Mathematical modelling of MSW incineration on a travelling bed

Numerical Simulation Study on Combustion of Low Calorific Value Waste Blended with Biomass

Numerical simulations of a 600 t/day waste incinerator was carried out using the fluid dynamic incinerator code and Fluent to evaluate the effect of biomass blending on furnace temperature, pollutant generation, and selective noncatalytic-reduction (SNCR) denitrification when treating low calorific-value waste. The results show that as the biomass blending ratio increases, the water content gradually decreases, the calorific value increases, and the maximum temperature of the incinerator gradually increases from 1227 to 1408 K, while the content of exported NOx increases from 579 to 793 mg/Nm3; during the combustion of low-quality waste, the residence time of the flue gas in the high-temperature region (above 1123 K) is 1.62 s. When the biomass blending ratio exceeds 20%, the residence time of the flue gas in the high-temperature region is more than 2 s, which can effectively curb the generation of dioxin. When the biomass blending ratio is 20%, and the normalized stoichiometric ratio (2nurea/nNO) of urea injected into the SNCR is 1.1, the NOx concentration at the outlet is 230.08 mg/Nm3, which satisfies the NOx emission standard of less than 250 mg/Nm3.

A two-fluid model simulation of an industrial moving grate waste incinerator

CFD modelling and simulation is an effective means of optimizing the design and operation of moving grate waste incinerators. Conventional approach models the grate combustion and the furnace combustion separately by using an in-bed/over-bed coupling procedure. In this paper, a comprehensive two-fluid reacting model that integrates the gas-solid grate incineration and the gas turbulent combustion in one scheme is developed for industrial incinerators. Realistic grate geometry and direct simultaneous coupling of the fuel bed and the freeboard gas phase are realized. According to different treatments of the solid phase, the whole incinerator is divided into three regions, namely the packed bed region, the fall region and the furnace region. The kinetic theory of granular flow (KTGF) is introduced to describe the rheological properties of waste particles, and the Ergun model is used for the gas-solid drag. Thermal conversion of wastes is characterized by the heterogeneous reactions of moisture evaporation, devolatilization, char-O₂ combustion and the homogeneous reactions of hydrocarbons combustion. Distributions of temperatures and gas species are predicted and validated by measurements. Particle properties are calculated to reveal the grate incineration characteristics. Effects of waste throughput on the incineration are also investigated. Overall, the present model provides a new methodology of in-bed and over-bed integration for the moving grate incinerator simulation.

CFD Steady Model Applied to a Biomass Boiler Operating in Air Enrichment Conditions

A numerical model is proposed to perform CFD simulations of biomass boilers working in different operating conditions and analyse the results with low computational effort. The model is based on steady fluxes that represent the biomass thermal conversion stages through the conservation of mass, energy, and chemical species in the packed bed region. The conversion reactions are combined with heat and mass transfer submodels that release the combustion products to the gas flow. The gas flow is calculated through classical finite volume techniques to model the transport and reaction phenomena. The overall process is calculated in a steady state with a fast, efficient, and reasonably accurate method, which allows the results to converge without long computation times. The modelling is applied to the simulation of a 30 kW domestic boiler, and the results are compared with experimental tests with reasonably good results for such a simple model. The model is also applied to study the effect of air enrichment in boiler performance and gas emissions. The boiler operation is simulated using different oxygen concentrations that range from 21% to 90% in the feeding air, and parameters such as the heat transferred, fume temperatures, and emissions of CO, CO2, and NOx are analysed. The results show that with a moderated air enrichment of 40% oxygen, the energy performance can be increased by 8%, CO emissions are noticeably reduced, and NOx remains practically stable.

