Co-pyrolysis of biomass and coal in a free fall reactor

An experimental study on co-pyrolysis of biomass and coal was performed in a free fall reactor under atmospheric pressure with nitrogen as balance gas. The coal sample selected was Dayan lignite, while the biomass used was legume straw. The operation temperature was over a range of 500-700 °C, and the blending ratio of biomass in mixtures was varied between 0 and 100 wt.%. The results indicated that there exist synergetic effects in the co-pyrolysis of biomass and coal. Under the higher blending ratio conditions, the char yields are lower than the theoretical values calculated on pyrolysis of each individual fuel, and consequently the liquid yields are higher. Moreover, the experimental results showed that the compositions of the gaseous products from blended samples are not all in accordance with those of their parent fuels. The CO₂ reactivities of the chars obtained from the co-pyrolysis under the higher blending ratio (around 70 wt.%) conditions are about twice as high as those of coal char alone, even higher than those of biomass alone.     

TGA and macro-TGA characterisation of biomass fuels and fuel mixtures

The thermal behaviour of selected biomass fuels and mixtures as wood, demolition wood, coffee waste and glossy paper was investigated using a thermogravimetric analyzer (TGA) and a macro-thermobalance (macro-TGA). A kinetic model, involving first-order independent parallel reactions, was applied to results obtained from pyrolysis TGA experiments. The pyrolysis rate was considered as the sum of the main biomass pseudo-components, namely cellulose, hemicellulose and lignin. Additionally, the thermal behaviour of the same fuels was investigated at combustion conditions in the TGA, including ignition behaviour. The thermogravimetric analysis showed that each single fuel had pyrolysis and combustion characteristics based on its own main pseudo-components (hemicellulose, cellulose and lignin). The pyrolysis and combustion characteristics of selected fuel mixtures and the gas composition analysis from macro-TGA experiments showed respectively quantitative and qualitative summative behaviour based on the single fuels.     

Pyrolysis of mixtures based on the oil shale and brown coal of Belarus and the characterization of the resulting fuel products

A comparative analysis of the quality of pyrolysis products obtained from oil shale, brown coal, and their mixtures showed that the combined processing of mixed fuels is promising because it provides an opportunity to expand the domestic raw materials base of solid fuels.     

Use of TMA to predict deposition behaviour of biomass fuels

Deposits formation on boiler heat transfer surfaces is one of the main operational problems associated to biomass co-combustion. This paper proposes a novel method to rank deposition propensity of biomass fuels. The method combines thermo-mechanical analysis (TMA) with ash leaching procedures and is applied here to a set of biomass fuels. Based on the TMA penetration and shrinkage traces obtained for the original and the leached ashes, the fuels are classified regarding their deposition tendencies. The results have been successfully validated with chemical analysis of the liquid fraction from ash leaching. Initial results for the behaviour of coal-biomass mixtures are also presented.     

Pyrolysis kinetics and combustion characteristics of waste recovered fuels

Alternative fuels, such as biomass and refuse derived fuels tend to play an increasingly important role in the European energy industry. Co-firing fuels derived from non-hazardous waste streams have the potential of covering a significant part of the future demand on co-incineration capacities, which is expected to increase due to the implementation of the 2000/76 EC landfill Directive. However, their combustion behaviour has not yet been fully investigated, because of the difficulty to define representative fuel characteristics simulating accurately all the fuel fractions. In the present study, refuse derived fuel behaviour was investigated by thermogravimetry under pyrolysis and combustion conditions. A non-isothermal thermogravimetric analyser (TA Q600) operated at ambient pressure was used for both the pyrolysis and combustion experiments. The devolatilisation of the waste samples was investigated at a temperature range of 30-1000 °C with the constant heating rate of 20 °C/min and for particle sizes between 150 and 250 μm. Combustion tests were realized under the same heating conditions. The independent parallel, first order, reactions model was elaborated for the kinetic analysis of the pyrolysis results. The thermal degradation of the refuse derived fuel samples was modeled assuming four parallel reactions corresponding to the devolatilisation of cellulose, hemicellulose, lignin and plastics. Increased activation energies were calculated for the plastics fraction. Lignin presented the lowest contribution in the pyrolysis of the samples. Slightly increased combustion reactivities were found for the waste fuel samples compared to lignite. It is concluded that waste recovered fuels can be used in existing combustion facilities either alone or in combination with coal and future investigations should focus on the operational behaviour of large-scale facilities when exploiting these waste species.     

