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.     

Improvements in the performance of a medium-pressure-boiler through the adjustment of inlet fuels in a refinery plant

Hydrogen has been considered as a promising alternative for fossil fuel in recent years because it is very “clean”. Fossil fuel generates CO₂, CO, SO_x, unburned hydrocarbon and particles during combustion, while hydrogen only yields NO_x. In this study, a medium-pressure boiler with 130 ton/h boiler loading in a full-scale plant was studied with two inlet hydrogen-rich refinery gas (RG)/fuel oil (FO) volumetric flow rate ratios (inlet RG/FO ratio) and two residual O₂ concentration (vol.%) in flue gases (2%, 4%) to evaluate their influence on the emissions of NO_x and CO₂, flue gas temperatures and boiler efficiencies. The result shows significant improvements in both boiler efficiencies and emissions of air pollutants. By increasing the inlet RG/FO ratio from 1:5 to 1:1.5, the fuel cost was reduced by 11%, NO_x emission down by 12%, and the CO₂ emission 20,200 ton lower per year was achieved. Thus, better economic operating conditions for the boiler are suggested at inlet RG/FO ratio = 1:1.5 with the residual O₂ concentration in flue gases = 2%.     

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

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

Analysis of the behaviour of pollutant gas emissions during wheat straw/coal cofiring by TG-FTIR

The behaviour of pollutant gas emissions during the firing of wheat straw and coal blends was examined experimentally by using thermogravimetric analysis (TGA). Typical anthracite coal and wheat straw in central China were selected in this study. The ratio of coal to wheat straw by mass was set as 10:90, 15:85, 40:60 and 60:40 and the firing was carried using simulated air with oxygen and nitrogen gases. The emission characteristics of gas pollutants such as HCl, SO₂, CO₂ and NO_x were determined by coupled Fourier transform infrared (FTIR) measurements. The results showed that HCl, SO₂, CO₂ and NO_x emissions were closely related to the volatile combustion and char reacting stages. HCl emission was mainly released during the volatile combustion at the temperature between 220 and 450 °C. The profiles of HCl against temperature exhibit a single-peak, and the HCl peak occurred at 310 °C for all blends no matter what the ratio. The emission profiles of SO₂, and NO_x against temperature had the characteristic of two peaks. The first peak occurred around 320 °C for all blends, and however the second peak shifted towards higher temperatures as the coal content was increased in the blends. The study showed that combining the straw and coal can produce better emission control by reducing the magnitude of the peak releases. The analysis showed that the blended sample with 40% coal and 60% straw by mass produced the lowest levels of HCl, NO_x and SO₂ gas emissions. The CO₂ emission was mainly produced in the char combustion stage and purely increased with the carbon content in the blends.     

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

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

Electron beam flue gas treatment (EBFGT) technology for simultaneous removal of SO2 and NOx from combustion of liquid fuels

Electron beam flue gas treatment technology was applied for removal of SO₂ and NO_x from flue gas, emitted from combustion of high-sulfur fuel oils. The detailed study of this process was performed in a laboratory by irradiating the exhaust gas from the combustion of three grades of Arabian fuels with an electron beam from accelerator (800 keV, max. beam power 20 kW). SO₂ removal is mainly dependent on ammonia stoichiometry, flue gas temperature and humidity and irradiation doses up to 8 kGy. NO_x removal depends primarily on irradiation dose. High removal efficiencies up to 98% for SO₂ and up to 82% for NO_x were obtained under optimal conditions. The flue gas emitted from combustion of high-sulfur fuel oils, after electron beam irradiation, meets the stringent emission standards for both pollutants. The by-product, which is a mixture of ammonium sulphate and nitrate, can be used as a fertilizer as such or blended with other components to produce commercial agricultural fertilizer.     

Optimization of coal reburning in a 1 MW tangentially fired furnace

This paper deals with an experimental study on the influence of coal reburn on NO_x reduction efficiency, unburned carbon in fly ash and the furnace temperature distribution along the height in a 1 MW (heat input power) tangentially firing furnace with multiple low NO_x control technologies. Several variables associated with the reburn system have been investigated in the experiment which includes the air stoichiometry in reburn zone, the location of reburn burner and reburn coal fineness. The optimum location of reburn nozzles has been found where NO_x reduction efficiency is highest. With the decrease of reburn coal size (average diameter from 53.69 μm to 11.47 μm), NO_x reduction efficiency increases slightly, but the burnout performance of coal is improved noticeably. In the process of coal reburning, the temperature of flue gas is 70–90 °C lower in primary combustion, but 130–150 °C higher at the top of furnace as compared to baseline.     

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.     

