NOx emission from a 71 MWe pressurized fluidized bed combustor

Flue gas NO_x concentration was measured at the outlet of gas turbine (the inlet of de-NO_x catalyst) of a 71 MWe pressurized fluidized bed combustor. The effect of operating parameters on NO_x emission was approximated by assuming a linear combination of three independent parameters, concentration of O₂ in the flue gas, SO₂ removal efficiency, and dense bed temperature. Sensitivity factors for different type of coals were obtained. The relationship between the sensitivity factors and fuel ratio (fixed carbon/volatile matter ratio) was obtained. NO_x emission was well approximated by the present approximation, even for coals/coal mixtures that were not used to determine the sensitivity factors. NO_x emissions for coal/coal mixtures and combustor operation under fly ash recycle conditions can be predicted approximately from the equation derived from other coals under conditions without fly ash recycle.     

Evaluation of the use of coal volatiles as reburning fuel for NOx reduction

In this study, computational fluid dynamic (CFD) and kinetic models were used to investigate the relative performances of coal volatiles and natural gas reburning. This modeling approach considers fluid dynamic and non-isothermal effects, which were not considered in past laboratory flow reactor studies. The commercial CFD code FLUENT 6.1 was used to predict the residence times and temperatures for reburning tests in the down-fired combustor (DFC), a 0.5 MMBTU/h research combustor at The Pennsylvania State University. To predict NO_x concentrations within the combustor, this data was then applied to an advanced reburning kinetic model used in past studies. For equal firing rates and stoichiometric ratios, reburning using methane yielded lower concentrations of NO_x (and, therefore, better NO_x reduction performance) than reburning using coal volatiles. The coal volatiles give increased flame temperature over natural gas, which apparently offsets the increased reburn zone hydrocarbon radical yield of coal volatiles over natural gas.     

Coal characterisation for NOx prediction in air-staged combustion of pulverised coals

A series of world-traded coal samples has been tested using the Imperial College high temperature wire mesh apparatus (HTWM) in order to assess the relationship between high temperature (1600°C) char nitrogen content and NO_x formation in Hemweg Power Station (in the Netherlands) using deep furnace air staging. A linear relationship between high temperature char nitrogen and NO_x formation has been confirmed. These results suggest that high temperature char N content is the main factor limiting NO_x emissions with deep air-staged combustion.Char N and (hence apparently deep air-staged NO_x) can be predicted with an accuracy of approximately ±20% for most coals from the coal proximate and ultimate analysis—but this might not be sufficient for stations operating close to their emission limits. Measuring high temperature char N directly reduces the likely uncertainty in deep air-staged NO_x emissions for coals (and most blends) to approximately ±10%. Its use should be considered on a routine basis for coal selection on plants employing this technology.     

Modelling coal combustion: the current position

At the present time, computer models for coal combustion are not sufficiently accurate to enable the design of combustion plant or the selection of a coal based on combustion behaviour. Most comprehensive combustion models can predict with reasonable accuracy flow fields and heat transfer, but usually with a much lesser degree of accuracy than the combustion of the coal particles through to char burnout. Many research programmes are aimed at developing a much more accurate predictive tool for assessing coals specially fired in burners or furnaces employing a range of NO_x abatement technologies. Some of the current developments in CFD coal combustion modelling are outlined here. Particular attention is paid to the first step, where the devolatilisation pre-processor code is used to compute the pyrolysis rate, the yields and the composition of volatiles and char. These parameters are used as inputs to the devolatilisation and volatile combustion sub-models, where various options can be used, and also the char burnout sub-models. The accuracy of the sub-models is examined using data from four well-studied coals, three from the UK and one from the US. The main network devolatilisation codes are compared with experimental data. Two char combustion models have also been investigated in order to compare char burnout predictions and the development of char morphology and surface area during burnout are considered. The applications of these sub-models to two combustion situations were considered. These involve reactions in a drop tube furnace and a low NO_x industrial burner and in both cases, the model predictions were compared with experimental measurements.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Combustion characteristics of coal in a mixture of oxygen and recycled flue gas