Possibility of using paper sludge in co-firing applications

The possibility of co-processing paper sludge with coal in power plants for power production and useful products was investigated as an alternative to the disposal option. The thermal behaviour of the fuels and their blend during pyrolysis and combustion processes was studied, kinetic models were developed and the compatibility of each component in the blend was evaluated. The experiments were conducted in a thermogravimetric analysis system, at non-isothermal heating conditions, over the temperature range 25-850 °C. The effect of the inorganic constituents of the fuels and their mixture on thermal conversion characteristics, reactivity, slagging and fouling propensities and environmental pollution was examined.The thermochemical reactivity of the two fuels was different in both nitrogen and air. Devolatilization of paper sludge occurred earlier and with a higher rate, while its combustion was hindered by the high content of ash. When the two fuels were mixed their pyrolysis or combustion reactivities did not substantially change. A first-order parallel reactions model for pyrolysis and a power low model for combustion fitted the experimental results accurately. The kinetic parameters of the blend could be predicted from the data of the individual components. Co-firing paper sludge with subbituminous coal might somehow improve the slagging/fouling potential of the coal. However, if the mineral matter of paper sludge is partly removed before use, then the combustion behaviour of the mixture could resemble that of coal alone and the overall efficiency of the process would increase.     

The low-shear rheology and sedimentation stability of coal—oil dispersions

Stable coal—oil mixtures can be prepared by grinding coal particles in fuel oil. These products have been prepared by the British Petroleum Company plc and are referred to as Coal—Oil Dispersions (COD).

One of the major problems associated with the production of COD is the rapid assessment of the length of time the coal particlesa are likely to remain in suspension under a particular set of storage conditions. This paper describes a number of measurements of the low-shear rheology and sedimentation stability of a series of CODs prepared by grinding two types of coal in two different fuel oils.

The results suggest that two types of COD are possible. One type exhibits complex rheological properties at low shear rates and does not produce a coal sediment, even after prolonged storage at 80 °C under dynamic conditions. The other exhibits near Newtonian behaviour and appears to form a sedimented layer of coal during storage.     

Co-combustion of coal and olive oil industry residues in fluidised bed

Recently new environmental regulations of fossil fuels have further increased interest in the use of waste and biomass for energy generation. Co-combustion is generally viewed as the most cost-effective approach to biomass and wastes utilisation by the electric utility industry.The aim of this paper is to assess the feasibility of co-firing coal and a very specific biomass waste from the olive oil industry: foot cake, in a fluidised bed. This waste is quite difficult material to be used in combustion process, due to its high moisture content and alkaline content in ashes.Two different Spanish coals were selected for this study: a lignite and an anthracite. The combustion tests were carried out in the CIEMAT bubbling fluidised bed pilot plant. In order to study the effect of different parameters on the emissions and combustion efficiency, the tests were done using different operating conditions: furnace temperature, share of foot cake in the mixtures and coal type.The pilot plant tests show that the combustion of foot cake/lignite or anthracite mixtures in bubbling fluidised bed is one way to utilise this biomass residue in energy generation. The presence of foot cake in the mixtures has not any significant effect on the combustion efficiency. SO₂ and NO_x emissions decrease when the amount of foot cake in the mixtures increases, while N₂O emission increases.     

