to predict the formation of nitric oxide in a three-dimensional, W-shaped boiler furnace burning pulverized fuel. The model includes complex interactions in gas-particle turbulent flow, heat transfer, gaseous chemical reaction, coal combustion, and NO_x reaction chemistry. Because fuel nitrogen is released in proportion to burnout of pulverized coal particles, the particles are treated in a Lagrangian framework in order to track burning pulverized coal particles through the gas continuum. The results show capability of the model to describe NO_x emissions under different operating conditions for full and partial loads.     

Effect of methane reforming before combustion on emission and calorimetric characteristics of its combustion process

In this study, combustion and emission characteristics of methane mixed with steam (CH₄/H₂O) and the products of methane reforming with steam (CO/H₂/H₂O) were compared. Four fuel compositions were analysed: CH₄+H₂O, CH₄+2H₂O, and products of complete methane reforming in these mixtures, respectively. A comparison was carried out through the numerical model created via Ansys Fluent 2019 R2. A combustion process was simulated using a non-premixed combustion model, standard k-ϵ turbulence model and P-1 radiation model. The combustor heat capacity for interrelated fuel compositions was kept constant due to air preheating before combustion. The inlet air temperature was varied to gain a better insight into the combustion behaviour at elevated temperatures. The effect of steam addition on the emission characteristics and flame temperatures was also evaluated. NO_x formation was assessed on the outlet of the combustion zone. The obtained results indicate that syngas has a higher combustion temperature than methane (in the same combustor heat capacity) and therefore emitted 27% more NO_x comparing to methane combustion. With the air inlet temperature increment, the pollutant concentration difference between the two cases decreased. Steam addition to fuel inlet resulted in lesser emissions both for methane and syngas by 57% and 28%, respectively. In summary, syngas combustion occurred at higher temperature and produced more NO_x emissions in all cases considered.     

Experimental study on combustion of torrefied palm kernel shell (PKS) in oxy‐fuel environment

Oxy‐combustion of biomass can be a major candidate to achieve negative emission of CO₂ from a pulverized fuel (pf)‐firing power generation plants. Understanding combustion behavior of biomass fuels in oxy‐firing conditions is a key for design of oxy‐combustion retrofit of pulverized fuel power plant. This study aims to investigate a lab‐scale combustion behavior of torrefied palm kernel shell (PKS) in oxy‐combustion environments in comparison with the reference bituminous coal. A 20 kW_th‐scale, down‐firing furnace was used to conduct the experiments using both air (conventional) and O₂/CO₂ (30 vol% for O₂) as an oxidant. A bituminous coal (Sebuku coal) was also combusted in both air‐ and oxy‐firing condition with the same conditions of oxidizers and thermal heat inputs. Distributions of gas temperature, unburned carbon, and NO_x concentration were measured through sampling of gases and particles along axial directions. Moreover, the concentrations of SO_x and HCl were measured at the exit of the furnace. Experimental results showed that burnout rate was enhanced during oxy‐fuel combustion. The unburnt carbon in the flue gas was reduced considerably (~75%) during combustion of torrefied PKS in oxy‐fuel environment as compared with air‐firing condition. In addition, NO emission was reduced by 16.5% during combustion of PKS in oxy‐fuel environment as compared with air‐firing condition.