The combustion of coal in a mixture of pure O₂ and recycled flue gas is one variant of a novel combustion approach called oxy-fuel combustion. With the absence of N₂, this approach leads to a flue gas stream highly enriched in CO₂. For many applications, this flue gas stream can then be compressed and sequestered without further separation. As a result, oxy-fuel combustion is an attractive way to capture CO₂ produced from fossil fuel combustion. When coal is burned in this O₂ and CO₂ rich environment, its combustion characteristics can be very different from conventional air-fired combustion. In CETC-O, a vertical combustor research facility has been used in the past years to investigate the combustion characteristics of several different coals with this variant of oxy-fuel combustion. This included flame stability, emissions of NO_x, SO_x and trace elements, heat transfer, in-furnace flame profiles and flue gas compositions. This paper will report some of the major findings obtained from these research activities.     

Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a bubbling 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 bubbling fluidized bed combustor. After starting the sludge feed, emissions of NO_x increased with time, but N₂O emissions changed only slightly. After a certain amount of sludge was burned, the fuel was switched from sludge to coal. Emissions of NO_x from coal combustion after sludge combustion were higher than those before sludge combustion. These results suggest that the accumulation of sludge ash influenced NO_x emissions. A simple model of ash accumulation and removal was proposed. The transient change in NO_x after starting co-combustion was explained using the model presented here.     

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.     

Numerical investigation on the flow, combustion and NOx emission characteristics in a 500 MWe tangentially fired pulverized-coal boiler

The characteristics of the flow, combustion, temperature and NO_x emissions in a 500 MW_e tangentially fired pulverized-coal boiler are numerically studied using comprehensive models, with emphasis on fuel and thermal NO_x formations. The comparison between the measured values and predicted results shows good agreement, which implies that the adopted combustion and NO_x formation models are suitable for correctly predicting characteristics of the boiler. The relations among the predicted temperature, O₂ and CO₂ mass fractions are discussed based on the calculated distributions. The predicted results clearly show that NO_x formation within the boiler highly depends on the combustion processes as well as the temperature and species concentrations. The results obtained from this study have shown that overfire air (OFA) operation is an efficient way to reduce the NO_x emissions of the pulverized-coal fired boiler. Air staging combustion technology (OFA operation) adopted in this boiler has helped reduce fuel NO_x formation as well as thermal NO_x formation under the present simulated conditions. The decrease in the formation of fuel NO_x is due to the decreased contact of the nitrogen from the fuel with the oxygen within the combustion air, while the decrease in thermal NO_x formation is caused by a decrease in temperature. The detailed results presented in this paper may enhance the understanding of complex flow patterns, combustion processes and NO_x emissions in tangentially fired pulverized-coal boilers, and may also provide a useful basis for NO_x reduction and control.     

Modelling the combustion of pulverized biomass in an industrial combustion test furnace

A CFD model that simulates the combustion of biomass in existing pf coal fired furnaces has been developed and model results for the combustion of a typical wood in a 1 MW industrial test facility have been presented. The model is primarily based on coal combustion submodels using an Eulerian–Lagrangian frame of reference. Biomass specific constants that define the submodels have been investigated and employed in the simulation. In particular, potassium release during biomass combustion and the formation of NO_x have been simulated. Numerical predictions have been compared with some experimental measurements that have been taken and reasonably good agreement has been achieved.     