An investigation of the grindability of two torrefied energy crops

The process of torrefaction alters the physical properties of biomass, reducing its fibrous tenacious nature. This could allow increased rates of co-milling and therefore co-firing in coal fired power stations, which in turn would enable a reduction in the amount of coal used and an increase in the use of sustainable fuels, without the need for additional plant. This paper presents an experimental investigation of the pulverisation behaviour of two torrefied energy crops, namely: willow and Miscanthus. A multifactorial method approach was adopted to investigate the three process parameters of temperature, residence time and particle size, producing fuels treated using four different torrefaction conditions. The untreated and torrefied fuels were subjected to standard fuel analysis techniques including ultimate analysis, proximate analysis and calorific value determination. The grindability of these fuels was then determined using a laboratory ball mill and by adapting the Hardgrove Grindability Index (HGI) test for hard coals. After grinding, two sets of results were obtained. Firstly a determination similar to the HGI test was made, measuring the proportion of sample passing through a 75 μm sieve and plotting this on a calibrated HGI chart determined using four standard reference coals of known HGI values. Secondly the particle size distributions of the entire ground sample were measured and compared with the four standard reference coals. The standard fuel tests revealed that temperature was the most significant parameter in terms of mass loss, changes in elemental composition and energy content increase. The first grindability test results found that the untreated fuels and fuels treated at low temperatures showed very poor grindability behaviour. However, more severe torrefaction conditions caused the fuels to exhibit similar pulverisation properties as coals with low HGI values. Miscanthus was found to have a higher HGI value than willow. On examining the particle size distributions it was found that the particle size distributions of torrefied Miscanthus differed significantly from the untreated biomass and had comparable profiles to those of the standard reference coals with which they had similar HGI values. However, only the torrefied willow produced at the most severe conditions investigated exhibited this behaviour, and the HGI of torrefied willow was not generally a reliable indicator of grindability performance for this energy crop. Overall it was concluded that torrefied biomass can be successfully pulverised and that torrefied Miscanthus was easier to grind than torrefied willow.     

Gasification and co-gasification of biomass wastes: Effect of the biomass origin and the gasifier operating conditions

Air gasification of different biomass fuels, including forestry (pinus pinaster pruning) and agricultural (grapevine and olive tree pruning) wastes as well as industry wastes (sawdust and marc of grape), has been carried out in a circulating flow gasifier in order to evaluate the potential of using these types of biomass in the same equipment, thus providing higher operation flexibility and minimizing the effect of seasonal fuel supply variations. The potential of using biomass as an additional supporting fuel in coal fuelled power plants has also been evaluated through tests involving mixtures of biomass and coal–coke, the coke being a typical waste of oil companies. The effect of the main gasifier operating conditions, such as the relative biomass/air ratio and the reaction temperature, has been analysed to establish the conditions allowing higher gasification efficiency, carbon conversion and/or fuel constituents (CO, H₂ and CH₄) concentration and production. Results of the work encourage the combined use of the different biomass fuels without significant modifications in the installation, although agricultural wastes (grapevine and olive pruning) could to lead to more efficient gasification processes. These latter wastes appear as interesting fuels to generate a producer gas to be used in internal combustion engines or gas turbines (high gasification efficiency and gas yield), while sawdust could be a very adequate fuel to produce a H₂-rich gas (with interest for fuel cells) due to its highest reactivity. The influence of the reaction temperature on the gasification characteristics was not as significant as that of the biomass/air ratio, although the H₂ concentration increased with increasing temperature.     

Heating value of biomass and biomass pyrolysis products

Studies conducted on the heating value of various types of biomass components and their pyrolysis products such as char, liquids and gases are presented. Heating values of chars are comparable with those of lignite and coke; heating values of liquids are comparable with those of oxygenated fuels such as methanol and ethanol, which are much lower than those of petroleum fuels. Heating values of gases are comparable with those of producer gas or coal gas and are much lower than that of natural gas. It is also found that the heating values of products are functions of the initial composition of biomass; correlations are developed to express these. Also, correlations are developed which explain the influence of ash elements on heating values of the pyrolysis products and on percentage distribution of energy in the products.     

