Experimental investigation of NO reburning during oxy-coal burner staging

This study presents an investigation into the impact of varied burner staging environments on an oxy-fuel flame and the rate of the NO formation and destruction processes. The experimental data was extracted from the use of a 250 kWth down-fired combustion test facility with a scaled-down model of an industrial low-NOx burner (LNB). Two oxy-coal combustion regimes were investigated by varying a fixed flow of oxidant between the secondary and tertiary registers, so as to impact the stoichiometry in the fuel-rich region and flame structure, and using various NO recycling regimes, to test the impact of these different burner configurations on NO reburning. The data was collected by monitoring key emissions in the flue gas and in the flame, as well as temperatures throughout the furnace and the unburned carbon content of the ash. A detailed investigation encompassing the impact of secondary oxidant proportion for different oxidants on NO emissions, together with the quantification of recycled NO destruction, is discussed. This investigation finds that 85 % to 95 % of the recycled NO is destroyed at a range of burner configurations using OF 27 and OF 30 at 170 kWth. In addition to this, NO formation and carbon burnout are found to be significantly affected with changing burner configurations. Further to this, OF 30 flames appear to be more sensitive to burner configuration than OF 27 flames with regards to both NO formation and destruction, possibly due to the decreased density of the OF 30 oxidant. Radial profiles of two burner configurations at OF 27 and OF 30, as well as an axial profile of two burner configurations at OF 30, are analysed. The profiles appear to show that burner staging aids in controlling the products of NO reburning, hence maximising the destruction of recycled NO

Selective non-catalytic reduction – Fe-based additive hybrid technology

Fe-based additives can be used to improve coal combustion and reduce NOx emissions; further to this, iron oxide (Fe2O3) has been found to interact with ammonia. Therefore, it is critically imperative to understand and assess the impact of the Fe-based additive on the use of ammonia based selective non-catalytic reduction (SNCR) and to evaluate the economic feasibility of such a combination for full-scale use. Experiments were performed using a 100 kWth down fired-combustion test facility burning pulverised coal over three Fe-based additive concentrations, while the ammonia input was varied between normalised stoichiometric ratios 0-3. This study finds evidence of an interaction between the Fe-based additive and SNCR. The interaction leads to greater ammonia utilisation and an increased NOx reduction due to the SNCR of >10%. The interaction is theorised to be pseudo-catalytic with the fuel additive providing an active site for ammonia to reduce NO. Using Carnegie Mellon University’s ‘Integrated Environmental Control Model’ (IECM), this has been shown to create an economically viable opportunity to increase SNCR effectiveness

Bio-CCS: co-firing of established greenfield and novel, brownfield biomass resources under air, oxygen-enriched air and oxy-fuel conditions

As demand for electricity and atmospheric CO concentrations rise technologies that reduce the environmental impact of generating electricity are sought. Within the many options a combination of co-firing of biomass and carbon capture and storage (Bio-CCS) could present a negative-emission process. This work investigates co-firing of a novel brownfield and two conventional greenfield biomass reserves with coal in oxygen-enriched conditions which may enhance the efficiency of post-combustion capture units. A 20kW furnace is used to assess combustion characteristics in a range of scenarios. Results suggest oxidant staging during oxygen-enriched co-firing can exhibit lower NO emissions while achieving high combustion efficiencies

Fuel additive technology - NOx reduction, combustion efficiency and fly ash improvement for coal fired power stations

Fuel additive technology is based on the use of a solid, fuel additive (iron, aluminium, calcium and silicon based oxides), to reduce NOx emission, improve the quality of fly ash and result in 1-3% coal savings for pulverised coal combustion. The findings in this study have been mainly based on extensive experimentation on 100 kWth down fired-combustion test facility (CTF) and partially on a commercial 260 tons/h steam producing water tube pf boiler. International Innovative Technologies (IIT) developed this additive based technology for the combined effect of reducing NOx from the combustion of hydrocarbon fuels (mainly coal) and more specifically to improve the combustion process of fossil fuels resulting in an ash by product with improved loss on ignition and lower carbon content. The improvement in the combustion thermal efficiency of the commercial 260 tons/h steam producing boiler has been calculated as per the direct calculation method of EN BS12952-15:2003 standard. © 2014 Elsevier Ltd. All rights reserved