The effect of O2 enrichment on NOx formation in biomass co-fired pulverised coal combustion

Oxygen enrichment of the combustion air in pulverised coal combustion for power plant is seen as a possible retrofit measure to improve CO₂ scrubbing and capture. This technique produces a reduced volume of flue gas with higher CO₂ concentration than normal air combustion that will contributes to the enhancement of amine scrubbing plant efficiencies. We report in this article the results of a study at the small pilot scale into the effect of these combustion modifications on the formation of NO_x and associated carbon burnout changes. Experiments were performed using a Russian coal, typical of that used in some UK power stations with shea meal and Pakistani cotton stalk as biomass fuels co-fired at a fraction of 15%th. The down-fired pulverised coal combustor was operated at 20 kWth under air-staged conditions for NO_x control and the secondary and over-fire air flows were both enriched by up to 79% (100% O₂) for a range of splits giving a 35% overall O₂ concentration for full enrichment. When the same enrichment process was applied to biomass/coal combustion different behaviour was observed with respect to NO_x formation. We have shown that oxygen enrichment can achieve benefits of improved carbon burnout with a positive impact on NO_x emissions over and above the primary aim of increasing CO₂ concentration in the flue gas for enhanced capture efficiencies. With all other conditions of overall stoichiometry, OFA levels and O₂ enrichment levels remaining the same, NO_x levels at 22% OFA initially increased over the range of secondary air enrichment, particularly for shea meal/coal co-firing. At 31% OFA the trends were to lower NO_x at high enrichment levels. However, co-firing with shea meal initially showed an increase in NO_x emission at lower levels of enrichment (up to 40% O₂) followed by overall lower NOx emissions at 100% O₂ in the secondary air. The results show that NO_x emissions can either increase or decrease depending on the operating conditions. The differences in behaviour are attributed, not only to the effects of enrichment on the stoichiometry of the near-burner zone, but also on the flame dynamics and intensity of combustion related to the associated reductions in gas velocity and swirl intensity by the transition from air to pure O₂ in the secondary oxidant stream.     

The effect of combustion conditions in a full-scale low-NOx coal fired unit on fly ash properties for its application in concrete mixtures

The wide implementation of low-NO_x combustion technologies in pulverized coal combustion can lead to higher levels of carbon in fly ash and increase the adsorptivity toward surfactants of the carbon. Consequently, the air entraining agent (AEA) requirements of the fly ash used for concrete production increases, which can complicate the stabilization of entrained air. In this study, a low-NO_x tangential fired 875 MW_th power plant burning bituminous coal have been operated under extreme conditions in order to test the impact of the operating conditions on fly ash adsorption behavior and NO_x formation. It was found that the AEA adsorption of the fly ash was reduced up to five times compared to reference operation, when the plant was operated with minimum furnace air staging, three levels of burners instead of four and without recycled flue gas. The lower AEA requirements of the fly ash at these conditions were primarily caused by a reduction in total carbon content, while the AEA adsorptivity of the residual carbon was lowered to about 60% of reference value. The tested operation mode, however, increased the NO_x level in the flue gas before the DeNO_x plant by 60% compared to reference operation.     

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.     

NOx generation from the combustion of Australian brown and subbituminous coals

The formation of NO_x during the combustion of pulverized brown and subbituminous coals from Victoria and Queensland respectively was investigated in an entrainment reactor. As no NO₂ was detected, all the NO_x was present in the form of NO. The brown coals exhibited a significantly greater potential for NO emission under fuel-lean conditions than did the subbituminous coal, even though the latter coal had a higher nitrogen content. However, under fuel-rich conditions the conversion of coal nitrogen to NO for the subbituminous coal was higher than for the brown coals. The differences in conversion efficiency may have been related in part to the nature and reactivity of the volatile nitrogen species. Reactivity differences between the chars produced from the brown and subbituminous coals may also have accounted for different extents of removal of NO. There was a significant reduction in the amount of NO emitted when brown coal was added to a combustion gas stream containing an appreciable quantity of NO before coal injection.     

