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.     

Numerical investigation of Solid Recovered Fuels’ co-firing with brown coal in large scale boilers – Evaluation of different co-combustion modes

In the current work the co-combustion of Solid Recovered Fuels’ (SRFs’) with brown coal in large scale pulverised coal boilers under different operational conditions is numerically investigated. In order to overcome the difficulty of the complex, inhomogeneous nature of waste recovered fuels, SRF is modelled as a mixture of two different fractions, the biogenic and the plastic one. For each fraction different combustion mechanisms are presented, whilst for the first time the proposed combustion mechanism of the plastic fraction is incorporated in a commercial CFD code and validated against available experimental data. A 600 MW_e brown coal boiler is simulated as a reference and its operational characteristics are compared with parameterised scenarios of SRF co-firing conditions. Based on the numerical results, the optimum co-firing concepts regarding the more efficient operation of the boiler (hot spots and fuel’s burnout) are identified, decreasing the environmental impact of the boiler’s emissions.     

Biomass co-firing in a pressurized fluidized bed combustion (PFBC) combined cycle power plant: A techno-environmental assessment based on computational simulations

The co-utilization of coal with biomass and biomass waste in a pressurized fluidized bed combustion (PFBC) system is a promising power generation option for addressing various areas of concern relating to the anthropogenic sources of harmful emissions, the global reliance on fossil fuel and the overall energy supply issues. In this study, coal with a wide range of biomass and biomass waste types such as straw, willow chips and switch grass as well as miscanthus and olive pits are fired in an advanced PFBC system. The produced gases and the evolved heat energy are employed to run a combined cycle. To understand the behavior of the proposed system, detailed computational simulations are carried out utilizing various feedstock mixtures ranging from 100% coal to 40% biomass. The results of the simulations are used to show the effect of co-firing on the technical and environmental performance of the power plant.The results show that the main parameters affecting the overall power plant efficiency are the co-firing ratios and the specific properties of the chosen biomass/waste types. Furthermore, the investigation indicates that the steam cycle output reacts more sensitive to the fuel configurations than the gas turbine cycle. As expected, the increased fraction of biomass or waste significantly reduces net CO₂ emissions, and has a beneficial influence on SO_x emissions. NO_x emissions tend to rise for all biomass types, except the high moisture content willow chips, with increasing co-firing fraction.     

Waste tyre rubber as a secondary fuel for power plants

Approximately 1 billion waste tyres are generated worldwide each year, with the US producing 300 million and the EU 260 million tyres, representing an enormous waste management problem. At the same time, increasingly stringent emission control targets are being imposed on electric power generating plants. The development of science and technology for clean coal combustion is crucial for a sustainable environment which is dependent on a mix of energy production systems. In this pilot scale study we have shown that tyre rubber can be fired with pulverised coal and may have a role to play in co-firing configurations in full scale power plant boilers as wastes are beginning to feature in ‘fuel switching’ scenarios for CO₂ mitigation. Utilisation of waste tyres in a coal combustion plant can, in one step, reduce NO emissions and recover energy from waste tyres, efficiently. Therefore, through this process, a problem waste stream is effectively utilised to help solve a major environmental pollution problem. We present data demonstrating reburning and co-firing configurations utilising waste tyre rubber. Low levels of NO emission (up to 80% reduction at reburning fuel fractions <12%th) can be achieved, when using a lower volatile South African coal as the primary fuel. The results for tyre reburning are compared with the performance of a suite of reburning fuels with differing volatile hydrocarbon contents. Direct co-firing of tyre with coal can also reduce NO levels but the degree of reduction is dependent on the reactivity of the coal and the prevailing combustion conditions in the primary zone of the mixed fuel flame.     

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).     

Utilization of fly ash coming from a CFBC boiler co-firing coal and petroleum coke in Portland cement

Fly ash coming from a circulating fluidized bed combustion (CFBC) boiler co-firing coal and petroleum coke (CFBC fly ash) is very different from coal ash from traditional pulverized fuel firing due to many differences in their combustion processes, and thus they have different effects on the properties of Portland cement. The influences of CFBC fly ash on the strength, setting time, volume stability, water requirement for normal consistency, and hydration products of Portland cement were investigated. The results showed that CFBC fly ash had a little effect on the strength of the Portland cement when its content was below 20%, but the strength decreased significantly if the ash content was over 20%. The water requirement for normal consistency of cement increased from 1.8% to 3.2% (absolute increment value) with an addition of 10% CFBC fly ash; and the free lime (f-CaO) content of CFBC fly ash affected the value of increasing. The setting time decreased with an increase of CFBC fly ash content. The volume stability of the cement was qualified even when the content of SO₃ and f-CaO reached 4.48% and 3.0% in cement, respectively. The main hydration productions of cement with CFBC fly ash were C-S-H (hydrated calcium silicate), AFt (ettringite), and portlandite.     