Reactivity during bench-scale combustion of biomass fuels for carbon capture and storage applications

Reactivities of four biomass samples were investigated in four combustion atmospheres using non-isothermal thermogravimetric analysis (TGA) under two heating rates. The chosen combustion atmospheres reflect carbon capture and storage (CCS) applications and include O2O2 and CO2CO2-enrichment. Application of the Coats–Redfern method assessed changes in reactivity. Reactivity varied due to heating rate: the reactivity of char oxidation was lower at higher heating rates while devolatilisation reactions were less affected. In general, and particularly at the higher heating rate, increasing [O2O2] increased combustion reactivity. A lesser effect was observed when substituting N2N2 for CO2CO2 as the comburent; in unenriched conditions this tended to reduce char oxidation reactivity while in O2O2-enriched conditions the reactivity marginally increased. Combustion in a typical, dry oxyfuel environment (30% O2O2, 70% CO2CO2) was more reactive than in air in TGA experiments. These biomass results should interest researchers seeking to understand phenomena occurring in larger scale CCS-relevant experiments

NOx control in coal combustion by combining biomass co-firing, oxygen enrichment and SNCR

There has been renewed interest in evaluating NOx emission control by selective non-catalytic reduction (SNCR) combined with biomass co-firing to meet impending enforcement of NOx emission limits for power generation plant. Oxygen enrichment for the concentration of CO 2 in the flue gas has been observed in this work to have benefits for NOx emission control. This paper presents new information on the effect of combining biomass co-firing with SNCR under various oxygen enriched and air-staging conditions performed in the 20 kW combustion facility. Biomass has a higher tendency to generate CO and produced better reductions in NO x emission with and without using SNCR. NO reduction of around 80% were attained using SNCR for 15% and 50% blending ratios of biomasses at 21% overall O2 concentration for unstaged combustion. Whereas, a range of 40-80% NO reductions were attained for coal (Russian Coal) and 15% co-fired biomasses with 3.1-5.5% overall O2 concentration at 22-31% levels of flame staging. Moreover, it was found that better NOx removal efficiency was attained for higher NOx emission baselines under both oxygen enriched and normal firing conditions. However, SNCR NOx control for both coal or coal-biomass blends was observed to produce higher NOx reductions during O2 enrichment, believed to be due to the self-sustained NOx reduction reactions. Hence, NOx control by SNCR, oxygen enriched co-firing in power station boilers would result in lower NOx emissions and higher CO2 concentration for efficient scrubbing with better carbon burnouts. © 2012 Elsevier Ltd. All rights reserved

The use of kaolin and dolomite bed additives as an agglomeration mitigation method for wheat straw and miscanthus biomass fuels in a pilot-scale fluidized bed combustor

Renewable biomass fuels are frequently used for power generation. Biomass ash causes bed agglomeration in fluidized bed boilers due to the formation of alkali silicate melts. Very few prior studies have tested dolomite and kaolin bed additives for agglomeration mitigation with agricultural biomasses. In this work, pelletized miscanthus and wheat straw were tested in a pilot-scale 65kWth fluidized bed combustor with varying dosages of dolomite and kaolin on a silica sand bed. Neither additive improved defluidization time with wheat straw, whereas additive use at all dosages prevented bed defluidization with miscanthus. Agglomerates were studied through a novel, detailed SEM/EDX analysis across structural features. SEM/EDX analysis presented evidence of chemical reaction between both additives and fuels. Potassium in ash migrated into kaolin particle at depths of up to 60 μm. With dolomite, calcium and magnesium raised melt temperatures. Thermochemical modelling of the ash and additive combinations predicted that additive use would substantially reduce ash melt formation. It is proposed that the wheat straw pellet acted as a “ready-made” agglomerate structure due to release of molten ash to the pellet surface which bed material then sticks to, hence the lack of change to defluidization time regardless of additive use. Future studies into this behaviour would improve additive use

