Co-combustion of pulverized coal and solid recovered fuel in an entrained flow reactor - General combustion and ash behaviour

Co-combustion of a bituminous coal and a solid recovered fuel (SRF) was carried out in an entrained flow reactor, and the influence of additives such as NaCl, PVC, ammonium sulphate, and kaolinite on co-combustion was investigated. The co-combustion experiments were carried out with SRF shares of 7.9 wt.%, 14.8 wt.% and 25 wt.%, respectively. The effect of additives was evaluated by maintaining the share of secondary fuel (mixture of SRF and additive) at 14.8 wt.%. The experimental results showed that the fuel burnout, NO and SO₂ emission in co-combustion of coal and SRF were decreased with increasing share of SRF. The majority of the additives inhibited the burnout, except for NaCl which seemed to have a promoting effect. The impact of additives on NO emission was mostly insignificant, except for ammonium sulphate which greatly reduced the NO emission. For SO₂ emission, it was found that all of the additives increased the S-retention in ash. Analysis of the bulk composition of fly ash from different experiments indicated that the majority of S and Cl in the fuels were released to gas phase during combustion, whereas the K and Na in the fuels were mainly retained in ash. When co-firing coal and SRF, approximately 99 wt.% of the K and Na in fly ash was present in water insoluble form such as aluminosilicates or silicates. The addition of NaCl, PVC, and ammonium sulphate generally promoted the vaporization of Na and K, resulting in an increased formation of water soluble alkalis such as alkali chlorides or sulphates. The vaporization degree of Na and K was found to be correlated during the experiments, suggesting an interaction between the vaporization of Na and K during pulverized fuel combustion. By collecting deposits on an air-cooled probe during the experiments, it was found that the ash deposition propensity in co-combustion was decreased with increasing share of SRF. The addition of NaCl and PVC significantly increased the ash deposition propensity, whereas the addition of ammonium sulphate or kaolinite showed a slight reducing effect. The chlorine content in the deposits generally implied a low corrosion potential during co-combustion of coal and SRF, except for the experiments with NaCl or PVC addition.     

硫酸铵和尿素抑制飞灰合成二(口恶)英

二(口恶)英(PCDD/Fs)排放制约了当前焚烧技术在垃圾处置和资源化利用领域的应用和推广。研究了硫酸铵和尿素在不同温度下对垃圾焚烧飞灰合成PCDD/Fs的抑制能力。实验结果表明:高温下(650℃)硫酸铵和尿素对PCDD/Fs合成具有更高的抑制效率,但高温下飞灰合成PCDD/Fs量远低于低温下(350℃)的生成量;PCDD/Fs的降低量均主要来自于PCDFs的减少,并且低温下TeCDF减少量最大,而高温下减少最多的同系物为OCDF。由于实际烟道中,高温处投入的抑制剂会进入低温段继续产生抑制作用,所以仍需要更多的研究正确评价抑制剂的抑制效果和最优使用温度。     

烧结过程持久性有机污染物生成机理研究进展

烧结过程持久性有机污染物问题凸显,已然成为钢铁环保领域的重要研究内容。本文从二英类基本性质入手,简述了烧结工艺过程基本特征,并介绍了烧结过程二英类污染物的排放水平和分布,指出烧结过程二英类污染物的治理关键环节在于烟气。其后,重点论述了烧结过程二英类的生成机理及影响因素,指出二英类主要生成空间位于烧结床内部,从温度、碳源、氯源、氧、催化剂等方面进行深入分析和比较,为烧结过程持久性有机污染物相关研究提供理论基础。在此基础上,对目前二英类污染物源头减排技术进行梳理,提出对于烧结过程复杂工况下抑制剂的控污机理和烧结矿性能影响等相关问题仍有待深入研究,并提出烧结过程二英类生成抑制技术将成为重要研究方向。     

Influences of coal size and coal-feeding location in co-firing with rice husks on performance of a short-combustion-chamber fluidized-bed combustor (SFBC)