Coprocessing of a Turkish lignite with a cellulosic waste material: 2. The effect of coprocessing on liquefaction yields at different reaction pressures and sawdust/lignite ratios

Most of the research works done for alternative energy sources have shown that, in general, coprocessing of coal with biomass-type wastes has a positive effect on the liquefaction yields and these materials are increasingly studied as coliquefaction agents for the conversion of coal to liquid fuels. Addition of biomass waste materials to coal is known to be synergetic in that it improves the yields and quality of liquid products produced from coal under relatively mild conditions of temperature and pressure. This paper reports the coprocessing of a Turkish lignite with sawdust in the category of biomass-type waste material. The experiments have been conducted in a stainless-steel reactor, and temperature and tetralin/(lignite+sawdust) ratio were kept constant at 350 °C and 3:1 (vol/wt), respectively. This is the first time that the influence of reaction pressures on coliquefaction yields was investigated. In addition, the influence of the sawdust/lignite ratios on coprocessing conversion and product distribution was also investigated under the same reaction conditions. The runs were carried out at 10, 25, 40, 55, and 70 atm initial cold hydrogen pressure values and at 0.5:1, 0.75:1, 1:1, 1.25:1, and 1.5:1 sawdust/lignite (wt/wt) ratio values.     

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment

Three kinds of subbituminous and bituminous coals with added potassium hydroxide were heated at 523 K in a nitrogen stream to transform thermally stable organic sulfur to reactive species. Extents of total sulfur removal were 27–52% during the course of alkali treatment, while weight loss was 8–13%. The extent of total sulfur removal was linearly proportional to the internal surface area of the parent coal. The parent coals and alkali-treated samples were pyrolysed rapidly in a free-fall reactor in a nitrogen stream at 1233 K. Under these conditions the alkali-treated samples lost more organic sulfur than did the parent coals. The observed changes in the content of sulfur forms were successfully simulated kinetically. The combined process of rapid pyrolysis with alkali leaching was effective for reduction of organic sulfur, except for a high-rank coal of small internal surface area.     

Some recent advances in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal

The brown coal in the Latrobe Valley, Victoria, Australia, has many unique structural features and properties. The brown coal has a very low ash yield and contains highly dispersed alkali and alkaline earth metallic (AAEM) species, either as carboxylates forming part of its organic matter or as NaCl dissolved in its moisture. Owing to its unique structural features and properties, the brown coal behaves very differently from many other solid fuels such as biomass, bituminous coals and anthracites. For example, the highly reactive nature of its volatiles, the vulnerable nature of its nascent char and the presence of finely distributed AAEM species mean that the volatile–char interactions, a common phenomenon in all gasifiers, especially in the fluidised-bed gasifiers, would influence almost every aspect of its pyrolysis and gasification behaviour. Some recent progress in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal will be reviewed in this paper. After a brief account of the effects of AAEM species on the pyrolysis yields, the factors influencing the volatilisation of AAEM species will be summarised. This will be followed by the discussion of the factors influencing the reactivity of brown coal char and the catalytic reforming/cleaning of volatiles and gasification products by char-supported catalysts. The effects of dewatering/drying on the pyrolysis behaviour of Victorian brown coal and the conversion of pollutant-forming elements will be mentioned briefly. The progress in the fundamental understanding of the pyrolysis and gasification behaviour of Victorian brown coal has laid solid foundation for the further development of advanced gasification technologies for the clean and efficient utilisation of this cheap but important resource.     

Flow properties of biomass and coal blends

Co-firing of coal/biomass blends in the existing coal fired power plants is an attractive option for reducing the greenhouse emissions. However, fuel processing and handling problems associated with coal/biomass blends restrict the widespread application of the co-firing technology. In this study, flow properties of typical Australian coal and biomass as well as their blends were systematically studied. The flow property data obtained from this study provided an insight into the underlying phenomena responsible for some of the problems often encountered in handling of coal/biomass blends. The flow properties of the coal and biomass blends were found to be dependent upon the form of biomass being used. We found that blending coal with sawdust reduced the likelihood of flow stoppage because sawdust particles lowered the bulk strength (cohesive strength) of the mixture from that of coal alone while maintaining more or less the same frictional properties as the parent coal. On the contrary, blends of coal and woodchip exhibited frictional characteristics far greater than the parent coal while showing bulk strengths similar to coal. As such, blends of woodchips and coal were found to be more susceptive to flow stoppage.     