Modelling grate combustion of biomass and low rank fuels with CFD application

A reactor cascade model is used for the numerical simulation of the fixed bed combustion of biomass, municipal solid waste and other low rank fuels. Non-spherical particle geometries are accounted for via their plain specific surface. The model is able to reproduce key quantities like ignition delay time, ignition rate, reaction front velocity and mass conversion rate in close agreement with experimental data. This provides a basis to transfer the fixed bed results to the situation of continuous combustion on forward acting grates. A simplified numerical procedure is presented for this purpose. It allows to estimate the structure of a fuel bed moving on the grate as well as the overbed gas temperature and composition. These data serve as input for separate CFD applications which simulate the gas flow in small scale and in industrial scale power plants together with the associated heat transfer processes. Validation of the modelling is presented for lab scale and bench scale experiments as well as for an industrial municipal solid waste incinerator.

A mathematical model for municipal solid waste management - A case study in Hong Kong

With the booming economy and increasing population, the accumulation of waste has become an increasingly arduous issue and has aroused the attention from all sectors of society. Hong Kong which has a relative high daily per capita domestic waste generation rate in Asia has not yet established a comprehensive waste management system. This paper conducts a review of waste management approaches and models. Researchers highlight that mathematical models provide useful information for decision-makers to select appropriate choices and save cost. It is suggested to consider municipal solid waste management in a holistic view and improve the utilization of waste management infrastructures. A mathematical model which adopts integer linear programming and mixed integer programming has been developed for Hong Kong municipal solid waste management. A sensitivity analysis was carried out to simulate different scenarios which provide decision-makers important information for establishing Hong Kong waste management system.

Modelling and simulation of wood chip combustion in a hot air generator system

This study focuses on modelling and simulation of horizontal moving bed/grate wood chip combustor. A standalone finite volume based 2-D steady state Euler–Euler Computational Fluid Dynamics (CFD) model was developed for packed bed combustion. Packed bed combustion of a medium scale biomass combustor, which was retrofitted from wood log to wood chip feeding for Tea drying in Sri Lanka, was evaluated by a CFD simulation study. The model was validated by the experimental results of an industrial biomass combustor for a hot air generation system in tea industry. Open-source CFD tool; OpenFOAM was used to generate CFD model source code for the packed bed combustion and simulated along with an available solver for free board region modelling in the CFD tool. Height of the packed bed is about 20 cm and biomass particles are assumed to be spherical shape with constant surface area to volume ratio. Temperature measurements of the combustor are well agreed with simulation results while gas phase compositions have discrepancies. Combustion efficiency of the validated hot air generator is around 52.2 %.

Effect of ash content on the combustion process of simulated MSW in the fixed bed

This paper experimentally and numerically investigates the effects of ash content on the combustion process of simulated Municipal Solid Waste (MSW). A fixed-bed experimental reactor was utilized to reveal the combustion characteristics. Temperature distributions, ignition front velocity, and the characteristics of gas species' release were measured and simulated during the combustion process. In the present work, the two-dimensional unsteady mathematical heterogeneous model was developed to simulate the combustion process in the bed, including the process rate model as well as NOx production model. The simulation results in the bed are accordant with the experimental results. The results show that as ash content increases, the lower burning rate of fuel results in char particles leaving the grate without being fully burned, causing a loss of combustible material in the MSW in a fixed bed and therefore reducing the combustion efficiency and increasing the burning time of the MSW.

Study on the behavior of heavy metals during thermal treatment of municipal solid waste (MSW) components