This study examined the combustion characteristics and performance of rice husks co-fired with coal in a short-combustion-chamber fluidized-bed combustor (SFBC) with a 225 kW_th capacity. Rice husks were the main fuel, and coal was a supplementary fuel in the experiments. The effects of coal size (< 5 mm and 5-10 mm) and coal-feed location (above or below a recirculating ring) on combustion performance were investigated. Various co-combustion tests of rice husks with coal were performed, with different thermal percentages (10, 15, 20, and 25%) of coal. The results were compared to firing 100% rice husks alone. With the assistance of a stirring blade and a recirculating ring, good combustion was feasible without using any inert materials mixed into the bed.Combustion efficiency in an excess of 98% was readily achievable. CO and SO₂ emissions (at 6% O₂) were in the range 64-104 and 10-22 ppm, respectively, while NO_x emissions were in the range of 208-281 ppm. Although the CO and SO₂ emissions were acceptable, combustion of 100% rice husks and co-combustion with < 20% coal failed to comply with Thai NO_x emission limits. Therefore, to minimize NO_x emissions (208-244 ppm, at 6% O₂), coal of both sizes was introduced below the recirculating ring. The results demonstrated that the thermal percentage of coal in the fuel mixture should be 20-25%.     

Effects of operational parameters on emission performance and combustion efficiency in small-scale CFBCs

A well-designed CFBC can burn coal with high efficiency and within acceptable levels of gaseous emissions. In this theoretical study effects of operational parameters on combustion efficiency and the pollutants emitted have been estimated using a developed dynamic 2D (two-dimensional) model for CFBCs. Model simulations have been carried out to examine the effect of different operational parameters such as excess air and gas inlet pressure and coal particle size on bed temperature, the overall CO, NO_x and SO₂ emissions and combustion efficiency from a small-scale CFBC. It has been observed that increasing excess air ratio causes fluidized bed temperature decrease and CO emission increase. Coal particle size has more significant effect on CO emissions than the gas inlet pressure at the entrance to fluidized bed. Increasing excess air ratio leads to decreasing SO₂ and NO_x emissions. The gas inlet pressure at the entrance to fluidized bed has a more significant effect on NO_x emission than the coal particle size. Increasing excess air causes decreasing combustion efficiency. The gas inlet pressure has more pronounced effect on combustion efficiency than the coal particle size, particularly at higher excess air ratios. The developed model is also validated in terms of combustion efficiency with experimental literature data obtained from 300 kW laboratory scale test unit. The present theoretical study also confirms that CFB combustion allows clean and efficient combustion of coal.     

Emissions monitoring during coal waste wood co-combustion in an industrial steam boiler

Co-combustion tests were performed in a 13.8 MW_th industrial steam boiler, using Greek lignite from Ptolemais reserve, natural waste wood, MDF residues and power poles. Fuel blends were prepared by mixing single waste wood components with lignite in various concentrations. Oxygen concentration and emissions of CO, SO₂ and NO_x were continuously monitored, during the co-combustion tests. Gaseous and particulate samples were collected and analysed for heavy metals, dioxins and furans according to standard methods. The results showed that co-combustion is technically feasible provided that agglomeration problems could be confronted. Low emissions of toxic pollutants were obtained during the co-combustion tests, below the legislative limit values. The lowest values of dioxins and furans were observed during combustion of fuel blends containing MDF, possibly due to inhibition by some nitrogenous components in MDF. No direct correlation was found between emitted PCDD/F and metal compounds, especially copper. Among the measured metals in the flue gases, zinc was the most prominent, while iron was mainly observed in the solid ash samples.     

Biomass-coal co-combustion: opportunity for affordable renewable energy

This investigation explores the reasons for and technical challenges associated with co-combustion of biomass and coal in boilers designed for coal (mainly pulverized coal) combustion. Biomass-coal co-combustion represents a near-term, low-risk, low-cost, sustainable, renewable energy option that promises reduction in net CO₂ emissions, reduction in SO_x and often NO_x emissions, and several societal benefits. Technical issues associated with cofiring include fuel supply, handling and storage challenges, potential increases in corrosion, decreases in overall efficiency, ash deposition issues, pollutant emissions, carbon burnout, impacts on ash marketing, impacts on SCR performance, and overall economics. Each of these issues has been investigated and this presentation summarizes the state-of-the-art in each area, both in the US and abroad. The focus is on fireside issues. While each of the issues can be significant, the conclusion is that biomass residues represent possibly the best (cheapest and lowest risk) renewable energy option for many power producers.     

