M. Pourkashanian

Combustion of pulverised coal and biomass

This review paper is concerned with the current status of the understanding of the combustion of pulverised coal and pulverised biomass from the viewpoint of computer modelling. Whilst a knowledge of the underpinning science is of vital importance its translation into applicable computer useable equations or computer data base libraries is of vital importance. A review is given of the current status of sub-models for the combustion of pulverised coal. Much of the information available for coal is transferable to biomass combustion although there are still areas where there is a lack of information.

Modelling coal combustion: the current position

Abstract At the present time, computer models for coal combustion are not sufficiently accurate to enable the design of combustion plant or the selection of a coal based on combustion behaviour. Most comprehensive combustion models can predict with reasonable accuracy flow fields and heat transfer, but usually with a much lesser degree of accuracy than the combustion of the coal particles through to char burnout. Many research programmes are aimed at developing a much more accurate predictive tool for assessing coals specially fired in burners or furnaces employing a range of NO x abatement technologies. Some of the current developments in CFD coal combustion modelling are outlined here. Particular attention is paid to the first step, where the devolatilisation pre-processor code is used to compute the pyrolysis rate, the yields and the composition of volatiles and char. These parameters are used as inputs to the devolatilisation and volatile combustion sub-models, where various options can be used, and also the char burnout sub-models. The accuracy of the sub-models is examined using data from four well-studied coals, three from the UK and one from the US. The main network devolatilisation codes are compared with experimental data. Two char combustion models have also been investigated in order to compare char burnout predictions and the development of char morphology and surface area during burnout are considered. The applications of these sub-models to two combustion situations were considered. These involve reactions in a drop tube furnace and a low NO x industrial burner and in both cases, the model predictions were compared with experimental measurements.

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

In many Eastern European countries the emission of pollutants from coal fired domestic and small commercial heating plants is a serious problem. One alternative method to reduce emissions is by burning coal with biomass that is by co-combustion, which would reduce the amount of net CO2 produced, the rate of reduction of coal reserves, and the overall amounts of pollutants. The effects of co-combustion of coal and biomass on the levels of pollutant formation have been studied for a 30 kW domestic boiler. Of particular interest are the emissions of PAH and VOC since it has been shown that these emissions are lowered during co-combustion. The relation between boiler design fuel composition and measured emission profiles for VOC and PAH is discussed in detail. Outputs from modelling of the emissions and devolatilisation characteristics of the fuels have been compared to measured values. Analytical methods have been developed for characterisation of the initial devolatilisation products from both coal and biomass fuels and the relation to modelling is discussed. The fate of these initial devolatilisation products and the effect of boiler design on the formation of PAH and soot in the post combustion zones is addressed.

Prediction of ash deposition on superheater tubes from pulverized coal combustion

A numerical model has been developed to describe the main deposition processes of ash particles with a predetermined chemical composition. Thermophoresis and inertial impaction are considered, the latter being the predominant process for particles > 10 μm. A single drop-tube furnace and a horizontal p.f.-fired furnace have been modelled in this study. Predicted results show that the denser (iron-rich) particles are more inclined to deposit than less dense (silica-rich) particles. Boiler aerodynamics are also shown to be a controlling factor in the slagging process. Ash layer formation is shown to enhance the deposition process further owing to the increased surface temperature of the deposit. Inclusion of a variable deposit temperature results in good agreement with experimentally measured values. The effect of gas temperature is shown to influence the deposition process owing to the effects on particle viscosity. Predicted results show good agreement with both laminar drop tube and p.f.-fired furnaces for fouling coals, but it has not yet been possible to model non-fouling coals accurately, since ash erosion and deposit breakage have not been considered.

Modelling pulverised coal combustion using a detailed coal combustion model

ABSTRACT The ability to assess the combustion behaviour of internationally traded coals and accurately predict flame characteristics, stable species concentration, unburned carbon and pollutant emissions is of importance to the power generating industry. Despite recent advances in coal combustion modelling detailed understanding is still lacking on the exact role of the coal maceral content on the combustion process. Here, a CFD-based coal combustion model that includes these effects has been developed to try to improve the predictive capability. A computational simulation of a 1 MW (thermal), pulverised fuel combustion test furnace, which was designed to replicate the time-temperature history of a full-scale furnace, was performed. This furnace also contained a triple-staged low-NOx swirl burner. A number of simulations were made using a number of coals in order to calculate NOx and the unburned carbon-in-ash, the latter being a sensitive test for the accuracy of the char combustion model.