Laboratory experiments were conducted to investigate the volatilization behavior of heavy metals during pyrolysis and combustion of municipal solid waste (MSW) components at different heating rates and temperatures. The waste fractions comprised waste paper (Paper), disposable chopstick (DC), garbage bag (GB), PVC plastic (PVC), and waste tire (Tire). Generally, the release trend of heavy metals from all MSW fractions in rapid-heating combustion was superior to that in low-heating combustion. Due to the different characteristics of MSW fractions, the behavior of heavy metals varied. Cd exhibited higher volatility than the rest of heavy metals. For Paper, DC, and PVC, the vaporization of Cd can reach as high as 75% at 500 °C in the rapid-heating combustion due to violent combustion, whereas a gradual increase was observed for Tire and GB. Zn and Pb showed a moderate volatilization in rapid-heating combustion, but their volatilities were depressed in slow-heating combustion. During thermal treatment, the additives such as kaolin and calcium can react or adsorb Pb and Zn forming stable metal compounds, thus decreasing their volatilities. The formation of stable compounds can be strengthened in slow-heating combustion. The volatility of Cu was comparatively low in both high and slow-heating combustion partially due to the existence of Al, Si, or Fe in residuals. Generally, in the reducing atmosphere, the volatility of Cd, Pb, and Zn was accelerated for Paper, DC, GB, and Tire due to the formation of elemental metal vapor. TG analysis also showed the reduction of metal oxides by chars forming elemental metal vapor. Cu2S was the dominant Cu species in reducing atmosphere below 900 °C, which was responsible for the low volatility of Cu. The addition of PVC in wastes may enhance the release of heavy metals, while GB and Tire may play an opposite effect. In controlling heavy metal emission, aluminosilicate- and calcium-based sorbents can be co-treated with fuels. Moreover, pyrolysis can be a better choice for treatment of solid waster in terms of controlling heavy metals. PVC and Tire should be separated and treated individually due to high possibility of heavy metal emission. This information may then serve as a guideline for the design of the subsequent gas cleaning plant, necessary to reduce the final emissions to the atmosphere to an acceptable level.

Removal of toxic and alkali/alkaline earth metals during co-thermal treatment of two types of MSWI fly ashes in China

This study aims to vaporize heavy metals and alkali/alkaline earth metals from two different types of fly ashes by thermal treatment method. Fly ash from a fluidized bed incinerator (HK fly ash) was mixed with one from a grate incinerator (HS fly ash) in various proportions and thermally treated under different temperatures. The melting of HS fly ash was avoided when treated with HK fly ash. Alkali/alkaline earth metals in HS fly ash served as Cl-donors to promote the vaporization of heavy metals during thermal treatment. With temperature increasing from 800 to 900°C, significant amounts of Cl, Na and K were vaporized. Up to 1000°C in air, less than 3% of Cl and Na and less than 5% of K were retained in ash. Under all conditions, Cd can be vaporized effectively. The vaporization of Pb was mildly improved when treated with HS fly ash, while the effect became less pronounced above 900°C. Alkali/alkaline earth metals can promote Cu vaporization by forming copper chlorides. Comparatively, Zn vaporization was low and only slightly improved by HS fly ash. The low vaporization of Zn could be caused by the formation of Zn2SiO4, ZnFe2O4 and ZnAl2O4. Under all conditions, less than 20% of Cr was vaporized. In a reductive atmosphere, the vaporization of Cd and Pb were as high as that in oxidative atmosphere. However, the vaporization of Zn was accelerated and that of Cu was hindered because the formation of Zn2SiO4, ZnFe2O4 and ZnAl2O4 and copper chloride was depressed in reductive atmosphere.

Numerical study of radiation effect on the municipal solid waste combustion characteristics inside an incinerator

Due to its advantages of high degree volume reduction, relatively stable residue, and energy reclamation, incineration becomes one of the best choices for Municipal Solid Waste (MSW) disposal. However, detailed measurements of temperature and gas species inside a furnace are difficulty by conventional experimental techniques. Therefore, numerical simulation of MSW incineration in the packed bed and gas flow field was applied. In this work, a three dimensional (3-D) model of incinerator system, including flow, heat transfer, detailed chemical mechanisms, and non-gray gas models, was developed. Radiation from the furnace wall and the flame formed above the bed is of importance for drying and igniting the waste. The preheated air with high temperature is used for the MSW combustion. Under the conditions of high temperature and high pressure, MSW combustion produces a variety of radiating gases. The wavelength-depend radiative properties of flame adopted in non-gray radiation model were obtained from a statistical narrow-band model. The influence of radiative heat transfer on temperature, flow field is researched by adiabatic model (without considering radiation), gray radiation model, and non-gray radiation model. The simulation results show that taking into account the non-gray radiation is essential.

