V. Nasserzadeh

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.

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.

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.

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.

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.

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%.