The oxidative reactivity of coal chars in relation to their structure

During the oxidation of porous coal chars the internal surface area can increase as a function of the degree of conversion due to pore growth and the opening up of sealed internal pores or cavities. Consequently, rate expressions for their oxidation are more accurately described in terms of the intrinsic reactivity, where differences in surface area and porosity can be taken into account. The oxidative reactivity of coal chars is complicated by a number of different factors, which are explored in this paper. These include (i) the development of the pore structure during devolatilisation of the coal, (ii) the ash content, its distribution in the carbon matrix and effect on reactivity, (iii) the extent of the H and N functional groups present in the solid matrix, and their interrelation with residual volatile species which are present, (iv) the extent of the graphitic nature of the carbon surface and (v) the active surface area available for reaction.

Modelling NOx formation in coal particle combustion at high temperature: an investigation of the devolatilisation kinetic factors

Coal combustion computational fluid dynamic (CFD) models are a powerful predictive tool in combustion research. In existing coal combustion CFD models, the process is described by three kinetic rates: coal devolatilisation, volatile combustion and char combustion. A general, representative devolatilisation rate for coal is a matter of some contention, and measured rates depend upon the type of experimental system employed in their determination. Thus the reported rates vary considerably, causing difficulties in the choice of rate expression for CFD modelling applications. In this investigation, a laminar flow CFD model of a drop-tube furnace was used to assess the influence of global devolatilisation rates on overall combustion behaviour, and in particular, NOx emissions. The rates chosen include some of the common expressions employed by researchers in the field. Analysis, and comparison of the modelling results with those of the experimental, indicated that a single-step devolatilisation rate can give satisfactory profiles. This rate can be calculated from the tar release rate using a network model such as FG-DVC (functional group, depolymerisation, vaporisation and cross-linking), together with the nitrogen partitioning between gas and char during pyrolysis. The use of these single-step models result in good predictions of NOx, and the inclusion of soot/NOx interactions can improve the model significantly to give an excellent agreement with experimental results.

The combustion of coal and some other solid fuels

This paper outlines some of the key issues currently being debated regarding the combustion of coal and of some biomass materials. It attempts to summarize the present approaches toward the quantification of the fundamental processes of solid fuel combustion for use in computer models. Some aspects of the various chemical and physical processes are included, such as heating-up of particles, devolatilization, and subsequent char formation. Of particular interest is the prediction of char properties, such as composition, surface areas, and morphology, since these impact on char combustion. The available models for devolatilization, char formation, and heterogeneous, oxidation are considered for application to both coal and biomass, and a description in terms of the fundamental molecular processes of a coal particle is explored. Char combustion models, including global intrinsic reaction rates, are outlined. The latter has clear advantages, but requires inputs of char physical properties which, at present, cannot be predicted a priori . Intimately linked with the processes of devolatilization and char combustion are NO x formation and carbon burnout, and special attention is given to these important factors. Finally, the general methods for modeling coal reaction processes are outlined, and illustrative examples are given with regard to the current situation in the application of these submodels to computer modeling of pulverized and coal bed combustion.

Review of the Computational Fluid Dynamics Modeling of Fuel Cells

This paper presents a review of the current situation in the computational fluid dynamics (CFD) modeling of fuel cells and highlights the significant challenges that lie ahead in the development of a comprehensive CFD model for fuel cell applications. The paper focuses on the issues concerned with solid oxide fuel cells and proton exchange membrane fuel cells because these are the two most poplar and probably the most promising types of fuel cells for both stationary and transport applications. However, the general principles presented in this paper are applicable to all types of fuel cells.

Burn-out of pulverised coal and biomass chars☆

The burn-out of carbon in pulverised fired power stations is commercially important. Interest in the burn-out of biomass chars is growing because biomass is increasingly being co-fired with coal to reduce the carbon dioxide emissions. The significance of carbon burn-out is that it is linked with the efficiency of the plant and the suitability of the coal ash for construction purposes. Residual carbon in ash has generally increased in recent years because of the influence of the lower temperatures and slower mixing resulting from the use of low NOx burners. The amount of unburned carbon is thus a function of the plant design and operating conditions but it is also linked to the ease of combustion of the coal and the char formed. These latter factors are related to the properties of the coal and this paper attempts to quantify the impact of certain coal and char properties on carbon-out. An approach for assessing biomass combustion performance is also discussed.