Agglomeration and the effect of process conditions on fluidized bed combustion of biomasses with olivine and silica sand as bed materials : pilot-scale investigation

Bubbling fluidized bed combustion of biomass has benefits of fuel flexibility, high combustion efficiency, and lower emissions. Bed agglomeration is where bed particles adhere together with alkali silicate melts and can lead to unscheduled plant shutdown. This pilot-scale study investigates performance and agglomeration when varying fuel (white wood, oat hull waste, miscanthus, wheat straw), bed height, bed material, and includes a novel spatial analysis of agglomerates from different bed locations. White wood was the best performing fuel and did not undergo bed defluidization due to its low ash content (0.5% mass as received), whereas wheat straw (6.67% mass as received ash) performed worst (defluidization times <25 min). Olivine was a superior bed material to silica sand, with 25%+ longer defluidization times with the worst performing fuel (wheat straw). Calcium-rich layers formed at olivine particle surfaces within wheat straw ash melts, and capillary action drew potassium silicate melt fractions into olivine particle fractures. An analysis of agglomerate samples from different bed spatial locations following tests with oat hull waste showed that ash layers on agglomerates retrieved further from the landing point of fuel onto the bed had reduced potassium and elevated calcium, likely due to reduced availability of fresh fuel ash for reaction with bed material

Comparing fuel additives for fireside corrosion inhibition in pulverised fuel boilers using thermodynamic modelling

This study presents a method for comparing corrosion inhibiting fuel additives using the thermodynamic modelling software FactSageTM. Two biomass ashes were investigated while using a range of loadings of three additives: Fe-based additive and two coal ashes, one with an 80% Si and Al contents and one with less than 50% of Si and Al contents but having a significant Ca content. The metrics used for analysis were the formation of various corrosive compounds and by-products. The Fe-based additive could inhibit the formation of corrosive species but not as well as either of the coal ashes, as it was key to increase the Si and Al content of the deposits. The coal ash with the greatest Si and Al content proved most capable of inhibiting fireside corrosion, while the contaminants present in the other coal ash proved detrimental to reducing the production of certain corrosive species. This method can be used as a qualitative predictive tool by research and development teams to quickly and economically understand the consequences of utilising fuel additives on ash chemistry or of changes in proportion/sources of fuel in a power plant

Detection of onset of agglomeration in a bubbling fluidized bed biomass combustor using reactive Eulerian computational fluid dynamics

The choice of a type of combustion technology to be used for heat or power generation depends on economic, technical, operational and fuel availability constraints. The benefits associated with the evolving market driven by the fluidised bed combustion (FBC) technology cannot be overlooked especially when gauged at 65 GWth of worldwide installed capacity alongside added benefits of handling fuel variation, low pollutant emissions and high combustion efficiency. Biomass or biomass waste will continue to have a vital role to play in the future FBC technology-based power generation. Biomass often contains high levels of inorganic species that can form sticky agglomerates posing a significant risk to boiler operation resulting in unscheduled outages. This added complexity of the behaviour of the fuel and bed material mix highlights the requirement for simulation models to identify agglomeration to help improve the overall performance and reliability of FBC technology. To resolve this problem, this research devised a simulation strategy for the detection of agglomeration using the Eulerian–Eulerian approach. The developed modelling strategy is validated with the experimental data available in literature for two-dimensional simplified geometry of a pilot-scale fluidised bed combustor. The model results were found promising and robust to predict bed defluidisation times and other parameters consistent with the experimental data