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.     

Catalytic effect of biomass ash on CO, CH4 and HCN oxidation under fluidised bed combustor conditions

In fluidised bed combustion heterogeneous reactions catalysed by the bed material, CaO, and char are significant for the emission levels for instance of NO, N₂O, and CO. The catalysts present in the bed affect significantly the selectivity of HCN and NH₃ oxidation, which are known as precursors of NO_x (i.e. NO and NO₂) and N₂O emissions from solid fuel combustion. Thus the catalytic activity of biomass ashes may also be responsible for the negligible N₂O emissions from biomass combustion due to the presence of a large amount of solids in fluidised bed combustion, homogeneous oxidation may be suppressed within the bed by the quenching of the radicals. For this reason the catalytic oxidation of hydrocarbons and CO on the bed material may be of significance for the total burnout within the fluidised bed combustor.Within this study the effect of different ashes from spruce wood, peat, and for comparison bituminous coal on the oxidation of CH₄, CO, and HCN was studied. The different ashes were shown to have a strong catalytic activity for the oxidation of CH₄, CO, and HCN. In HCN oxidation the selectivity towards NO is high, whereas very little N₂O is formed. The activity of the ashes is strongly dependent on the fuel, which may be explained by their composition.The kinetics of the oxidation of CO and HCN in the temperature range relevant for fluidised bed combustion, i.e. 800-900 °C, has been evaluated for spruce wood ash.     

A techno-economic analysis of the application of continuous staged-combustion and flameless oxidation to the combustor design in gas turbines

The impact of NO_x reduction technologies upon a gas turbine power station has been investigated using the ECLIPSE process simulator. Technical, environmental and economic assessments were performed, based upon a model of the simple cycle gas turbine, fuelled by natural gas.The technologies assessed were: a) selective catalytic reduction (SCR); b) continuous staged air combustion (COSTAIR); and c) flameless oxidation method (FLOX).The SCR method produced a 90% reduction of NO_x emissions, at an additional penalty to the electricity cost of 0.19–0.20 p/kWh, over the base case of simple cycle with standard combustor.The COSTAIR method reduced 80.4% of NO_x emissions, at an additional electricity cost of 0.03–0.04 p/kWh, over the base case; but 0.16–0.17 p/kWh less than the SCR method at a slightly higher level of NO_x emissions.The FLOX method generated 92.3% less of NO_x emissions, at an additional electricity cost of 0.08–0.11 p/kWh, over the base case; but 0.09–0.11 p/kWh less than the SCR method at a lower level of NO_x emissions.A sensitivity analysis of the Break-Even Selling Price (BESP) of electricity and the Specific Investment (SI) versus the cost of different burner systems shows that the SCR system had the highest values for BESP and SI; and the COSTAIR system had the lowest.The results show that the use of these non-standard burners could offer an effective method of reducing NO_x emissions considerably for simple cycle gas turbine power plants with minimal effect on system capital cost and electricity selling price, and were also cheaper than using SCR.     

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.     

Chemical kinetic modelling of the effect of NO on the oxidation of CH4 under fluidized bed combustor conditions

The ignition and burnout of the volatiles in fluidized bed combustor are essential for its performance and emissions. NO_x are known to sensitize the oxidation of hydrocarbons, CO, and H₂. This effect is relevant especially for fluidized bed combustors, which are operated at relatively low temperatures (i.e. about 850 °C). Different reaction mechanisms and modifications to existing mechanisms have been proposed in the literature to account for these low temperature interactions of NO_x and hydrocarbons. In this work, an existing widely used reaction mechanism is adapted and tested for its capability to describe the NO sensitized oxidation of CH₄ under conditions relevant to fluidized bed combustion. NO lowers the ignition temperature to about 300 °C under the conditions investigated. Three different oxidation paths for the oxidation of CH₄ have been identified and discussed. Their relative importance strongly depends on combustion temperature, indicating that the presence of NO_x significantly affects the oxidation of the volatiles in fluidized bed combustion.     