Numerical and experimental studies on effects of moisture content on combustion characteristics of simulated municipal solid wastes in a fixed bed

In order to reveal the features of the combustion process in the porous bed of a waste incinerator, a two-dimensional unsteady state model and experimental study were employed to investigate the combustion process in a fixed bed of municipal solid waste (MSW) on the combustion process in a fixed bed reactor. Conservation equations of the waste bed were implemented to describe the incineration process. The gas phase turbulence was modeled using the k-ε turbulent model and the particle phase was modeled using the kinetic theory of granular flow. The rate of moisture evaporation, devolatilization rate, and char burnout was calculated according to the waste property characters. The simulation results were then compared with experimental data for different moisture content of MSW, which shows that the incineration process of waste in the fixed bed is reasonably simulated. The simulation results of solid temperature, gas species and process rate in the bed are accordant with experimental data. Due to the high moisture content of fuel, moisture evaporation consumes a vast amount of heat, and the evaporation takes up most of the combustion time (about 2/3 of the whole combustion process). The whole bed combustion process reduces greatly as MSW moisture content increases. The experimental and simulation results provide direction for design and optimization of the fixed bed of MSW.

CFD simulation of MSW combustion and SNCR in a commercial incinerator

A CFD scheme was presented for modeling municipal solid waste (MSW) combustion in a moving-grate incinerator, including the in-bed burning of solid wastes, the out-of-bed burnout of gaseous volatiles, and the selective non-catalytic reduction (SNCR) process between urea (CO(NH2)2) and NOx. The in-bed calculations provided 2-D profiles of the gas-solid temperatures and the gas species concentrations along the bed length, which were then used as inlet conditions for the out-of-bed computations. The over-bed simulations provided the profiles of incident radiation heat flux on the top of bed. A 3-dimensional benchmark simulation was conducted with a 750 t/day commercial incinerator using the present coupling scheme incorporating with a reduced SNCR reduction mechanism. Numerical tests were performed to investigate the effects of operating parameters such as injection position, injection speed and the normalized stoichiometric ratio (NSR) on the SNCR performance. The simulation results showed that the distributions of gas velocity, temperature and NOx concentration were highly non-uniform, which made the injection position one of the most sensitive operating parameters influencing the SNCR performance of moving grate incinerators. The simulation results also showed that multi-layer injections were needed to meet the EU2000 standard, and a NSR 1.5 was suggested as a compromise of a satisfactory NOx reduction and reasonable NH3 slip rates. This work provided useful guides to the design and operation of SNCR process in moving-grate incinerators.

Waste incineration in rotary kilns: a new simulation combustion tool to support design and technical change

This article presents a tool based on a simplified model developed for the combustion processes in a rotary kiln incinerator (slightly inclined rotating primary combustion chamber). The model was developed with the aim of supporting the design phase of the incinerator combustion chamber and, at the same time, of investigating possible technical changes in existing plants in order to optimise the combustion process and the dimension of the rotary kiln (length, diameter) as a function of the characteristics of the fed waste. The tool has been applied and the obtained results compared with a real incineration plant operating on healthcare waste located in Rome (Italy). The mass and thermal balances were taken into account, together with kinetic parameters for the combustion of the specific waste stream. The mass balance considered only the major mass components (carbon, hydrogen, oxygen, nitrogen and sulphur). The measured external temperatures appear to be in good agreement with the simulated results. A sensitivity analysis of the plant under different operating conditions was carried out using different input flow rates and excess air ratios, and an assessment was made of the refractory and insulator properties of the kiln's behaviour. Some of the simulated results were used during the periodical maintenance to improve the refractory characteristics in order to reduce the fret and corrosion process.

Simulation of co-incineration of sewage sludge with municipal solid waste in a grate furnace incinerator

Incineration is one of the most important methods in the resource recovery disposal of sewage sludge. The combustion characteristics of sewage sludge and an increasing number of municipal solid waste (MSW) incineration plants provide the possibility of co-incineration of sludge with MSW. Computational fluid dynamics (CFD) analysis was used to verify the feasibility of co-incineration of sludge with MSW, and predict the effect of co-incineration. In this study, wet sludge and semi-dried sludge were separately blended with MSW as mixed fuels, which were at a co-incineration ratios of 5 wt.% (wet basis, the same below), 10 wt.%, 15 wt.%, 20 wt.% and 25 wt.%. The result indicates that co-incineration of 10 wt.% wet sludge with MSW can ensure the furnace temperature, the residence time and other vital items in allowable level, while 20 wt.% of semi-dried sludge can reach the same standards. With lower moisture content and higher low heating value (LHV), semi-dried sludge can be more appropriate in co-incineration with MSW in a grate furnace incinerator.

