Grate Combustion Research Articles

Experimentation of Grate Combustion of Municipal Solid Waste in Ouagadougou: Influence of Primary and Secondary Air Flow on Thermal Potential

The present work concerns the study of the influence of combustion air flows on the thermal potential during the combustion of a 2 kg mixture of solid model waste from the city of Ouagadougou. This mixture consists primarily of wood, cardboard and plastic. The prototype model is a natural laterite internal-walled grate furnace. During these investigations, we were interested in the temperature variation measured by 4 thermocouples K1 to K4 placed respectively from the waste bed to the top of the furnace. These variations are considered according to a variable primary and secondary air flow. The maximum primary and secondary airflow is 3000 L/min. Measurements are taken throughout the experiment at a step of 18 seconds and a maximum duration of 3 minutes for each case of flow considered. Maximum temperatures reaching 450oC are observed following the injection of secondary air only. This is explained by the supply of oxygen making the combustion more complete. The results show that the injection of primary and secondary air at a flow rate of 1500 L/min gives a more stable temperature result, i.e. 362,18oC 186,82oC 109,27oC 72,41oC respectively for the thermocouples K1, K2, K3, K4. An increase in the primary and secondary air flow to a maximum of 3000 L/min leads to a drop in temperatures due to the dilute internal atmosphere. These results allow to set the optimal operating parameters of the furnace in order to produce more heat for its recovery into electricity through a heat exchanger and a steam turbine.

CFD Modelling and Optimization of Solid Waste Combustion in a 1 MW Fixed Bed Combustion Chamber

The global population and energy consumption grow each year. Therefore, the tendency to use biofuels instead of fossil fuels assist the global requirement. Also, enhancing of the combustion efficiency and minimizing pollutant emissions during the combustion of municipal solid waste (MSW). The present study employed a CFD model to simulate solid fuel combustion in a grate combustion chamber which was commonly used in small-capacity boilers. computational fluid dynamics technique was used to simulate complex solid fuel reactions where 1 MW boiler was supposed for wood and coal combustion. The achievements found that 10%-dried MSW was used as fuel. The results of the study were related to the average gas flow temperatures and composition in the furnace outflow section, as well as the high combustion temperatures of fossil fuels. The results for the average temperature and average chemical molecular fractions (CO, O2, CO2, H2O, and N2) generated at the exit of the combustion process were positive. Thus, it was found that the thermal efficiency of the boiler was significantly affected by the amount of air supplied to the system at a temperature of 273 K and atmospheric pressure. We observed variations in flame morphology, temperature profiles, and corresponding numerical values.

Potassium release and mitigation by additives in different biomass combustion systems

Deposition formation on heat-exchanging surfaces and corrosion limit the use and efficiency of solid biomass combustion systems. The main influencing factors on these ash-related problems are the potassium content, the form in which the potassium is bound to the fuel matrix, and the temperature in the combustion system. To avoid these deposition problems, choosing the right fuel for a particular combustion system and vice versa is important.In this work, the temperature in fluidized bed, grate firing, and suspension combustion systems is analyzed by using the example of the power plants Altenstadt and Avedøre, as well as a European municipal solid waste incineration plant. Additionally, the temperature-resolved potassium release of ten different biofuels is analyzed in an electrothermal vaporization unit (ETV) connected to an inductively coupled plasma optical emission spectrometer (ICP-OES). The potential of additives to mitigate the potassium release is analyzed by adding kaolin and coal fly ash to miscanthus, torrefied wood, and beechwood.The results show that hardly any potassium is released into the gas phase at the typical temperatures of fluidized bed systems. Most of the potassium release occurs between 1000 °C and 1400 °C, the typical temperature range of grate combustion systems. This shows that reducing the temperature of the fuel on the grate could drastically reduce the potassium release. However, due to the negative effect on the efficiency, other measures, such as using additives for capturing potassium species, are favored. At the temperatures in suspension-fired combustion systems, almost all potassium is released into the gas phase, and additives cannot mitigate the release at these temperatures. Additives can shift the potassium release of miscanthus and torrefied wood while showing no positive effect when mixed with beechwood.All in all, this work enables a quantitative comparison of various biofuels and a rough evaluation of the suitability of a fuel regarding the deposition formation for different combustion systems.