Soot oxidation on a catalytic NOx trap: Beneficial effect of the Ba–K interaction on the sulfated Ba,K/CeO2 catalyst

The Ba,K/CeO₂ catalyst is active both for NO_x trapping and soot combustion. In this work we report a Ba–K interaction that prevents K sulfation when NO_x is present, thus preserving the activity of K towards soot combustion during the working period of the trap. This effect is originated in the K₂SO₄(s) + Ba(NO₃)₂(s) → 2KNO₃(s) + BaSO₄(s) reaction, which is thermodynamically favored. In the absence of NO_x, the soot combustion reaction is strongly depressed by SO₂ whereas when NO_x is present both the sulfated and the non-sulfated catalysts present similar TPO patterns.     

CO2 capture using oxygen enhanced combustion strategies for natural gas power plants

This paper describes a series of experiments conducted with natural gas in air and in mixtures of oxygen and recycled flue gas, termed O₂/CO₂ recycle combustion. The objective is to enrich the flue gas with CO₂ to facilitate its capture and sequestration. Detailed measurements of gas composition, flame temperature and heat flux profiles were taken inside CANMET's 0.3 MWth down-fired vertical combustor fitted with a proprietary pilot scale burner. Flue gas composition was continuously monitored. The effects of burner operation, including swirling of secondary stream and air staging, on flame characteristics and NO_x emissions were also studied. The results of this work indicate that oxy-gas combustion techniques based on O₂/CO₂ combustion with flue gas recycle offer excellent potential for retrofit to conventional boilers for CO₂ emission abatement. Other benefits of the technology include considerable reduction and even elimination of NO_x emissions, improved plant efficiency due to lower gas volume and better operational flexibility.     

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.     

Flame and radiation characteristics of gas-fired O2/CO2 combustion

This paper presents an experimental study on the flame properties of O₂/CO₂ combustion (oxy-fuel combustion) with focus on the radiation characteristics and the burn-out behaviour. The experiments were carried out in a 100 kWth test unit which facilitates O₂/CO₂ combustion with real flue gas recycle. The tests comprise a reference test in air and two O₂/CO₂ test cases with different recycled feed gas mixture concentrations of O₂ (OF 21 @ 21 vol.% O₂, 79 vol.% CO₂ and OF 27 @ 27 vol.% O₂, 73 vol.% CO₂). In-furnace gas concentration, temperature and total radiation (uni-directional) profiles are presented and discussed. The results show that the fuel burn-out is delayed for the OF 21 case compared to air-fired conditions as a consequence of reduced temperature levels. Instead, the OF 27 case results in more similar combustion behaviour compared to the reference conditions in terms of in-flame temperature and gas concentration levels, but with significantly increased flame radiation intensity. The information obtained from the radiation and temperature profiles show that the flame emissivity for the OF 21 and OF 27 cases both differ from air-fired conditions. The total emissivity and the gas emissivity of the OF 27 and the air-fired environment are discussed by means of an available model. The gas emissivity model shows that the increase in radiation intensity (up to 30%) of the OF 27 flame compared to the air flame can partly, but not solely, be explained by an increased gas emissivity. Hence, the results show that the OF 27 flame yields a higher radiative contribution from in-flame soot compared to the air-fired flame in addition to the known contribution from the elevated CO₂ partial pressure.     

Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation

Coal combustion with O₂/CO₂ is promising because of its easy CO₂ recovery, extremely low NO_x emission and high desulfurization efficiency. Based on our own fundamental experimental data combined with a sophisticated data analysis, its characteristics were investigated. It was revealed that the conversion ratio from fuel-N to exhausted NO in O₂/CO₂ pulverized coal combustion was only about one fourth of conventional pulverized coal combustion. To decrease exhausted NO further and realize simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x, a new scheme, i.e. O₂/CO₂ coal combustion with heat recirculation, was proposed. It was clarified that in O₂/CO₂ coal combustion, with about 40% of heat recirculation, the same coal combustion intensity as that of coal combustion in air could be realized even at an O₂ concentration of as low as 15%. Thus exhausted NO could be decreased further into only one seventh of conventional coal combustion. Simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x could be realized with this new scheme.     

Characterisation of biomass and coal co-firing on a 3 MWth Combustion Test Facility using flame imaging and gas/ash sampling techniques

Co-firing of biomass with pulverised coal at existing coal power stations remains a practical option available to power plant operators and is being widely adopted as one of the main technologies for reducing greenhouse gas emissions. However, there is a range of technological problems that are not well understood. This paper presents experimental investigations into the co-firing of pulverised coal directly co-milled with 5–20% biomass on a 3 MWth Combustion Test Facility. A number of combustion parameters, including flame temperature and oscillation frequency and particle size distribution, were measured under a range of co-firing conditions. The gas species within the flame and fly ash in flue gas were also sampled and analysed. The experimental data collected are used to study the impact of biomass additions to pulverised coal on the combustion characteristics of the co-firing process. The relationships between the flame characteristics, gas species and ash deposition of the furnace are investigated. The results suggest that, due to the varying physical and chemical properties of the biomass fuels, the biomass additions have impact on the combustion characteristics in a very complicated way. It has been found that the biomass addition to coal would improve the combustion efficiency because of the lower CO concentrations and higher char burnout level in co-firing. In addition, NO_x emission has been found closely linked to the flame stability, and SO_x emission reduced in general for all co-firing cases.     