Minimizing fuel and environmental costs for a variable-load power plant (co-)firing fuel oil and natural gas: Part 1. Modeling of gaseous emissions from boiler units

This work was aimed at modeling of major gaseous emissions (NO_x, SO₃, SO₂, CO₂) from boiler units of a power plant firing (or co-firing) fuel oil and natural gas for variable operating conditions (load and load-related variables: excess air, flue gas recirculation, etc.). The emission rate of the pollutants for the co-firing was estimated for a particular boiler using these characteristics for the burning of each fuel in the boiler on its own and taking into account energy fractions (contributions) of fuel oil and natural gas to the boiler heat input. The gaseous emissions (in terms of emission concentrations, emission rates and specific emissions) from a 200-MW boiler unit firing low-S fuel oil and from a 310-MW boiler unit firing (or co-firing) medium-S fuel oil and natural gas were estimated and compared for 50–100% unit loads based on actual fuel properties and load-related operating variables of these units. Upper limit for the energy fraction of medium-S fuel oil was determined for the 310-MW boiler unit co-firing the two fuels with the aim to meet the national emission standard for SO₂.     

Circulating fluidized bed combustion of a high-sulphur eastern canadian coal

Combustion tests were carried out with Minto coal in combination with three different limestones in the University of British Columbia (UBC) pilot scale (152 mm square x 7.3 m tall) circulating fluidized bed combustion (CFBC) unit. Operating conditions were chosen to be typical of those employed in large-scale CFBC power boilers. Recycling of fine particles captured by the secondary cyclone was found to be of considerable importance in increasing sulphur capture, enhancing combustion efficiency and reducing the amount of calcium sulphide in the solids residues. NO_x emissions increased as the Ca:S ratio increased. Local gas concentrations inside the reactor were strongly influenced by the core-annulus solids distribution patterns which characterize circulating fluidized beds.     

Experimental investigation on the combustion behaviour of pre-dried Greek lignite

The present paper includes the results of the combustion tests with Greek dried lignite performed at a 1 MWth semi industrial scale pulverized coal combustion facility. Scope of the campaign is the investigation of the combustion behaviour of Greek lignite, i.e. temperature fields, ignition, burnout, emissions, as well as slagging and fouling tendency, while firing with varying levels of recirculated flue gas. Dry coal co-firing conditions in a large scale boiler are simulated by adjusting the volume flow of recirculated flue gas.Two test series representing different boiler operation modes are performed. During the first series the maximum flue gas temperature increase, when co-firing dry coal, is determined, while in the second test series the needed load decrease, in order to keep constant furnace outlet temperature in dry coal co-firing conditions is recorded. A detailed measurement set is carried out including temperature profiles, emissions, fuel, fly ash sampling and slagging and fouling investigations through the installation of dedicated deposition probes.The anticipated increase of the furnace temperature profiles by decreasing the inserted recirculated flue gas is confirmed by the experimental results. No clear trend of dry coal co-combustion on the emissions' behaviour is noticed, while dry coal firing appears to have a moderate effect on the deposition behaviour of Greek lignite. These preliminary investigations indicate that no significant operational problems are expected during a potential future demonstration of dry lignite co-firing in a Greek large scale boiler.     

Influence of particle shape and internal thermal gradients of biomass particles on pulverised coal/biomass co-fired flames

The practice of employing pulverised coal/biomass co-firing in power plants is gradually increasing. This is mainly because of the benefits associated in reducing the coal based CO₂ and biomass based SO_x and NO_x emissions. However, biomass is difficult to mill due to its fibrous texture and this results in the presence of large particles of different shapes which influence the combustion characteristics. Existing computational fluid dynamics (CFD) models often ignore thermal gradients within the particles and this leads to inaccuracy in the combustion process modelling. In this paper, a CFD sub-model for the heat transfer within large particles is developed. The model is validated for the heating up, moisture release and devolatilisation of single wood particle measurements that are available in the literature. The impact of fuel particle sizes on the combustion characteristics has been investigated in terms of ignition, devolatilisation and char combustion in a co-firing case of an industrial combustion test facility. The predictions, while considering the internal thermal gradients with particle size and shape distribution, were identified to be in excellent agreement with measured data. The code was worked well when coupled with ANSYS FLUENT and with a negligible amount of extra time for the computations.     