The Experimental Analysis of Biochar Combustion in a Traveling Grate Furnace

The current energy policy targets reducing energy dependence and minimizing pollutant emissions. Therefore, with the growing interest in using biomass as an alternative energy source, conducting scientific studies on its behavior and optimizing the respective conversion systems has become imperative. The present study focuses on investigating the combustion of biochar pellets in a laboratory-scale traveling grate furnace at three different bed temperatures: 700, 750, and 800 °C. The biochars were obtained via the carbonization process of Pinus pinaster, Acacia dealbata, and Cytisus scoparius pellets. The biochar combustion was studied using a moving-bed carbon particle burning model, supported by kinetic information which was obtained via the combustion of the same biochars in a bubbling fluidized bed. The diffusive parameter which was representative of this traveling grate combustion technology was determined, particularly the bed bypass factor. The combustion tests were carried out with the incomplete combustion of the char pellets. In general, the increase in biochar size led to a decrease in the bypass factor. However, the furnace temperature did not influence this parameter.

Selection of Ideal MSW Incineration and Utilization Technology Routes Using MCDA for Different Waste Utilization Scenarios and Variable Conditions

Liability to prevent the consequences of an unhealthy situation due to accumulating toxic and hazardous emissions caused by open dumping of municipal solid waste with increasing urbanization has necessitated a renewed thinking on waste disposal. Grate-fired incineration systems were adopted by urban management in the past and present, but with criticism due to the formation of airborne emissions. Improved combustion methods like fluidized beds are now propagated because of current requirements like efficient energy recovery potential, stricter emission norms, adaptability with urban growth, adaptability to co-firing with other waste like biomass, edible oil wastes or industrial effluent, and integration with conventional energy generation. Such a comprehensive and futuristic approach is more sustainable for the community. A multi-criteria decision-making tool is used to identify the best technology option between grate combustion and fluidized bed combustion for disposing and energy recovery from waste. A total of 10 different collection and disposal options involving two combustion methods, namely, grate combustion and fluidized bed method, are considered. Utilization of the energy is done for three end uses, namely, power generation, water distillation, and district cooling. Two different regions in an arid climate zone are considered for this study under two types of scenarios, namely with recycling and without recycling. The different options are prioritized based on their overall ranking using five major performance factors.

Reconstruct the biomass particles fields in the particle-fluid problem using continuum methods by applying the physics-informed neural network

The motion of particles in the moving grate combustion chamber is used as the case study. These problems are categorized as particle-fluid problems. They are typically solved using Lagrangian-Eulerian methods, one of which is the coupling between the discrete element method (DEM, which is applied to the particles phase) and the computational fluid dynamics method (CFD, which is applied to the fluid phase). The current study's objective is to avoid coupling and instead, focusing on using the CFD method only. There are dense piles of particles moving on the grates in the biomass combustion chamber. We assumed the dense particles' behaviors similar to the fluid, and then, applied the fluid governing equations to the particles phase. The virtual fields of the velocities, pressure, and density are specified for the particles' phase. Afterward, the physics-informed neural network (PINN) is used to reconstruct particles' fields and additionally to investigate the capability of the predicted fields to satisfy the fluid governing equations. This model has the benefit of reconstructing the particles' fields without the need for boundaries and initial conditions. The precision of the model is assessed by comparing the test data set with the exact data obtained from the eXtended discrete element method (XDEM is an in-house software). It is demonstrated that the trained neural network delivered high accuracy and is capable of predicting all outputs with an error value of less than 2 percent. Additionally, to choose the optimum architecture for the neural network, the effect of the number of hidden layers and neurons is studied.

Fluidised-bed incineration bottom ash as the sole precursor of alkali-activated binders: A comparison with bottom ash from grate incinerators

The novel formulation of alkali-activated binders (AABs) using incineration bottom ash (IBA) from fluidised bed (FB) combustion technology as a sole precursor was assessed. In addition, IBA-FB was compared with IBA from moving grate (IBA-MG) combustion technology. The AABs formulated with IBA-FB (AAB-FB) and IBA-MG (AAB-MG) were also evaluated from a physicochemical, mechanical, and environmental perspective. The results revealed that specific surface area in IBA-FB hinders the retention of kneading water of fresh pastes, affecting the workability, consistency, porosity, and mechanical strength of AAB-FB. Moreover, the low calcium content of IBA-FB requires that the formulated AABs must be cured at higher curing temperatures to ensure the formation of (N,C)ASH gels, while the high calcium content of IBA-MG promotes the formation of CSH gels at room temperature. The compressive strength results demonstrated that AAB-MG (≈ 11 MPa) and AAB-FB (≈ 8 MPa) could be used for non-structural purposes.