Effect of carbon dioxide on flame propagation of pulverized coal clouds in CO2/O2 combustion

CO₂/O₂ combustion of pulverized coal is one of the promising new technology in order to reduce the emission of CO₂ and NO_x from coal combustion furnaces. However, several experiments with pulverized coal burners show that temperature and stability of pulverized coal flame is reduced in this condition. CO₂ has distinctive thermodynamic and optical property compared with that of other gas, and it is important to know the effect of CO₂ on the flame stability of pulverized coal. In this study, effect of CO₂ on flame propagation velocity of pulverized coal clouds were studied experimentally using micro-gravity condition, and also numerically considering detailed radiation heat exchange using Monte Carlo method.Experiments were made by using spherical chamber with inner diameter of 200 mm. Micro-gravity condition was used in order to achieve uniform pulverized coal cloud in a chamber. Flame propagation velocity was measured from the photographic image of the flame front by using high speed camera. Results show that flame propagation velocity of pulverized coal cloud in CO₂/O₂ mixture gas decreases to about 1/3–1/5 of that in N₂/O₂ mixture gas at the same oxygen concentration. By using Ar/O₂ mixture gas, it is revealed that thermal diffusivity of gas seems to have a large effect on flame propagation velocity. From the numerical analysis using Monte Carlo method, effect of absorption of radiation by CO₂ gas is proven to be relatively small compared with that of thermodynamic property especially for heat capacity of CO₂. Consequently, it is clarified that reduction of flame stability in CO₂/O₂ combustion is mainly due to the larger heat capacity of CO₂ gas.     

Coal combustion in O2/CO2 mixtures compared with air

As part of CO₂ abatement strategies for climate change, we are investigating coal combusion behaviour in various O₂/CO₂ mixtures and in air. The goal is to simulate conditions of coal combustion with flue gas recirculation in order to maximize the CO₂ concentration in the flue gas prior to its recovery. A western Canadian sub‐bituminous coal and a U.S. eastern bituminous coal were investigated. Thermal input was set at 0.21 MW with a flue gas oxygen concentration of 5 vol%. Experiments were done using various O₂/CO₂ mixtures and air. The oxygen concentration ranged from 21% to 42%. Up to 95% CO₂ concentrations were achieved in the flue gas. This paper describes experimental results in terms of flame temperatures and pollutant emissions (NO_x', SO₂ and CO).     

Kinetic model for the NOx reduction process by potassium containing coal char pellets at moderate temperature (350–450 °C) in the presence of O2 and H2O

A four-step mechanism is proposed to describe the reduction of NO_x by potassium containing coal char pellets under O₂-rich atmospheres. The four-step mechanism includes the chemisorption of both O₂ (step 1) and NO_x (step 3) on the so-called free sites (C_f) of carbon, which generates surface oxygen complexes (denoted by (CO)^#). The step 2 considers the direct decomposition of (CO)^# to yield CO₂ and the step 4 the reaction between NO_x and (CO)^# to yield CO₂ and C_f. NO_x reduction isothermal reactions between 350 and 450 °C have been carried out with potassium containing coal char pellets (16.8% w/w of catalyst) under 0.2%NO_x/5%O₂/N₂ and 0.2%NO_x/2%H₂O/5%O₂/N₂. The NO_x reduction and sample conversion experimental profiles have been successfully simulated by the system of algebraic and differential equations deduced from the four-step mechanism, which indicates that the mechanism seems to be feasible. Experimental results pointed out that the selectivity of pellets towards NO_x reduction against O₂ combustion decreases with temperature. This is in agreement with the elemental step rate constants (k_{step number = 1, 2, 3, or 4}) predicted by the model, that is, k₁ and k₂ increased with temperature in a major extent than k₃ and k₄, which are the steps in which NO_x are involved. The selectivity also decreases when H₂O is present in the reactive mixture. This is due to the destabilisation of (CO)^# in the presence of H₂O, thus creating C_f through step 2 that react with O₂ (step 1) in a major extent than with NO_x (step 3), as it is also deduced from the elemental step rate constants predicted by the model.     

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.     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Resistance to sulfidation of the catalytic reduction of NOx over Ag/Al2O3 by controlling the loadings of Ag and Ag+

SO₂ strongly decreased the catalytic activities of low loading Ag/Al₂O₃ below 500 °C in selective catalytic reduction (SCR) of NO_x by propene with or without the assistance of non-thermal plasma (NTP), which was mainly attributed to the competition between SO₂ and NO. By controlling the loadings of Ag and Ag⁺ over alumina, the resistance of SO₂ was remarkably enhanced between 400 °C and 500 °C in thermal SCR. In the NTP-assisted SCR, most of the NO_x conversions were also apparently recovered from 250 °C to 500 °C.