Metal emissions from co-combustion of sewage sludge and coal/wood in fluidized bed

Co-combustion of sewage sludge with coal or wood has been investigated in the 12 MW_th circulating fluidized bed (CFB) boiler at Chalmers Technical University. The investigation focuses on emissions of trace metals from co-combustion compared to mono-combustion in CFB. The results show that co-combustion can be carried out in a CFB plant designed for the base fuel without exceeding EU emission limits for practically interesting sludge energy fractions. This was also the case for the mercury emission. In general, the ashes are enriched by trace elements with increasing share of sludge, especially in the case of wood when the ashes are dominated by the sludge properties also at small fractions of sludge addition. In the present tests the trace metals were trapped in the fly ashes regardless of base fuel.     

二英生成机理及减排方法研究进展

二 英作为一种具有剧毒并且对生态环境和人类健康有着巨大危害的持久性有机污染物,引起了国际社会的广泛关注,为了减少二 英对生态环境以及人类健康产生的潜在威胁,世界各国的学者对二 英理化性质以及生成机理进行了深入的研究。本文详细介绍了二 英相关的理化性质以及生成二 英的3种机理,包括高温气相机理、前体合成机理以及从头合成机理。高温气相机理属于高温同相合成反应,需在相对较高的温度下进行,合成量很小;前体合成机理和从头合成机理均属于低温异相催化合成反应,且在有飞灰的情况下,在较低的温度下就能合成大量的二 英,不同的燃烧状态对这两种合成机理有很大的影响; 而根据二 英的生成机理采取有效的过程控制方法可以极大地减少二 英的生成量,本文为抑制二 英的合成介绍了几种可行的过程控制方法。     

Co-combustion of peach and apricot stone with coal in a bubbling fluidized bed

In this study a bubbling fluidized bed combustor (BFBC) having an inside diameter of 102 mm and a height of 900 mm was used to investigate the co-combustion characteristics of peach and apricot stones produced as a waste from the fruit juice industry with coal. A lignite coal was used for co-combustion. On-line concentrations of O₂, CO, CO₂, SO₂, NO_X and total hydrocarbons (C_mH_n) were measured in the flue gas during combustion experiments. Variations of emissions of various pollutants were studied by changing the operating parameters (excess air ratio, fluidization velocity, and fuel feed rate). Temperature distribution along the bed was measured with thermocouples.     

Co-combustion of coal and biomass in a circulating fluidized bed combustor

In this research, co-combustion of coal and rice husk was studied in a circulating fluidized bed combustor (CFBC). The effects of mixed fuel ratios, primary air and secondary air flow rates on temperature and gas concentration profiles along riser (0.1 m inside diameter and 3.0 m height) were studied. The average particle size of coal from Maetah used in this work was 1,128 mm and bed material was sand. The range of primary air flow rates was 480–920 l/min corresponding to U _g of 1.0–2.0 m/s for coal feed rate at 5.8 kg/h. The recirculation rate through L-valve was 100 kg/hr. It was found that the temperatures along the riser were rather steady at about 800–1,000 degrees Celsius. The introduction of secondary air improved combustion and temperature gradient at the bottom of the riser, particularly at a primary air flow rate below 1.5 m/s. Blending of coal with biomass, rice husk, did improve the combustion efficiency of coal itself even at low concentration of rice husk of 3.5 wt%. In addition, the presence of rice husk in the feed stocks reduced the emission of both NO _x and SO₂.     