Investigation of the release behaviour of caesium and strontium during the incineration of municipal solid waste in fluidized beds

In this study the behaviour of the trace components caesium (Cs) and strontium (Sr) in a fluidized bed municipal waste incineration is investigated. Doped RDF (refuse derived fuel) was combusted and bed and fly ash concentration were measured for varied fuel injection temperatures and doping amounts. The distribution of the trace components was calculated and shows that Cs is mostly transferred to the fly ash fraction. The same was found to a lesser extend for Sr. The influence of both, injection temperature and doping amounts, was mostly inconclusive. The comparison of the distribution with data from grate combustion experiments shows a significantly higher transfer of Cs to the fly ash. This was even more pronounced for Sr and indicates a transfer of Sr mostly by entrainment of coarse particles in the flue gas. The experimental results give an indication for the release behaviour of Cs and Sr in fluidized bed municipal waste incineration and relevant influencing factors.

Stochastic gradient descent algorithm for the predictive modelling of grate combustion and boiler dynamics

Pressure and water level control are quite challenging in grate-fired boilers due to higher combustion lag. Fluctuation in the pressure and the water level is more evident in the coal-fired grate boilers due to the presence of lower volatile and higher char. This results in suboptimal operation and poor performance of the boiler. This paper presents a novel predictive and dynamic simulation model for the drum dynamics analysis of a grate-fired boiler by combining a data-driven model and a thermodynamic model. A data-driven methodology is employed for the estimation of combustion, heat transfers, and circulation performance of the boiler. A novel thermodynamic model is proposed for the boiler dynamics of a hybrid boiler. The proposed data-driven model has been integrated with the thermodynamic model to reduce the randomness and improve consistency. Pressure and water level errors are estimated by comparing the predicted value and experimental result and the multi-objective optimisation technique is employed for the minimisation of errors. The Stochastic Gradient Descent algorithm is proposed for its ability to quick learning and adaptation to variations in fuel and combustion characteristics. The model demonstrates good accuracy in predicting the combustion and boiler dynamics of a grate-fired boiler. The proposed model has good potential to be used for the reciprocating grate solid-fuel boiler control in fluctuating load conditions.

Reducing Emissions from Combustion of Grape Residues in Mixtures with Herbaceous Biomass.

The use of grape residues as a renewable energy source for combustion presents various problems. One of these is the excessive production of carbon monoxide and nitrogen oxides. Analyses and combustion tests were performed on white and red grape pomace as well as grape stems. To verify the possibility of a reduction in emissions, straw of Miscanthus sinensis was added to mixtures with red grape pomace. Emission concentrations of carbon monoxide and nitrogen oxides were determined on a grate combustion device with a nominal thermal output of 8 kW under steady-state conditions. In addition to these emission concentrations, the excess air factor and the flue gas temperature were monitored. The results show a high energy content in grape residues. In red grape pomace, the gross calorific value of dry matter reached 22.17 MJ kg−1. Unfavourable properties included high ash and nitrogen contents. During combustion tests on all types of grape residue, the emission concentrations of carbon monoxide were above the legal limit for the combustion of solid fuels. The addition of Miscanthus straw improved the behaviour during combustion. The maximum content of grape pomace in the mixture capable of meeting legislative emission requirements was 50% wt.

Modification of a 240 kWth grate incineration system for oxyfuel combustion of wood chips

The objective of this work is the application of the oxyfuel combustion technology for grate incineration systems to increase the CO2 concentration in the flue gas for higher efficiency when processing the gas for storage or utilization. Therefore, an experimental 240 kWth moving grate combustion facility has been modified to allow the investigation of the combustion behavior under various oxyfuel conditions. Subsequent to the modification, combustion experiments with dried wood chips under air and several oxyfuel atmospheres have been performed. During oxyfuel operation, the oxygen concentration in the gas supply streams towards the furnace has been set to 30 vol.-% dry with recycled flue gas being the rest of the gas composition. The oxygen distribution between primary and secondary zone has been varied (30%/70%, 40%/60%). Additionally, one case with increased oxygen concentration in the primary zone (40 vol.-% dry) has been investigated. The results show that oxyfuel grate incineration of the given fuel in this setup is feasible. However, the combustion behavior differs significantly between air and oxyfuel conditions in the investigated cases. The CO2 concentration in the dry flue gas could be increased to around 73 vol.-% with 5.7 vol.-% excess O2. Compared to the reference air cases the emission rates of CO were also increased during oxyfuel operation. However, it seems manageable given enough excess oxygen (∼7 vol.-% dry). Although the temperature at the end of the furnace is comparable among the cases, oxyfuel combustion leads to lower maximum temperatures along the combustion chamber. Differences between 150 °C and 200 °C have been measured among corresponding air and oxyfuel cases. Also, the temperatures of the grate and inside the fuel bed are reduced during oxyfuel operation even in the case with increased O2 concentration leading to the impression that the thermal strain on the grate is lower compared to air operation with the same fuel throughput.