Experimental investigation of NOx emissions in oxycoal combustion

This work presents the results of an experimental investigation on NO_x emissions from coal combustion in a pilot scale test facility. Three oxidiser atmospheres have been compared, namely air, CO₂/O₂, and O₂ enriched recirculated flue gas. NO_x emissions from two different combustion modes have been studied, swirl flame and flameless combustion. The influence of the burner oxygen ratio and the oxidiser O₂ concentration on NO_x formation and reduction have been analysed. With increasing burner oxygen ratio, an increase of NO_x emissions has been obtained for air and CO₂/O₂ in both, swirl flame and flameless combustion. In case of the swirl flame, flue gas recirculation leads to a reduction of NO_x emissions up to 50%, whereas in case of flameless combustion this reduction is around 40% compared to CO₂/O₂. No significant impact of the oxidiser O₂ concentration in the CO₂/O₂ mixture on NO_x emissions is observed in the range between 18 and 27 vol.% in swirl flames. An analysis of NO_x formation and reduction mechanisms showed, that the observed reduction of NO_x emissions by flue gas recirculation cannot be attributed to the reduction of recirculated NO_x alone, but also to a reduced conversion of fuel-N to NO.     

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.     

NOx and SOx emissions of a high sulfur self-retention coal during air-staged combustion

NO_x and SO_x emissions of air-staged combustion were investigated in a 1 MW tangentially-fired furnace combusting a high sulfur self-retention coal. Two variables including the air stoichiometric ratio of primary combustion zone and the relative location of over-fire air (OFA) injection ports were studied. These results suggest that NO_x reduction efficiency monotonically increases with increasing the relative location of OFA injection ports, and the lowest NO_x emissions are achieved when the air stoichiometric ratio of primary combustion zone is 0.85. In the meantime, SO_x emissions can be effectively reduced when the air stoichiometric ratio of primary combustion zone is 0.85 or 0.95, and SO_x emissions monotonically decrease with increasing the relative location of OFA injection ports.     

Application of the thermal DeNOx process to diesel engine DeNOx: an experimental and kinetic modelling study

Diesel DeNO_x experiments have been conducted using the selective noncatalytic ‘thermal DeNO_x’ process in a diesel fuelled combustion-driven flow reactor which simulated a single cylinder (966 cm³) and head equipped with a water-cooling jacket and an exhaust pipe. NH₃ was directly injected into the cylinder to reduce NO_x emissions. A wide range of air/fuel ratios (A/F=20-40) was selected for NO_x reduction where an initial NO_x of 530 ppm was usually maintained with a molar ratio (β=NH₃/NO_x) of 1.5.The results indicate that a 34% NO_x reduction can be achieved from the cylinder injection in the temperature range, 1100-1350 K. Most of the NO_x reduction occurs within the cylinder and head section (residence time<40 ms), since temperatures in the exhaust are too low for additional NO_x reduction. Under large gas quenching rates, increasing β values (e.g. 4.0) substantially increase the NO_x reduction up to 60%, which is comparable with those achieved under isothermal conditions. Experimental findings are analysed by chemical kinetics using the Miller and Bowman mechanism including both N/H/O species and CO/hydrocarbon reactions to account for CO/UHC oxidation effects, based on practical nonisothermal conditions. Comparisons of the kinetic calculations with the experimental data are given as regards temperature characteristics, residence time and molar ratio. In addition, the effects of CO/UHC and branching ratio (α=k₁/(k₁+k₂)) for the reaction NH₂+NO=products are discussed in terms of NO reduction features, together with practical implications.     

Mitigation of NOx emissions from a jatropha biodiesel fuelled DI diesel engine using antioxidant additives

Biodiesel offers cleaner combustion over conventional diesel fuel including reduced particulate matter, carbon monoxide and unburned hydrocarbon emissions. However, several studies point to slight increase in NO_x emissions (about 10%) for biodiesel fuel compared with conventional diesel fuel. Use of antioxidant additives is one of the most cost-effective ways to mitigate the formation of prompt NO_x. In this study, the effect of antioxidant additives on NO_x emissions in a jatropha methyl ester fuelled direct injection diesel engine have been investigated experimentally and compared. A survey of literature regarding the causes of biodiesel NO_x effect and control strategies is presented. The antioxidant additives L-ascorbic acid, α tocopherol acetate, butylated hydroxytoluene, p-phenylenediamine and ethylenediamine were tested on computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4 kW rated power. Results showed that antioxidants considered in the present study are effective in controlling the NO_x emissions of biodiesel fuelled diesel engines. A 0.025%-m concentration of p-phenylenediamine additive was optimal as NO_x levels were substantially reduced in the whole load range in comparison with neat biodiesel. However, hydrocarbon and CO emissions were found to have increased by the addition of antioxidants.     