Study on ash deposition under oxyfuel combustion of coal/biomass blends

Combustion in an O₂/CO₂ mixture (oxyfuel) has been recognized as a promising technology for CO₂ capture as it produces a high CO₂ concentration flue gas. Furthermore, biofuels in general contribute to CO₂ reduction in comparison with fossil fuels as they are considered CO₂ neutral. Ash formation and deposition (surface fouling) behavior of coal/biomass blends under O₂/CO₂ combustion conditions is still not extensively studied. Aim of this work is the comparative study of ash formation and deposition of selected coal/biomass blends under oxyfuel and air conditions in a lab scale pulverized coal combustor (drop tube). The fuels used were Russian and South African coals and their blends with Shea meal (cocoa). A horizontal deposition probe, equipped with thermocouples and heat transfer sensors for on line data acquisition, was placed at a fixed distance from the burner in order to simulate the ash deposition on heat transfer surfaces (e.g. water or steam tubes). Furthermore, a cascade impactor (staged filter) was used to obtain size distributed ash samples including the submicron range at the reactor exit. The deposition ratio and propensity measured for the various experimental conditions were higher in all oxyfuel cases. The SEM/EDS and ICP analyses of the deposit and cascade impactor ash samples indicate K interactions with the alumina silicates and to a smaller extend with Cl, which was all released in the gas phase, in both the oxyfuel and air combustion samples. Sulfur was depleted in both the air or oxyfuel ash deposits. S and K enrichment was detected in the fine ash stages, slightly increased under air combustion conditions. Chemical equilibrium calculations were carried out to facilitate the interpretation of the measured data; the results indicate that temperature dependence and fuels/blends ash composition are the major factors affecting gaseous compounds and ash composition rather than the combustion environment, which seems to affect the fine ash (submicron) ash composition, and the ash deposition mechanisms.     

State-of-the-art processes for manufacturing synthetic liquid fuels via the Fischer-Tropsch synthesis

Processes for manufacturing synthetic liquid fuels on the basis of the Fischer-Tropsch synthesis from alternative feedstock (natural gas, coal, biomass of various origins, etc.) are surveyed. State-of-the-art technology, companies that offer such processes, and the quality of products in comparison with their oil analogs, as well as economic features of the processes, are considered.     

煤与生物质的热解

综述了煤与生物质的热解研究现状，着重介绍了煤与生物质在各种不同气氛下的热解情况，并对煤与生物质的热解机理做了更进一步的探讨，指出对煤与生物质的共热解的研究具有重要的意义，而且可望成为获取高附加值的热解产品和洁净煤转化技术的新途径。     

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.     

TG-FTIR characterization of coal and biomass single fuels and blends under slow heating rate conditions: Partitioning of the fuel-bound nitrogen

The devolatilization behavior of a bituminous coal and different biomass fuels currently applied in the Dutch power sector for co-firing was experimentally investigated. The volatile composition during single fuel pyrolysis as well as during co-pyrolysis was studied using TG-FTIR characterization with the focus on the release patterns and quantitative analysis of the gaseous bound nitrogen species. It was shown that all investigated biomass fuels present more or less similar pyrolysis behavior, with a maximum weight loss between 300 and 380 °C. Woody and agricultural biomass materials show higher devolatilization rates than animal waste. When comparing different fuels, the percentage of fuel-bound nitrogen converted to volatile bound-N species (NH₃, HCN, HNCO) does not correlate with the initial fuel-N content. Biomass pyrolysis resulted in higher volatile-N yields than coal, which potentially indicates that NO_x control during co-firing might be favored. No significant interactions occurred during the pyrolysis of coal/biomass blends at conditions typical of TG analysis (slow heating rate). Evolved gas analysis of volatile species confirmed the absence of mutual interactions during woody biomass co-pyrolysis. However, non-additive behavior of selected gas species was found during slaughter and poultry litter co-pyrolysis. Higher CH₄ yields between 450 and 750 °C and higher ammonia and CO yields between 550 and 900 °C were measured. Such a result is likely to be attributed to catalytic effects of alkali and alkaline earth metals present in high quantity in animal waste ash. The fact that the co-pyrolysis of woody and agricultural biomass is well modeled by simple addition of the individual behavior of its components permits to predict the mixture's behavior based on experimental data available for single fuels. On the other hand, animal waste co-pyrolysis presented in some cases synergistic effects in gas products although additive behavior occurred for the solid phase.