Emissions during co-firing of RDF-5 with bituminous coal, paper sludge and waste tires in a commercial circulating fluidized bed co-generation boiler

Biomass fuel is the largest renewable energy resource and the fourth largest primary energy supply in the world. Because of its complex characteristics when compared to fossil fuel, potential problems, such as combustion system stability, the corrosion of heat transfer tubes, the qualities of the ash, and the emission of pollutants, are major concerns when co-firing the biomass fuel with fossil fuel in a traditional boiler. In this study, co-firing of coal with a biomass blend, including fuel derived from densified refuse, sludge, and waste tires, were conducted in a 130 ton/h steam circulating fluidized bed co-generation boiler to investigate the feasibility of utilizing biomass as a complemental fuel in a traditional commercial coal-fired boiler. The properties of the fly ash, bottom ash, and the emission of pollutants for various fuel ratios are analyzed and discussed in this study.     

Co-firing of coal and cattle feedlot biomass (FB) fuels. Part I. Feedlot biomass (cattle manure) fuel quality and characteristics

The use of cattle manure (referred to as feedlot biomass, FB) as a fuel source has the potential to solve both waste disposal problems and reduce fossil fuel based CO₂ emissions. Previous attempts to utilize animal waste as a sole fuel source have met with only limited success due to the higher ash, higher moisture, and inconsistent properties of FB. Thus, a co-firing technology is proposed where FB is ground, mixed with coal, and then fired in existing pulverized coal fired boiler burner facilities. A research program was undertaken in order to determine: (1) FB fuel characteristics, (2) combustion characteristics when fired along with coal in a small scale 30 kW_t (100,000 BTU/h) boiler burner facility, and (3) combustion and fouling characteristics when fired along with coal in a large pilot scale 150 kW_t (500,000 BTU/h DOE-NETL boiler burner facility). These results are reported in three parts. Part I will present a methodology of fuel collection, fuel characteristics of the FB, its relation to ration fed, and change in fuel characteristics due to composting. It was found that FB has approximately half the heating value of coal, twice the volatile matter of coal, four times the N content of coal on heat basis, and due to soil contamination during collection, the ash content is almost 9-10 times that of low ash (5%) coal. The addition of <5% crop residues had little apparent effect on heating value. The main value of composting for combustion fuel would be to improve physical properties and to provide homogeneity. The energy potential of FB diminished with composting time and storage; however, the DAF HHV is almost constant for ration, FB-raw, partially composted and finished composted. The fuel N per GJ is considerably high compared to coal, which may result in increased NO_x emissions. The N and S contents per GJ increase with composting of FB while the volatile ash oxide% decreases with composting. Based on heating values and alkaline oxides, partial composting seems preferable to a full composting cycle. Even though the percentage of alkaline oxides is reduced in the ash, the increased total ash percentage results in an increase of total alkaline oxides per unit mass of fuel. The adiabatic flame temperature for most of the biomass fuels can be empirically correlated with ash and moisture percentage.     

循环床锅炉燃烧份额分布的实验研究和理论分析

在循环流化床锅炉小型实验台上,研究了床温、过量空气系数、一二次风比例和煤种等因素对燃烧份额分布的影响,证实了循环流化床锅炉密相区处于欠氧燃烧状态,并且密相区产生的一氧化碳和部分挥发分被带到了稀相区进行燃烧。从流动和燃烧角度对实验结果进行了分析,并从密相区气固两相流行为出发,解释了循环流化床锅炉不同于鼓泡床的一些技术特点。     

Some implications of co-combustion of biomass and coal in a fluidized bed boiler

Laboratory combustion experiments were conducted to clarify some implications of co-firing coal with hog fuel and sludge in a power boiler. Combustion tests in a fixed bed stainless steel reactor at four temperatures (675, 725, 775, and 825 °C) under conditions simulating different moisture contents of hog-sludge blends indicated no problems with ignition. Burn-out of the coal reached 88–99%. However, the burn-out was very sensitive to the excess air, especially when co-firing wet hog fuel. Co-firing with coal will lead to higher sulfur dioxide emissions. Sulfation experiments were conducted in a fixed bed quartz reactor for five limestone particle size ranges (90–150 μm, 212–300 μm, 425–595 μm, 850–1000 μm, 2.0–3.4 mm) at the same temperatures as the combustion tests, with steam added to simulate the variation in the moisture content of the fuel mixture. The tests showed that the capacity of limestone to capture sulfur depends on temperature and particle size. The highest Ca utilization of the limestone was 51% (for the smallest particle size, at 825 °C). At 825 and 775 °C, the sulfur capture capacity of the limestone decreased significantly with increasing particle size, whereas at lower temperatures (725 and 675 °C), the Ca utilization was much less dependent on particle size. The sulfur capture capacity of the limestone was unaffected by the moisture content of the hog-sludge fuel. Calcium contained in the sand used as an inert in the power boiler may be capable of capturing small amounts of sulfur.     

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.     