Experimental investigation of nitrogen species release from different solid biomass fuels as a basis for release models

Experimental data on the release of NO_x precursors from solid biomass fuels during thermal conversion are necessary to study N release in general and to supply reliable data for the purpose of packed bed and gas phase conversion model development and validation. In this work the release of NO_x precursors was studied at a lab-scale pot furnace (batch reactor) by taking measurements during the conversion process of solid biomass in a packed bed. The investigations were carried out with relevant woody biomass fuels, which cover a broad range of fuel N contents: sawdust, bark, waste wood and MDF board. The most important NO_x precursor detected above the fuel bed under fuel rich conditions was NH₃, while HCN was almost insignificant with the exception of sawdust. NO was detected mainly under air rich conditions. Furthermore, the experimental data were utilised to derive release functions for the relevant NO_x precursors NO, NH₃ and HCN. The release functions were implemented in an in-house empirical packed bed combustion model, which serves as a basis for a subsequent CFD N species gas phase calculation.     

Estimation of NOx emissions from coal-fired utility boilers

CFD-based engineering models can be cost-effective tools in determining technical feasibility of clean coal-fired power generation technologies like NO_x mitigation strategies targeting the reduction of acid rain and smog. Their actual effectiveness depends on their capability to provide realistic engineering estimates without arbitrary adjustment of model parameters. This investigation focused on testing a CFD based NO_x model over a variety of coal type, firing configuration and boiler size ranging from 200 MW_e sub-critical to most modern 1000 MW_e ultra supercritical. In most cases, the NO_x estimates based on input data readily available from power plants were found within the range of measured data (with the worst estimate being 22% higher than the maximum measured NO_x level). The CFD results also indicated some sensitivity of the NO_x estimates to the ratio of volatile nitrogen to char nitrogen and the importance of NO reduction by char. However, this study showed that the locations of fuel-bound nitrogen evolution with respect to the stoichiometric condition within the boiler actually governed the overall NO emissions.     

Modelling of NOx adsorption over NOx adsorbers

Modelling of the phenomena involved during the adsorption of NO_x on NO_x trap catalysts was developed. The aim of the model is the prediction of the quantity of stocked barium nitrate as well as the emissions of NO and NO₂, as a function of time and temperature. The mechanism of the process is sounded on the adsorption of gas species (NO, NO₂, O₂) on platinum sites, equilibrium reaction between adsorbed species followed by the formation of Ba(NO₃)₂. This formation of barium nitrate is limited by the thermal decomposition reaction which liberates NO in the gas phase. The kinetic constant of decomposition of barium nitrate was determined by temperature programmed thermogravimetry on pure Ba(NO₃)₂, using the method of Freeman and Carroll. Other kinetic constants bound to the mechanism were estimated by fitting the results of the model to experimental results.The mechanism was validated for various values of the molar fraction of O₂, the molar fraction of NO and various values of the NO/NO₂ ratio in the gas entering the reactor. It was also tested with different catalyst compositions (variation of the platinum and BaO concentrations). The importance of oxygen in the process was clearly demonstrated as well as the promoting role of NO₂.     

Attempts to reduce NOx exhaust emissions by using reformulated biodiesel

Biodiesel is a renewable, domestically produced fuel that has been shown to reduce particulate, hydrocarbon, and carbon monoxide emissions from diesel engines. Under some conditions, however, biodiesel produced from certain feedstocks has been shown to cause an increase in nitrogen oxides (NO_x). This is of special concern in urban areas that are subject to strict environmental regulations. Although soy-based biodiesel may increase the emission of nitrogen oxides, it is the most easily accessible in North America. We investigated two routes to reformulate soy-based biodiesel in an effort to reduce nitrogen oxide emissions. In one of these, soy-oil methyl esters were modified by conversion of a proportion of the cis bonds in the fatty acid chains of its methyl esters to their trans isomers. In the other approach, polyol derivatives of soybean oil were transesterified to form soy methyl polyol fatty acid esters. The NO_x emissions of these modified biodiesels were then examined, using a Yanmar L100 single cylinder, four stroke, naturally aspirated, air cooled, direct injection diesel engine. Using either isomerized methyl oleate or isomerized soy biodiesel, at 20% blend level in petroleum diesel (‘B20’), nitrogen oxide emissions were elevated by between 1.5 and 3 percentage points relative to the combustion of a B20 blend of commercial biodiesel. Nitrogen oxide emissions were reduced in proportion to blend level during the combustion of polyol biodiesel, with a 20% blend in petrodiesel resulting in a reduction of about 4.5 percentage points relative to the emissions of a comparable blend of commercial soy biodiesel.     