The mechanism of the formation of soot and other pollutants during the co-firing of coal and pine wood in a fixed bed combustor

The co-firing of coal and biomass reduces the emission of pollutants by a mechanism which has been extensively studied but is still uncertain. Emissions were collected during the combustion in a fixed-bed furnace of Polish bituminous coal and pine wood, both individually and together, and it was observed that biomass produced less soot and burned at a lower temperature. Complementary analytical-scale combustion and pyrolysis experiments were carried out. The results of the analysis of emissions and reaction products, mainly by gas chromatography–mass spectrometry (GC–MS), but for large molecules by size exclusion chromatography (SEC), were interpreted so as to construct a reaction scheme for pollutant formation during co-firing. Evidence for three main routes to pollutant formation during co-combustion was adduced. Firstly, the presence of high MW material (from SEC) indicates escape of devolatilisation products round the outside of the flame. Secondly, the emission factors (ef) of PAH, alkyl-PAH, oxygen-containing PAH (O-PAC) and phenols are consistent with partial pyrolysis, while the high concentrations of the two-ring (naphthalenes and indene) PAH precursors evidently arise through radical reactions involving cyclopentadiene intermediates from phenols generated by pyrolysis of both coal and of biomass lignin. Thirdly, the concentrations of larger PAH are consistent with contributions from a hydrogen abstraction carbon addition (HACA) mechanism in which acetylene or butadiene formed in the flame are added to smaller PAH radicals. A kinetic model was applied to coal, biomass and coal/biomass co-combustion and highlighted the role of HACA in soot production during biomass combustion, but this route to soot was insufficient to model the higher yields of soot observed during coal combustion. In this latter case radical reactions involving either cyclopentadiene or condensation reactions of smaller PAH molecules initially formed in the pyrolysis stage to give ‘aromers’ are more important.     

Investigation of co-combustion of coal and olive cake in a bubbling fluidized bed with secondary air injection

In this study, a bubbling fluidized bed of 102 mm inside diameter and 900 mm height was used to burn olive cake and coal mixtures. Tunçbilek lignite coal was used together with olive cake for the co-combustion tests. Combustion performances and emission characteristics of olive cake and coal mixtures were investigated. Various co-combustion tests of coal with olive cake were conducted with mixing ratios of 25%, 50%, and 75% of olive cake by weight in the mixture. Operational parameters (excess air ratio, secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The results were compared with that of the combustion of olive cake and coal. Flue gas concentrations of O₂, CO, SO₂, NO_x, and total hydrocarbons (C_mH_n) were measured during combustion tests.For the setup used in this study, the optimum operating conditions with respect to NO_x and SO₂ emissions were found to be 1.35 for excess air ratio, and 30 L/min for secondary air flowrate for the combustion of 75 wt% olive cake and 25 wt% coal mixture. The highest combustion efficiency of 99.8% was obtained with an excess air ratio of 1.7, secondary air flow rate of 40 L/min for the combustion of 25 wt% olive cake and 75 wt% coal mixture.     

Circulating fluidised bed co-combustion of coal and biomass

Circulating fluidised bed combustion (CFBC) is receiving wide research attention in view its potential as an economic and environmentally acceptable technology for burning low-grade coals, biomass and organic wastes, and thereby mixtures of them. Designs of the existing fluidised bed boilers for biomass combustion are mainly based on experience from coal combustion because the mechanism of combustion of biomass in fluidised beds is still not well understood. A good understanding of the combustion and pollutant formation processes and the modelling of the combustor can greatly avoid costly upsets of the plants.In this paper, the performance of CFBC burning coal and biomass mixtures was analysed. Experimental results were obtained from the combustion of two kinds of coal with a forest residue (Pine bark) in two CFB pilot plants (0.1 and 0.3 MW_th). The effect of the main operating conditions on carbon combustion efficiency was analysed. Moreover, a mathematical model to predict the behaviour of the co-combustion of coal and biomass wastes in CFB boilers has been developed and validated. The developed model can predict the different gas concentrations along the riser (O₂, CO, CH₄, etc.), and the carbon combustion efficiency. The experimental results of carbon combustion efficiencies were compared with those predicted by the model and a good correlation was found for all the conditions used.     

Prediction of trace element volatility during co-combustion processes

The behaviour of some selected trace metals (Hg, Cd, As, Pb, Sb, Cr, Co, Cu, Mn, Ni and V) during co-combustion processes of bio-waste materials (sewage sludge, waste wood, refused derived fuel) and coal has been predicted by thermodynamic equilibrium calculations using the HSC-Chemistry 4.0 software. The influence of temperature, flue gas composition, trace element concentration and minor fly ash components on equilibrium composition was evaluated. For most of the elements, an increase in the HCl concentration favours the formation of gaseous species while increasing concentration of SO₂ in the gas composition enhances the formation of condensed species. Trace element interactions with minor fly ash components were predicted. From results obtained in this study it may be concluded that, from a thermodynamical point of view, the addition of a secondary fuel in combustion processes does not produce an increase in trace element emissions to the environment. Generally, trace elements are captured in ashes avoiding that these elements reach the stack.     