YG-75/5.29-M18型燃煤泥循环流化床锅炉的特点及应用

介绍了燃煤泥循环流化床锅炉在输送系统、喷枪结构、给料方式及分离器等方面的技术特点;论述了燃烧煤泥时对流化床温度、返料器温度、料层差压、炉膛差压和锅炉负荷等参数的控制与调整;应用此锅炉燃用煤泥或矸石,可降低生产成本,获得较好的经济效益。     

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.     

Investigation of sulphation behavior of two fly ash samples produced from combustion of different fuels in a 165 MWe CFB boiler

Emission of sulphur dioxide (SO₂) from combustion of fossil fuel is an important environmental issue. Circulating fluidized bed combustion (CFBC) technology can use limestone sorbent to achieve in situ SO₂ emissions control. This paper presents the chemical and physical analysis results of two fly ash samples derived from a 165 MWe CFBC boiler burning two different fuels with addition of limestone, as they pertain to sulphation behavior. One of the samples in this study was produced from combustion of a bituminous coal with high iron content, the other from firing of blended coal and petroleum coke fuel. The physical examination was conducted by scanning electron microscope (SEM) coupled with an energy dispersive X-ray (EDX) system for analysis of the surface structure or morphology of the sample, as well as the calcium and sulphur distribution. Some large particles derived from high-iron-content fuel were covered by dense iron shells; however, in general such a dense rim was found to not significantly impede the overall desulphurization performance in FBC in terms of the limestone utilization. The large particles (~ 100 μm in diameter) in both samples typically consisted of a CaSO₄ shell and an almost pure CaO core; however, numerous small particles of diameters of 10-20 μm consisted predominantly of CaO without sulphate shells. In particular, the emphasis of this investigation has been focused on the remaining capacity of the fly ash for reaction with sulphur dioxide and to clarify the effects of iron, both samples have been doped with additional iron content, and their sulphation behavior examined, and while both experienced a small reduction in sulphation capacity, the fly ash with the initial low iron content experienced the lowest reduction of sulphation capacity after doping, which is not supportive of the idea that iron has an important effect on sorbent capacity.     

Characterisation of large solid recovered fuel particles for direct co-firing in large PF power plants

Solid Recovered Fuels (SRF) are solid fuels prepared from high calorific fractions of non-hazardous waste materials intended to be co-fired in coal power plants and industrial furnaces (CEN/TC 343, Solid Recovered Fuels, 2003). They are composed of variety of materials of which some, although recyclable in theory, may have become in forms that made their recycling an unsound option. The SRF with an equivalent median diameter D₅₀ of 6.8 mm are to be directly co-fired in an existing pulverised coal power plant. In comparison to pulverised coal, the particle size distribution of the SRF is of several magnitudes higher, resulting in a different burnout behaviour. Size reduction of the SRF to a fraction similar to coal is not economically feasible. As such, the idea is to co-fire SRF without any further size reduction, and of course this proceeding bears the risk of incomplete combustion.Accordingly, the prediction of the burner levels at which the SRF should be injected and whether or not a complete combustion will be achieved under full and part load conditions are the primary objectives of this paper. In this work, laboratory experiments have been conducted to forecast the success of co-firing the SRF in a commercial pulverised coal power plant. It involves the analyses of the fuel and its intermediate chars, generated at conditions comparable to boiler conditions, to determine some characteristic parameters, namely the burnout time, the aerodynamic lift velocity (ALV), and the apparent densities. The information gathered from the lab experiments are correlated to boiler conditions to determine the possible distances they are likely to travel under various regimes, full load and part load, before they are completely consumed. Different scenarios are examined, and based on the results, the optimal boiler injection points are predicted.     

Firing of coal and biomass and their mixtures in 50 kW and 12 MW circulating fluidized beds - Phenomenon study and comparison of scales

The combustion dynamics of coal, wood chips and their mixture is investigated. Load change capability and the effect of the individual control variables, for example the mixture ratio of different fuels, on pilot-scale CFB boiler dynamics were also studied.Disturbances in fuel feeding cause fluctuations in the flue gas concentrations. Changes in the heating value are possible due to varying moisture content of the fuel. Both these disturbances affect the instantaneous firing rate in a boiler. Also the characteristics of the fuels have to be taken into consideration when designing boiler control systems. When co-firing two fuels with clearly distinct combustion characteristics, direct assumptions based on each fuel’s characteristics cannot be made about combustion behaviour of their mixture.Combustion experiments with coal and wood chips and their co-firing were carried out in a pilot-scale CFB reactor (VTT) and a large-scale CFB boiler (Chalmers). A comparison of the combustion in the two different size reactors, provides information about scaling. The combustion responses due to changes in the fuel feeding of the two circulating fluidized beds are analyzed by a dynamic model.