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 research on the estimation for the NOx emissive concentration of the pulverized coal boiler by the flame image processing technique

In this paper, flame image processing technique is used to estimate NO_x emissive concentration of the pulverized coal boiler. According to the brightness value of flame image pixels, the temperature distribution of the flame is calculated. The combustion characteristic parameters are picked up from the flame image. Based on the back propagation neural network, the model to estimate the NO_x concentration in the gas released from the power plant boiler has been developed. The experiments on the furnace flame monitor system of a power plant have testified that this estimation model can get the satisfying results.     

A technical and environmental analysis of co-combustion of coal and biomass in fluidised bed technologies

The use of biomass, which is considered to produce no net CO₂ emissions in its life cycle, can reduce the effective CO₂ emissions of a coal-fired power generation system, when co-fired with the coal, but may also reduce system efficiency.The technical and environmental analysis of fluidised bed technologies, using the ECLIPSE suite of process simulation software, is the subject of this study. System efficiencies for generating electricity are evaluated and compared for the different technologies and system scales.Several technologies could be applied to the co-combustion of biomass or waste and coal. The assessment studies here examine the potential for co-combustion of (a) a 600 MWe pulverised fuel (PF) power plant (as a reference system), (i) co-firing coal with straw and sewage sludge and (ii) using straw derived fuel gas as return fuel; (b) a 350 MWe pressurised fluidised bed combustion (PFBC) system co-firing coal with sewage sludge; (c) 250 MWe and 125 MWe circulating fluidised bed combustion (CFBC) plants co-firing coal with straw and sewage sludge; (d) 25 MWe CFBC systems co-firing low and high sulphur content coal with straw, wood and woody matter pressed from olive stones (WPOS); (e) 12 MWe CFBC co-firing low and high sulphur content coal with straw or wood; and (f) 12 MWe bubbling fluidised bed combustion (BFBC), also co-firing low and high sulphur content coal with straw or wood.In the large systems the use of both straw and sewage sludge resulted in a small reduction in efficiency (compared with systems using only coal as fuel).In the small-scale systems the high moisture content of the wood chips chosen caused a significant efficiency reduction.Net CO₂ emissions are reduced when biomass is used, and these are compared for the different types and scales of fluidised bed technologies. NO_x emissions were affected by a number of factors, such as bed temperature, amount of sorbent used for SO₂ capture and HCl emitted.     

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%.     

Impact of air staging along furnace height on NOx emissions from pulverized coal combustion

Experiments were carried out on an electrically heated multi-path air inlet one-dimensional furnace to assess NO_x emission characteristics of an overall air-staged (also termed air staging along furnace height) combustion of bituminous coal. The impact of main parameters of overall air-staged combustion technology, including burnout air position, air stoichiometric ratio, levels of burnout air (the number of burnout air arranged at different heights of the furnace), and the ratios of the burnout air flow rates and pulverized coal fineness of industrial interest, on NO_x emission were simulated to study in the experimental furnace, as well as the impact of air staging on the carbon content of the fly ash produced. These results suggest that air-staged combustion affects a pronounced reduction in NO_x emissions from the combustion of bituminous coal. The more deeply the air is staged, the further the NO_x emission is reduced. Two-level air staging yields a greater reduction in NO_x emission than single-level air staging. For pulverized coal of differing fineness, the best ratio between the burnout air rates in the two-level staging ranges from 0.6 to 0.3. In middle air-staged degree combustion with f_M = 0.75, pulverized coal fineness, R₉₀ (%), has a greater influence on NO_x emission, whereas R₉₀ has little influence on NO_x emission for deep air-staged degree with f_M = 0.61. Air-staged combustion with proper burnout air position has little effect on the burnout. For overall air-staged combustion, proper burnout air position and air-staged rate should be considered together in order to achieve high combustion efficiency.