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.     

A Techno-economic assessment of the reduction of carbon dioxide emissions through the use of biomass co-combustion

Using sustainably-grown biomass as the sole fuel, or co-fired with coal, is an effective way of reducing the net CO₂ emissions from a combustion power plant. There may be a reduction in efficiency from the use of biomass, mainly as a result of its relatively high moisture content, and the system economics may also be adversely affected.The economic cost of reducing CO₂ emissions through the replacement of coal with biomass can be identified by analysing the system when fuelled solely by biomass, solely by coal and when a coal-biomass mixture is used.The technical feasibility of burning biomass or certain wastes with pulverised coal in utility boilers has been well established. Cofiring had also been found to have little effect on efficiency or flame stability, and pilot plant studies had shown that cofiring could reduce NO_x and SO_x emissions.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, (i) cofiring 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 cofiring coal with sewage sludge; (c) 250 and 125 MWe circulating fluidised bed combustion (CFBC) plants cofiring coal with straw and sewage sludge; (d) 25 MWe CFBC systems cofiring low and high sulphur content coal with straw, wood and woody matter pressed from olive stones (WPOS); and (e) 12 MWe CFBC cofiring low and high sulphur content coal with straw.The technical, environmental and economic analysis of such 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. The capital costs of systems are estimated for coal-firing and also any additional costs introduced when biomass is used. The Break-even electricity selling price is calculated for each technology, taking into account the system scale and fuel used.Since net CO₂ emissions are reduced when biomass is used, the effect of the use of biomass on the electricity selling price can be found and the premium required for emissions reduction assessed. Consideration is also given to the level of subvention required, either as a Carbon dioxide Credit or as a Renewable Credit, to make the systems using biomass competitive with those fuelled only with coal.It would appear that a Renewable Credit (RC) is a more transparent and cost-effective mechanism to support the use of biomass in such power plants than a Carbon dioxide Credit (CC).     

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.     

Co-combustion of Korean anthracite with bituminous coal in two circulating fluidized bed combustors

The co-combustion characteristics for Korean anthracites and bituminous coals were determined in a lab-scale CFB reactor and the commercial scale Tonghae CFB Power Plant. In the lab-scale CFB combustion tests, the effluent rate of the emission gases, which can indicate the reactivity of the combustion, did not change appreciably when each coal burned. As the bituminous coal was added, however, the effluent rate of the emissions increased. The amount of the unburned carbon in ash decreased with increasing the ratio of the bituminous coal during the co-combustion. When the co-combustion was tested in the Tonghae CFB power plant, the temperatures at the upper part of the combustor and the cyclones, which were somewhat higher than designed and expected, could be reduced as the bituminous coal ratio increased. Consequently, more stable operation of the CFB boiler was achieved. The efficiency of the CFB boiler also increased due to increasing the reactivity of the combustion. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Measurement and prediction of the emission of pollutants from the combustion of coal and biomass in a fixed bed furnace

The effect of co-combustion of coal and biomass has been studied for a fixed bed appliance originally designed for coal and in widespread use in many parts of the world especially Eastern Europe. Organic, inorganic and gaseous emissions have been measured. Organic compounds have been determined for a range of fuel combinations. These include polycyclic aromatic hydrocarbons PAH, alkyl PAH, a range of oxygenated compounds (including phenols, aldehydes and ketones, oxygenated polycyclic aromatic compounds (O-PAC) and dioxins), polycyclic aromatic sulphur hydrocarbons (PASH), nitrogenated polycyclic aromatic compounds (N-PAC) and common volatile organic compounds (VOC). Inorganic species include trace heavy metals, as well as the gases, CO, CO₂, SO_x and NO_x. The concentration of the trace metals in the ash and fly ash have been compared to equilibrium calculations of the emission profiles during co-combustion.