Y.B. Yang

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.

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.

Development of an Incinerator Bed Model for Municipal Solid Waste Incineration

The growing awareness and concern over environmental problems associated with landfill has led to increased demand for incinerators to dispose of municipal waste. The design of an efficient incinerator requires fundamental knowledge of grate combustion and grate mixing, but for such a complex combustion problem, there has been no satisfactory model of the system as a whole. An experimental fixed bed reactor was used to investigate the incineration of simulated waste. Measurements of temperatures and gas compositions were made at several positions within the refuse bed. A mathematical model for combustion of the solid waste in a travelling grate incinerator was developed based on an unsteady-state static bed model. The predictions from the model were investigated and compared to the combustion experimental data carried out in the batch fixed bed incinerator. Although several simplifying assumptions were made during the model development, the final functional form of the predicted data corresponds relatively well with the measured data.

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.