J. Goodfellow

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.

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.

Investigation of Channel Formation Due to Random Packing in a Burning Waste Bed

Obtaining energy from sustainable sources such as waste and biomass has required a significant extension of combustion technology. Many of the advanced technologies are based on thermal treatment in gas-solid packed-bed systems such as gasifiers, incinerators and biomass furnaces. In this paper channel formation in a packed bed of fuel solids as a result of the random packing process has been investigated. Channelling causes a severely uneven distribution of the primary airflow through a packed fuel bed and results in poor combustion performance of the furnace. By assuming Furnas packing, a general relationship is derived between the bed porosity and the particle size distribution and the proposed methodology is tested against limited experimental data. A probability density function (PDF) of truncated Gaussian type is assumed for the random size distribution at local areas within the bed and the local bed porosity is calculated accordingly. Then by solving the fluid flow equations through the porous bed, flow rate profiles are obtained at the top surface of the bed. Two particulate systems were investigated as a function of change in bed height and pressure drop through the grate. Depending on bed height and pressure drop through the grate, maximum local flow rate at the top surface of the bed can be 1.5 ∼ 2 times higher than the minimum flow rate for the particulate system with a narrower size range (2.5 mm–18 mm) while the ratio of the maximum to minimum flow rate can reach as high as 8 ∼ 32 for the particulate system with a wider size range (0.677 mm–20 mm). Visualization of the velocity profile inside the bed reveals that flow passages are slightly curved in some areas but straight in others. The largest channel observed presents a ‘perfect’ straight passage of airflow running from the very bottom of the bed to the very top of the bed. Channelling inside a burning bed of solid waste in a large-scale travelling grate incinerator plant was also investigated using a unique in-house prototype instrument. The result shows that the combustion processes within the bed were dominated largely by the circles of formation and subsequent collapse of channels.

Optimisation study of a large waste-to-energy plant using computational modelling and experimental measurements

AbstractIn order to maximise the energy recovery efficiency of waste-to-energy plants, it is important to understand the physical processes that are occurring within the furnace. A mathematical model of the furnace section of a large waste-to-energy plant was constructed using FLIC to model combustion of the solid waste particles on the furnace grate and FLUENT to model the gas flow above the burning waste bed. The two models were coupled through their respective boundary conditions. A numerical simulation of the design-case setup of a large waste-to-energy plant was performed, which predicted the presence of a large flow recirculation zone in the radiation shaft. Further numerical simulations were performed using several different configurations of the secondary air jets, which revealed that the large flow recirculation in the radiation shaft could be avoided by changing the distribution of secondary air jets. Experimental measurements of the temperature profile within the burning waste bed of the plant ...

STUDY ON THE TRANSIENT PROCESS OF WASTE FUEL INCINERATION IN A FULL-SCALE MOVING-BED FURNACE

ABSTRACT The transient process in a full-scale municipal solid waste incinerator was investigated to assess the effect of the grate movement and waste feeding cycles. Inside-bed measurements of temperature and O2 were carried out and an 18-hour record of furnace temperature, airflow, and flue gas compositions was analyzed using discrete Fourier transform. The transient burning process was also simulated numerically by solving the governing equations for both the solid and gas phases. It is found that fluctuations in the furnace temperature and flue gas O2 level correspond to the waste feeding and grate movement cycles, either in phase or out of phase (depending on the length of the cycles), while combustion inside the bed is dominated by a series of big spikes and dips in both local temperature and O2 level. Theoretical calculations indicate that each grate movement causes a surge (by 35%) in the volatile release rate but a depression (by 24%) in the char burning rate. The overall bed-burning rate fluctuates between 90 and 110% of its average.

Cutting Wastes from Municipal Solid Waste Incinerator Plants

Major problems facing modern society include the provision of energy and means for waste disposal with the minimum generation of pollution and secondary wastes. Sustainable cities require the recovery of energy from that fraction of waste that cannot be economically reused or recycled. With up to 28 million tonnes of municipal solid waste having an average energy value of 10,000 kJ kg-1 being generated in the UK annually, incineration of with energy recovery is seen as the best available option for the safe disposal of municipal waste. Current efficient waste management strategies are also aimed at reducing the amount of waste from municipal solid waste incinerator plants by processing ash from new plants to approach the target of zero net waste material output and reducing the pollutants in flue gases by end-of-pipe treatment. Although pollutants in flue gases can be reduced to practically any desired level, the economic and environmental costs should be justified. Our research activities at Sheffield University Waste Incineration Centre are focussed on cutting down the amount of waste produced by these plants in the form of ash, slag and air pollutants. Our aim is to develop efficient yet cost-effective waste, energy and pollution management strategies to assist the incineration industries to control and operate future energy plants in the most efficient way possible.

Sintering of the APC Residue from Municipal Waste Incinerators

Air pollution control (APC) residues from municipal waste incineration contain considerable amounts of water-soluble heavy metal compounds which can cause environmental problems. Therefore, a satisfactory and efficient detoxification technology is required. A thermal process via sintering has been developed that is capable of forming two major fractions of sintered products — that is, a small fraction in which the volatile heavy metals are concentrated for recycling or disposal and a major solid particle fraction that is more stable. This result has been obtained by heating the APC ash up to 850 °C with a high efficiency processing system. This novel process has been demonstrated in a large experimental facility that includes a 100 kW regenerative burner. The approach is based on the application of a regenerative heating concept to achieve an economic and efficient process. Thus, the hazardous material which is causing a problem when landfilled can be converted to a better quality product either for utilization by the construction industry or for disposal at lower cost.

A novel measuring instrument for pyro-processes

A unique self-contained data acquisition unit consisting of multiple sensing elements and recording electronic components installed in a heat resistant capsule is currently being developed at SUWIC. This unit can be introduced into a process system with the raw feed material and experiences the same conditions as the material being processed, as it simultaneously measures the temperature, gas compositions and heat fluxes in the system. At the end of the process, the instrument can be recovered, and the data recorded and stored in its memory unit downloaded to a computer. This instrument is applicable to pyro-processes such as incineration, power generation and steel processing, as well as to the food industry.

Energy from waste : Current research and development in municipal waste incineration at Sheffield University Waste Incineration Centre (SUWIC)

SYNOPSIS In the UK, about 30 million tonnes of Municipal Solid Waste (MSW) is generated each year and the disposal of this waste has become a national problem. The growing scarcity of landfill sites for municipal solid waste disposal and the increasing environmental problems with landfill waste has led to more stringent regulations and high cost of waste disposal. The landfill tax in the UK is designed to encourage energy efficient waste management strategies aimed at reducing the amount of waste to be landfilled through incineration while maximising the recovery of the waste energy. Efficient recovery of energy from waste is therefore an important aim for modern incinerator design. The incineration of MSW is known to be a very complex process since the waste is poorly specified and its composition varies from moment to moment. Hence, research into waste combustion is fundamental to rational incinerator operation and the evolution of innovative processes such as co-incineration and integrated incinerator/gas turbine plants. This paper reviews the current research activities at Sheffield University Waste Incineration Centre (SUWIC) in MSW incineration.