Predictive Modelling of Grate Combustion and Boiler Dynamics

Pressure and air to fuel ratio control are extremely difficult in coal-fired grate boilers due to a significant lag in combustion. This leads to suboptimal operation of the boiler and poor efficiency of the plant. This also leads to higher level fluctuation. Fluctuation in pressure, water level and oxygen level are quite evident in the operation of coal-fired grate boilers in fluctuating load conditions. The present work aims to develop a predictive and dynamic simulation model of a coal-fired grate boiler for the prediction of pressure, and water level in fluctuating load conditions and its extension for the prediction of oxygen level. A data-driven approach has been used for the prediction of heat release, distribution of heat transfer, circulation analysis and airflow through the various dampers. This model has been integrated with the boiler dynamics model of a hybrid boiler. Errors in pressure and water level are measured for training data and the multi-objective optimisation method is used for the minimisation of errors. The Batch Gradient Descent method has been used for the minimisation of errors. The proposed integrated model is used for the estimation of heat release and the rate of combustion. Stochiometric combustion calculation is used to predict oxygen level by using the predicted value of airflow and rate of combustion. Root mean squared error is calculated for oxygen level and minimised by the Batch Gradient Descent algorithm. The model has good accuracy in the prediction of boiler pressure and water level and can be extended to improve the boiler controls of a solid fuel fired reciprocating grate boiler in extremely fluctuating load conditions.

Validation of a Numerical Approach for Simulation of the Thermal Decomposition Behaviour of Biomass in Grate Combustion Plants

The overall objective of the modelling work is to develop CFD aided design tools for optimum grate fired biomass-to-energy (BtE) and waste-to-energy (WtE) plants. An important part of this work has been to develop a flexible detailed transient fuel-bed model taking into account drying, pyrolysis, and char combustion/gasification, for different fuels (i.e. MSW fractions, and softwood and hardwood, including their bark and GROT (branches and treetops)). The fuel-bed model has been implemented in a CFD tool, ANSYS Fluent.The fuel bed consists of a large number of thermally thick particles. In this work, the fuel bed model is made up of representative particles, and the motion of every representative particle is individually tracked (Lagrangian tracking through Fluent's Discrete Phase Model). Thermochemical degradation and conversion of the representative particles are calculated by a thermally thick single particle model (SPM), with boundary conditions obtained from the solutions of the gas phase equations. The SPM model then provides sources to the gas-phase equations. In the modelling approach, the gas phase is solved using the Reynolds Averaged Navier-Stokes (RANS) equations. Under the given conditions, the gas flow in the bed is laminar.The developed model was validated against detailed experimental results from pyrolysis of dried spruce wood pellets in an electrically heated fixed bed reactor, with varying final pyrolysis temperature (600-800°C), heating rate (5-20 K/min) and purge gas composition (none, 100% N2 and 90/10% N2/O2). The experimental results included transient temperature measurements in different locations in the reactor and inside the pellets bed throughout the thermal decomposition process, as well as gas measurements of permanent gases.Through the CFD simulations, the main experimental trends could be reproduced, verifying the validity of the detailed modelling approach. This work is a step towards detailed modelling of biomass grate combustion units, which is required to improve their environmental and energetic performance.

Reaction mechanisms of wood ash for use as a partial cement replacement

An increase in the produced amounts of wood ashes (WAs) is seen worldwide. In this study, the potential use of two representative WAs as a partial cement replacement is investigated. The WAs originate from combustion of wood chips by grate combustion (WA1) and circulating fluidised bed combustion (WA2). Both WAs were found to possess hydraulic properties, no pozzolanic properties were detected. A substantial difference in strength development was observed between WA1 and WA2 attributed to the phases formed; WA2 contributing the most, ettringite in addition to other phases were formed, while gypsum formed in WA1 pastes.

Evaluation of BTEX adsorption in industrial effluent through the percolation method using rice husk ash as adsorbent material