Combustion modelling opportunities and challenges for oxy-coal carbon capture technology

Oxy-coal combustion is one of the leading technologies for carbon capture and storage. This paper presents a review of the opportunities and challenges surrounding the development of oxy-coal combustion models and discusses historical and recent advances in specific areas related to computational fluid dynamics (CFD), including char oxidation, radiation, pollutant formation and removal (Hg, NO_x and SO_x), and the impact of turbulence. CFD can be used to assess and optimise full-scale retrofit designs and to provide data on matching air-fired heat duties. In addition, CFD can also be used to improve combustion efficiency and identify potential reductions in corrosion, slagging, fouling and trace pollutant emissions. Transient simulations are becoming more computationally affordable for coal combustion, providing opportunities for model development. High concentrations of CO₂ and H₂O in oxy-coal can influence chemical kinetic rates, burnout and ash properties. The modelling can be improved by incorporating detailed kinetic mechanisms of gasification reactions. In addition, pollutant formation and removal mechanisms must be understood during oxy-coal firing to aid the selection of flue-gas cleaning strategies. Radiative heat transfer using spectral models for gaseous properties may be necessary in oxy-coal modelling because CO₂ and H₂O molecules have strong emission bands. Finally this review provides a coherent near-term and long-term oxy-coal specific CFD sub-models development strategy to simulate the complex oxy-coal combustion processes, heat transfer and pollutant emissions in power generation systems.     

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%.     

Co-firing characteristics of rice husk and coal in a cyclonic fluidized-bed combustor (Ψ-FBC) under controlled bed temperatures

This study extensively investigated temperature and emission characteristics, and the performance of co-firing rice husk with coal in a cyclonic fluidized-bed combustor (Ψ-FBC) of 125 kW_th nominal capacity. The Ψ-FBC integrated the distinct features of cyclonic/vortex and fluidized-bed combustion. Fluidization, without any inert material, can be accomplished by the stirring blades and vortex ring. The combustor was equipped with a multi-passes water coil to regulate the bed temperatures, varying 800-900 °C. Rice husk was co-fired with coal, a supplementary fuel, with coal blending ratios of 0-25% by thermal basis. The radial temperature profiles displayed vortex combustion along the wall, while the axial temperature profiles suggested a well-mixed condition in the lower part. The large depletion of O₂ and proliferation of CO in the lower part revealed vigorous combustion beneath the vortex ring. A reducing atmosphere appeared unfavorable to NO_x formation. The combustor showed satisfied E_c, mostly >98.5%. The optimum operating conditions with respect to NO_x emissions were: (1) the thermal percentage of coal not >20%, and (2) bed temperatures between 800 and 850 °C. Otherwise, NO_x emissions would exceed the regulations; even CO and SO₂ emissions were well acceptable.     

Optimization of air staging in a 1 MW tangentially fired pulverized coal furnace

This paper deals with an experimental study of air staging in a 1 MW (heat input power) tangentially fired pulverized coal furnace. The influences of several variables associated with air staging on NO_x reduction efficiency and unburned carbon in fly ash were investigated, and these variables included the air stoichiometric ratio of primary combustion zone (SR₁), the locations of over-fire air nozzles along furnace height, and the ratio of coal concentration of the fuel-rich stream to that of the fuel-lean one (RRL) in primary air nozzle. The experimental results indicate that SR₁ and RRL have optimum values for NO_x reduction, and the two optimum values are 0.85 and 3:1, respectively. NO_x reduction efficiency monotonically increases with the increase of OFA nozzle location along furnace height. On the optimized operating conditions of air staging, NO_x reduction efficiency can attain 47%. Although air staging can effectively reduce NO_x emission, the increase of unburned carbon in fly ash should be noticed.     

Burn-out of pulverised coal and biomass chars☆

The burn-out of carbon in pulverised fired power stations is commercially important. Interest in the burn-out of biomass chars is growing because biomass is increasingly being co-fired with coal to reduce the carbon dioxide emissions. The significance of carbon burn-out is that it is linked with the efficiency of the plant and the suitability of the coal ash for construction purposes. Residual carbon in ash has generally increased in recent years because of the influence of the lower temperatures and slower mixing resulting from the use of low NO_x burners. The amount of unburned carbon is thus a function of the plant design and operating conditions but it is also linked to the ease of combustion of the coal and the char formed. These latter factors are related to the properties of the coal and this paper attempts to quantify the impact of certain coal and char properties on carbon-out. An approach for assessing biomass combustion performance is also discussed.     

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.