Y. R. Goh

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.

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.

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.

A Novel Energy-Efficient Process Utilizing Regenerative Burners for the Detoxification of Fly Ash

During the last decade, the debate concerning incineration has focused mainly on potential risks from air emissions. Today, waste incineration will only gain full public acceptance if a high quality of ash residues can be guaranteed, particularly with respect to low levels of organic compounds such as polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans (PCDD/Fs) and the high elution stability of toxic heavy metals. Conventional thermal treatment techniques such as vitrification have been used in the past for detoxification of the ash residues from incinerators, but at a significantly high cost due to the massive energy consumption of the process. This paper addresses the problem of safe disposal of millions of tonnes of contaminated fly ash produced each year by developing an economically viable energy efficient process that can convert this toxic material into a non-toxic material. The decontaminated material can then be safely land-filled or used in construction applications such as in road foundations. This approach is based on the fact that sintering of the ash residue results in destruction of its toxic organic components, and also fixation of its heavy metal content. The novelty of the process includes the integration of the sintering process with the concept of regenerative heating. This is a very energy-efficient concept that results in typical fuel savings of 65% in industrial furnaces. A fully operational ash detoxifying pilot plant with a throughput suitable for continuous treatment of fly ash generated by a typical municipal solid waste incinerator plant was constructed and operated. An extensive series of analytical tests were carried out on both the raw and thermally treated ash samples. The results obtained showed an achievement of up to 96% reduction in the heavy metal leaching potential in the sintered ash. The sintering process de-volatilized metals having a high vapour pressure and effectively trapped other heavy metals in oxide sinters/melts. The reduction in leaching potential can be largely attributed to the formation of these stable oxides. This finding was verified by computational analysis that showed that up to 99.9% of some metals were converted into metal oxides during the sintering process. Analysis of the sintered ash samples/pellets for PCDD/Fs using standardized methods with GS-MS/MS equipment showed a major reduction in PCDD/Fs concentrations to below the detectable limit of 0.1 pg g − 1. These results are exceptionally encouraging since they eliminate the concern over the safety and suitability of the sintered ash material/pellets for re-use. Each cyclone was designed to provide enough residence time for the ash particles to be heated up to the sintering temperature of 850°C. High temperature conditionswere maintained throughout the cyclone thus helping to avoid any re-formation of toxic organic compounds such as PCDD/Fs. Detailed analysis showed that the use of heat regeneration in our sintering process provided significant energy and economic savings of up to 50%.

Formation and elimination of polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans from municipal solid waste incinerators

The emissions of polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans (PCDD/Fs) from incineration processes has become a major subject of considerable public and scientific concern in light of the evidence of their extreme toxicity. Efficient control of PCDD/F emissions from municipal solid waste (MSW) incineration plants requires fundamental knowledge of the pathways by which they are formed in these plants. Analytical tests conducted on several MSW incinerator residues confirmed that PCDD/Fs form in the heat recovery unit of the incinerator plant and are removed from the flue gas in the fabric filter. A mathematical model for predicting PCDD/F emissions from MSW incinerator plants was developed based on governing equations for their formation through the de novo synthesis mechanism in the post-combustion zone and their removal by the fabric filter. The PCDD/F concentrations and removal efficiencies computed using the mathematical model were also found to correspond remarkably well with plant test data.

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.