Jim Swithenbank

Tar reduction in pyrolysis vapours from biomass over a hot char bed

The behaviour of pyrolysis vapours over char was investigated in order to maximise tar conversion for the development of a new fixed bed gasifier. Wood samples were decomposed at a typical pyrolysis temperature (500 degrees C) and the pyrolysis vapours were then passed directly through a tar cracking zone in a tubular reactor. The product yields and properties of the condensable phases and non-condensable gases were studied for different bed lengths of char (0-450 mm), temperatures (500-800 degrees C), particle sizes (10 and 15 mm) and nitrogen purge rates (1.84-14.70 mm/s). The carbon in the condensable phases showed about 66% reduction by a 300 mm long char section at 800 degrees C, compared to that for pyrolysis at 500 degrees C. The amount of heavy condensable phase decreased with increasing temperature from about 18.4 wt% of the biomass input at 500 degrees C to 8.0 wt% at 800 degrees C, forming CO, H(2) and other light molecules. The main mode of tar conversion was found to be in the vapour phase when compared to the results without the presence of char. The composition of the heavy condensable phase was simplified into much fewer secondary and tertiary tar components at 800 degrees C. Additional measures were required to maximise the heterogeneous effect of char for tar reduction.

Effects of fuel devolatilisation on the combustion of wood chips and incineration of simulated municipal solid wastes in a packed bed

Detailed mathematical simulations as well as experiments have been carried out for the combustion of wood chips and the incineration of simulated municipal solid wastes in a bench-top stationary bed and the effects of devolatilisation rate and moisture level in the fuel were assessed in terms of ignition time, burning rate, reaction zone thickness, peak flame temperature, combustion stoichiometry and unburned gas emissions at the bed top. It is found that devolatilisation kinetic rate has a noticeable effects on the ignition time, peak flame temperature, CO and H2 emissions at the bed top and the proportion of char burned in the final stage (char burning only) of the combustion. However, it has only a minor effect on the other parameters. Reaction zone thickness ranges from 20 to 55 mm depending on the moisture level in fuel and an increase in the moisture level causes a shift of the combustion stoichiometry to more fuel-lean conditions.

Pyrolysis and combustion of oil palm stone and palm kernel cake in fixed-bed reactors

The main objective of this research was to investigate the main characteristics of the thermo-chemical conversion of oil palm stone (OPS) and palm kernel cake (PKC). A series of combustion and pyrolysis tests were carried out in two fixed-bed reactors. The effects of heating rate at the temperature of 700 degrees C on the yields and properties of the pyrolysis products were investigated. The results from the combustion experiments showed that the burning rates increased with an increase in the air flow rate. In addition, the FLIC code was used to simulate the combustion of the oil palm stone to investigate the effect of primary air flow on the combustion process. The FLIC modelling results were in good agreement with the experimental data in terms of predicting the temperature profiles along the bed height and the composition of the flue gases.

Mixing, Modelling and Measurements of Incinerator Bed Combustion

The safe disposal of municipal solid waste has now become an urgent environmental problem. The traditional method of landfilling waste has created so many environmental problems that countries including Denmark, Holland and Germany have imposed severe restrictions on landfilling burnable waste. With up to 1 tonne of municipal waste being generated by every individual annually in the UK, incineration is now at the forefront of combustion research, as developed countries recognize the environmentally friendly advantages of this technology. An efficient incinerator is not only assessed by the amount of heat recovery but also by the levels of emissions and quality of the ash it produces. Incinerator designs must therefore be fully optimized so that they can control emissions by reducing the production of harmful pollutants such as dioxins, furans, NOx and SOx. Hence, incinerator bed combustion is a vital area that urgently needs further investigation. At SUWIC, the present work concentrates on the development of a comprehensive and reliable model for the incinerator bed combustion process. The results from the incinerator burning bed model can then provide the much needed boundary conditions for Computational Fluid Dynamics (CFD) modelling of the gas phase reacting turbulent flow in the freeboard region of an incinerator. In addition to the development of the computational model, the work involves several parallel activities, including experimental investigations into waste combustion, solid mixing and prevention of slag formation and instrumentation development.

Modelling Waste Combustion in Grate Furnaces

The disposal of waste that cannot be minimized, recycled or reused is a huge international problem. In the UK, we currently landfill about 30 million tonnes of waste per year. This is environmentally unfriendly due to greenhouse gas emissions, etc., and squanders energy equivalent to about 25% of our current coal consumption. The incineration of material in energy-from-waste plants has received relatively little attention from combustion scientists and engineers in the past and this work is directed at rectifying this situation. Incinerators generally burn waste on a moving grate that transports and mixes it during combustion. The combustion process involves drying, devolatilization, gasification and char burn-out. Thus gasifiers and pyrolysers are subsets of this combustion problem. Mathematically modelling combustion on the grate requires the solution of the flow field in a reacting packed bed, including radiant heat transfer. Since the burning in the channels is mixing-limited, reactions also occur in the gas phase above the bed. The conditions evaluated at the surface of the bed are the boundary conditions for conventional computational fluid dynamic modelling of the mixing and reactions in the secondary combustion zone in the freeboard above the bed. This permits the evaluation and minimization of emissions such as dioxins to the point that dioxins from incinerators now only contribute 3% of the total UK dioxin emissions. The validation of our reacting bed modelling code (FLIC) has been achieved by measurements in a pot burner using various wastes. Furthermore, a small ‘ball instrument’ that has been specially developed to contain instruments has complemented these measurements by withstanding temperatures up to 1000°C for well over an hour. This novel device passes through industrial incinerator furnaces with the waste and records parameters such as oxygen, vibration and several temperatures onto a computer memory chip. The ball is recovered from the incinerator ash pit and the information is downloaded onto an Excel spreadsheet for detailed analysis. Incinerator combustion is obviously one of the most complex combustion physics/chemistry processes known. At the present time it is also industrially important, however it is now yielding its secrets to scientific study.