Mathematical modeling of MSW combustion and SNCR in a full-scale municipal incinerator and effects of grate speed and oxygen-enriched atmospheres on operating conditions

The rising popularity of incineration of municipal solid waste (MSW) calls for detailed mathematical modeling and accurate prediction of pollutant emissions. In this paper, mathematical modeling methods for both solid and gaseous phases were employed to simulate the operation of a 450 t/d MSW-burning incinerator to obtain detailed information on the flow and combustion characteristics in the furnace and to predict the amount of pollutant emissions. The predicted data were compared to on-site measurements of gas temperature, gas composition and SNCR de-NO(X) system. The major operating conditions considered in this paper were grate speed and oxygen concentration. A suitable grate speed ensures complete waste combustion. The predictions are as follows: volatile release increases with increasing grate speed, and the maximal value is within the range of 700-800 kg/m(2)h; slow grate speeds result in incomplete combustion of fixed carbon; the gas temperature at slow grate speeds is higher due to adequate oxygenation for fixed carbon combustion, and the deviation reaches 200K; NO(X) emission decreases, but CO emission and O(2) concentrations increase, and the deviation is 63%, 34% and 35%, respectively. Oxygen-enriched atmospheres promote the destruction of most pollutants due to the high oxygen partial pressure and temperature. The furnace temperature, NO production and CO emission increase as the oxygen concentration increases, and the deviation of furnace exit temperature, NO and CO concentration is 38.26%, 58.43% and 86.67%, respectively. Finally, oxygen concentration is limited to below 35% to prevent excessive CO and NO(X) emission without compromising plant performance. The current work greatly helps to understand the operating characteristics of large-scale MSW-burning plants.

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.

Mathematical modelling of sewage sludge incineration in a bubbling fluidised bed with special consideration for thermally-thick fuel particles

Fluidised bed combustor (FBC) is one of the key technologies for sewage sludge incineration. In this paper, a mathematical model is developed for the simulation of a large-scale sewage sludge incineration plant. The model assumes the bed consisting of a fast-gas phase, an emulsion phase and a fuel particle phase with specific consideration for thermally-thick fuel particles. The model further improves over previous works by taking into account throughflow inside the bubbles as well as the floating and random movement of the fuel particles inside the bed. Validation against both previous lab-scale experiments and operational data of a large-scale industrial plant was made. Calculation results indicate that combustion split between the bed and the freeboard can range from 60/40 to 90/10 depending on the fuel particle distribution across the bed height under the specific conditions. The bed performance is heavily affected by the variation in sludge moisture level. The response time to variation in feeding rate is different for different parameters, from 6 min for outlet H2O, 10 min for O2, to 34 min for bed temperature.

Experimental study on effects of moisture content on combustion characteristics of simulated municipal solid wastes in a fixed bed

A fixed-bed experimental reactor was employed to reveal the combustion characteristics in simulated municipal solid waste (MSW) beds. Temperature distributions, ignition front velocity, bed weight were measured during combustion and gas analysis was also performed for O2, CO2, CO, CxHy, NO, HCN. The moisture content in MSW was varied. For higher moisture content, the drying of material was finished much later and the ignition front temperatures, ignition front velocity and overall burning rate were found to drop. The average concentration of CO and CO2 from the bed was inversely proportional to the moisture level, and the highest concentrations of CO and CxHy were observed at middle level of investigated fuel moisture content. Measurements showed that NO and HCN emitted from the bed and reached the peaks simultaneously with the volatile matters, and the conversion ratio of fuel nitrogen to NO and average concentration of NO decreased with increasing moisture content.