The aromatic petroleum hydrocarbons (benzene, toluene, ethylbenzene and xylenes) are highly volatile and toxic to human health and the environment. Adsorption is a physical-chemical process widely used in effluent treatment in which a material called adsorbate has the capacity to retain components dispersed in an aqueous medium on its surface. Studies of waste recovery demonstrate potentialities and more noble uses than the final disposal. In this sense, the ashes generated from the combustion of rice husk can be used as a by-product with various applications; in the case of this work, as adsorbent material, due to its amorphism and its high surface area. This study aimed to evaluate the adsorption of the aromatic components of a petrochemical effluent (formation water) with the use of two types of rice husk ash generated from two different grate combustion processes in a filtration system under positive pressure. The ashes were segregated and characterized according to their physical, chemical, mineralogical, and microstructural properties. The adsorption procedure by percolation consisted of 1L of petrochemical effluent for 45 g of adsorbent material. The samples of raw effluent were analyzed by gas chromatography. The results proved that the percentage of removal of BTEX composts in the segregated ash I did not potentialize the effect of the adsorption, being higher in the raw ash, with values of 90.2; 94.9; 83.3 and 100 %, while in ash II the effect of the segregation influenced the adsorption process only for the benzene and toluene, with removal values that went from 55.08 to 69.14 % and 69.23 to 75 %, respectively, in both of the ashes studied. The results obtained were inferior to those recommended by the legislation. Thus, the different combustion temperatures of the husk influenced in the characteristics, adsorption capacity, and BTEX removal efficiency by the adsorbate. Therefore, the positive filtration system can be used as a final polishing step in the treatment of petrochemical effluents, replacing conventional systems that use activated carbon.

Phase development and mechanical response of low-level cement replacements with wood ash and washed wood ash

A significant increase is seen worldwide in the amounts of wood ashes (WAs) produced by the heating and power industry. This study investigates the potential utilisation of two different WAs as low-level cement replacements, in an untreated and a washed version. Phase development and mechanical response were investigated. Results indicated ettringite formation mainly to contribute to the strength development until excessive formation. Indications of an optimum SO3/C3A ratio for mixtures with a low-level cement replacement with WA was between 0.4 and 0.5. Washed WA originating from grate combustion appeared to be most promising of the tested WAs in cement-based materials.

Evaluation of Co and Nox Emissions in Real-Life Operating Conditions of Herbaceous Biomass Briquettes Combustion

Abstract The issue of carbon monoxide and nitrogen oxides emissions into the atmosphere is very current. This article thus focuses on the assessment of elemental composition of selected herbal biomass species and emission concentrations during combustion in a commonly available grate combustion device for briquetted fuel. In tests, emission concentrations were evaluated in contrast to the oxygen concentration in flue gas and flue gas temperatures. Samples of camelina (Camelina sativa), giant miscanthus (Miscanthus gigantheus), reed canary grass (Phalaris arundinacea L.) and sorghum (Sorghum bicolor) were used. Elementary and stoichiometric combustion analyses were conducted for these samples (LECO AC-600 semi-automatic calorimeter, CHN628 + S elemental analyser and LECO TGA-701 analyser). Analyses of C, H, N and S concentrations and calorific values showed that samples of briquetted herb biomass had insignificant differences and could be used for energy purposes without limitation. The limiting factor was the high amount of ash, which amounted to 6.59% of dry weight in reed canary grass briquette sample. Furthermore, a high percentage (1.91% wt.) of nitrogen was observed in reed canary grass briquettes. Such a high amount of nitrogen during combustion tests resulted in an increase in NOx emission levels in flue gas of the Phalaris arundinacea L. sample, in which it reached the maximum concentration of 375.20 mg·m−3. The combustion tests showed that even under steady-state conditions, high concentrations of carbon monoxide could not be avoided for tested briquetted biofuels without active regulation of the combustion process.

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.

Demonstrating Fuel Design To Reduce Particulate Emissions and Control Slagging in Industrial-Scale Grate Combustion of Woody Biomass

The demand for increased overall efficiency, improved fuel flexibility, and more stringent environmental legislations promotes the development of new fuel- and technology-related concepts for the b ...

A numerical investigation of NOx concentration at the outlet of a coal-fired chain grate boiler

ABSTRACTA 3D computational fluid dynamic (CFD) simulation is performed on a 35t/h grate boiler to investigate the compound combustion, consisting of grate firing of lump coal on the fire grate and suspension combustion of pulverized coal sprayed in the middle of the stoker. A grate boiler-based low-nitrogen combustion method is proposed in the present study. The influences of this technology on the boiler temperature field and flue gas components as well as NOxare studied by comparing two different combustion modes before and after the transformation. Results show that the compound combustion technology can significantly improve the boiler efficiency and reduce the NOx concentration at the outlet. Therefore, the proposed method may have favorable applications in different industries. It is found that the boiler NOx emission can reach an optimal status, when the pulverized coal feed rate of the burner and the proportion of pulverized coal are set to 20m/s and 20%, respectively. It is observed that when the forward spinning arrangement form is adopted, the emission can be reduced up to 58.08%. The NOx at the outlet of a 35t/h boiler was reduced by 35.71% after retrofitting, which is in consistency with the obtained results.