THERMAL REACTION MODELING OF A LARGE MUNICIPAL SOLID WASTE INCINERATOR

In a mass burn waste incinerator, the waste undergoes combustion in the bed on the grate and the products of incomplete combustion are destroyed in the gas plenum above the bed. Thus, it is essential to consider the reaction in both the waste bed and the gas plenum when evaluating the combustion performance using computational fluid dynamics (CFD). However, solving the two-phase reacting bed of waste is not straightforward in CFD simulations. This paper presents the combustion and gas flow of a waste incinerator using a new simulation approach that incorporates two models: FLIC for the reacting bed of solid fuels and FLUENT for the turbulent reacting flow of the gas plenum. The two models were linked through the boundary conditions on the waste bed in such a manner that FLIC provides the gas properties leaving the bed as an inlet condition of the gas flow and FLUENT produces the radiation transferred to the bed. The predicted results were compared with on-site measurements in the plant by a novel instrument that collected the temperature, oxygen concentration, and motion data while tumbling with the waste particles within the bed. The measured data were highly fluctuating, but FLIC satisfactorily predicted the overall trend of temperature and oxygen concentration including the upper and lower boundaries of the violent fluctuations. The FLIC/FLUENT combined simulation provided crucial information on the nature of combustion and flow characteristics such as the ignition and burnout points of waste, the rates of each combustion process, and the subsequent gas flow pattern in the combustion chamber.

Simulation of Channel Growth in a Burning Bed of Solids

Packed beds of solid particles are widely used in solid fuel combustion, waste incineration, drying and filtration processes etc. where gases flow through the beds at a certain superficial velocity. Non-uniformity in the bed porosity causes uneven distribution of the gas flow rate across the bed, and in extreme cases free-flowing channels or passages can be formed in the beds, downgrading the performance of the bed processes. In this paper, experimental observations of channelling in the burning beds of municipal solid waste incinerators are described and detailed numerical simulation of the growth of an individual burning channel is carried out.

Modeling coal-fired cyclone combustors

A mathematical model based on the finite-difference solution of momentum, enthalpy, and species conservation equations has been constructed to describe the process of coal combustion in cyclone furnaces. Coal particles are tracked through the flow and the evolution and combustion of volatiles calculated. Most of the char then burns on the walls of the chamber, while held in a molten ash layer, at a rate principally determined by the rate of diffusion of oxygen. The predictions of the model are compared with experimental data obtained in the late 1950s at the British Coal Utilization Research Association on a small experimental combustor. Flow and heat transfer data are closely predicted; comparison with the combustion efficiency data cannot be rigorous but no major inconsistencies emerge.

Design optimization of a large municipal solid waste incinerator

Abstract This paper discusses a number of design modifications and changes in operational conditions which have a major influence on the overall performance of the large Sheffield municipal solid waste incinerator plant (30 MW) which incorporates a heat recovery system for district heating. The effect of installing four different baffle configurations inside the radiation shaft and various primary and secondary air distribution patterns have been investigated using mathematical modelling, in attempt to eliminate the existing recirculation zone inside the shaft and at the same time obtain optimum combustion conditions which would minimize the emission of pollutants and reduce maintenance costs at the plant. A mathematical model of the finite difference type was employed to predict the three dimensional reacting flows within these modified incinerator designs. The κ - e submodel has been employed for turbulence, together with a combustion model of the Magnussen type. A Lagrangian model was used to estimate the residence times for different cases. The results obtained from the modelling work demonstrate that the suggested modified design produces longer residence times in the incinerator, improves the temperature profile at the exit, reduces the concentration of products of incomplete combustion and increases the overall boiler efficiency by nearly 8%. Using these modelling results a preliminary economic assessment of the potential benefits of the suggested modified incinerator design compared to the original design has been made assuming the energy recovered is sold as steam. The calculation indicated that because of significant savings in running and maintenance costs, an 18% reduction in net waste disposal cost may be achieved.

The effects of particle size distribution and refractive index on fly-ash radiative properties using a simplified approach

Abstract The radiative properties of a fly-ash polydispersion are calculated using a simplified approach based on Mie theory. The experimental data of fly-ash complex refractive index of Goodwin (Infrared optical constants of coal slags, Technical Report T-255, Stanford University, California) are employed in the calculation to take into account the wavelength-dependence of optical constants. The uncertainty in representing the particle size distribution is addressed explicitly. Due to this uncertainty, the uncertainty of the wavelength-integrated Planck mean absorption and scattering coefficients can be over 10%. The use of wavelength-independent optical constants for fly-ash yields unacceptable results of Planck mean coefficients.