Mathematical modelling of particle mixing effect on the combustion of municipal solid wastes in a packed-bed furnace

Packed bed combustion is still the most common way to burn municipal solid wastes. In this paper, a dispersion model for particle mixing, mainly caused by the movement of the grate in a moving-burning bed, has been proposed and transport equations for the continuity, momentum, species, and energy conservation are described. Particle-mixing coefficients obtained from model tests range from 2.0x10(-6) to 3.0x10(-5)m2/s. A numerical solution is sought to simulate the combustion behaviour of a full-scale 12-tonne-per-h waste incineration furnace at different levels of bed mixing. It is found that an increase in mixing causes a slight delay in the bed ignition but greatly enhances the combustion processes during the main combustion period in the bed. A medium-level mixing produces a combustion profile that is positioned more at the central part of the combustion chamber, and any leftover combustible gases (mainly CO) enter directly into the most intensive turbulence area created by the opposing secondary-air jets and thus are consumed quickly. Generally, the specific arrangement of the impinging secondary-air jets dumps most of the non-uniformity in temperature and CO into the gas flow coming from the bed-top, while medium-level mixing results in the lowest CO emission at the furnace exit and the highest combustion efficiency in the bed.

Improvement of operating conditions in waste incinerators using engineering tools

Operation parameters such as waste feed rate, air supply, and temperature of the gas in incineration plants should be carefully determined for various situations, which include seasonal and annual changes in fuel characteristics, and performance change of the hardware. These changes may cause off-design point operation of the incinerators, which results in many problems in operation of the flue gas treatment system, low-oxygen in the combustion chamber, thermal damage of the incinerator wall, and so on. In this study, an engineering approach using computational tools along with field tests and observation is presented. For computational tools, a 0-dimensional model for heat and mass balance, computational fluid dynamics (CFD), and a global prediction model for dioxin are employed. They play a key role in diagnosing incineration systems and evaluating changes in operating conditions. The typical results of each tool are reported, and examples of improvement in operating performance are described.

Simulation of the flue gas cleaning system of an RDF incineration power plant

Because of the stringent pollutant emission standards introduced with the European Union guidelines for waste incineration, it is very important to optimize the flue gas cleaning systems which are able to result in a low environmental impact according to the emission limits. In this paper a thermochemical model has been proposed for the simulation of the flue gas cleaning system of an RDF incineration plant. The model simulates the operation of the flue-gas treatment section and the combustion section by using a simplified approach. The combustion includes the grate incinerator and the post-combustion chamber, while the cleaning section includes the NO(x) reduction process (urea injection) and the scrubbing of SO(2) and HCl (Ca(OH)(2) as sorbent). The modelling has been conducted by means of ASPEN PLUS code. The simulation results have been validated with the operating data. The model proposed by the authors can be a useful tool in both evaluating the efficiency of the gas cleaning system by verifying the environmental pollution of an incinerator power plant in nominal operating conditions and in forecasting the efficiency of the cleaning system in off-design operating conditions.

Converting moving-grate incineration from combustion to gasification - numerical simulation of the burning characteristics

Waste incineration is a politically sensitive issue in the UK. The major current technology is based on direct combustion of wastes in a moving-grate furnace. However, general public opinion prefers non-direct burning technologies. Waste gasification is one of those nearest technologies available. By reducing the primary air-flow rate through the grate of a packed-bed system, operation of the existing solid-waste incineration equipment can be easily converted from combustion mode to gasification mode without major modification of the hardware. The potential advantages of this are lower dust carry-over in the flue gases, lower bed temperature (and therefore lower NO(x) formation in the bed), simplified gas-treatment procedures and lower running cost, among other benefits. The major disadvantages are, however, reduced throughput of the wastes and possibly higher carbon in the ash at exit. In this study, numerical simulation of both combustion and gasification of municipal solid wastes in a full-scale moving grate furnace is carried out employing advanced mathematical models. Burning characteristics, including burning rate, gas composition, temperature and burning efficiency as a function of operating parameters are investigated. Detailed comparisons between the combustion mode and gasification mode are made. The study helps to explore new incineration technology and optimise furnace operating conditions.