Grate Firing Research Articles

Результаты испытаний топливной присадки на котле со слоевым сжиганием угля

Результаты испытаний топливной присадки на котле со слоевым сжиганием угля

Numerical Simulation of Nitrogen Oxide Emission Characteristics of a Retrofitted Chain Grate Boiler

Raw coal is burned in furnaces or chain grate boilers. In this work, we studied the chamber combustion mechanism in a chain grate boiler (CGB) with a capacity of 20 t/hr. A mathematical model was built using Computational Fluid Dynamics (CFD). It was found that chamber combustion with swirl burners instead of grate firing has a positive effect on NO emissions. NO emissions dropped from 700 ppm in case 5 to 250 ppm in case 1. The calculated and measured NO emissions agreed well, with a discrepancy of less than 15%, except on plane 2 for four calculated cases. For the temperature field, the discrepancy between the measured and simulated values was below 12% for the four cases. These discrepancies between calculated and measured results are acceptable. This study provides a good idea for the design and reconstruction of CGBs. To reduce NO emissions, the reconstruction method can be used for CGBs instead of the grate firing system.

Influence of Oxidation–Reduction Layering on Fuel Nitrogen Oxide Emissions during a Char Grate-Fired Process

Previous studies have found that the nitric oxide (NO) trend above the fuel seam of a chain boiler is opposite that of a reciprocating grate boiler. We have researched the factors that influence fuel NO emissions during the char grate-fired process by changing the char/oxygen (O2) ratio in a small-scale one-dimensional fixed-bed system. With an increase in the char/O2 ratio compared to the mass loss of char, the advent of excess O2 is delayed and carbon monoxide (CO) is generated. As a result, oxidation–reduction layering of char occurs. Char grate firing leads to an increase in char mass of the complete oxidation layer, and a thicker ash layer covers the char surface. The reduction layer and low-O2 region of the oxidation layer favor NO reduction; therefore, NO emission is delayed or shows a double-peak trend. The ash layer covering and a long residence time at a high temperature increase the NO generation rate of the oxidation layer. However, the overall NO generation rate is controlled by the reduction...

Simulation of Reacting Moving Granular Material in Furnaces and Boilers An Overview on the Capabilities Of the Discrete Element Method

A brief overview is given on the capabilities and on the current limitations of the Discrete Element Method (DEM) coupled with Computational Fluid Mechanics (CFD) to simulate chemical reacting moving granular material. An approach to resolve the internal transport and reaction phenomena in particles of complex geometry is presented. Heat and mass transfer from and to the particles are accounted for as well as heat transfer between particles and between particles and a surrounding gas phase including radiation. Gas phase reactions outside the particle interact with inner particle processes.Examples will be shown to demonstrate the capabilities of DEM/CFD coupling. These examples are an industrial scale lime shaft kilns, the simulation of a domestic pellet stove and a grate firing system for the incineration of municipal waste. The advantages of a DEM/CFD approach will be highlighted but also the still existing drawbacks and limitations are discussed. The paper ends with an outlook on necessary developments to make DEM/CFD a standard engineering tool for chemically reacting granular material.

Combustion of Refined Renewable Biomass Fuel (RRBF) in a bubbling fluidized bed

The outlet of a mechanical biological treatment plant for mixed municipal solid waste is further processed to produce RRBF (Refined Renewable Biomass Fuel) within the frame of the EU Life+ project MARSS (Material Advanced Recovery Sustainable Systems). The input for the MARSS processing is dry and has small particle sizes below 40 mm, which makes it impossible to be burnt in standard grate firing systems. The main purpose of this study was to examine whether RRBF is a suitable fuel for bubbling fluidized bed combustion for the decentralised production of combined heat and power. RRBF was fed into a bubbling fluidized bed combustion plant with a nominal fuel input of 100 kW in 3 combustion tests. Proximate and ultimate analyses were performed for original RRBF and fly ash. The fuel analyses showed high ash contents between 25 and 37 weight-%, while the lower heating value lay in the range of 10.5–12.9 MJ kg−1. The ash softening temperature was above 1150 °C and therefore no bed agglomeration was observed. Combustion at around 900 °C could be maintained without preheating of the combustion air. The carbon content of the fly ash was about 1 %, which indicates complete combustion in the bubbling fluidized bed with sufficient residence time. The content of phosphorous in the fly ash was above 1 % and therefore this is an interesting material for prospective phosphorous recovery, potentially together with sewage sludge or ash from sewage sludge combustion. RRBF is a suitable solid fuel for fluidized bed combustion.

Measurements of temperature and gas composition within the burning bed of wet woody residues in a 4 MW moving grate boiler

Moving grate firing systems are widely used for biomass combustion. The characteristics of the fuel bed combustion in moving grate boilers are of practical interest as they are directly related to the release of pollutants and affect the furnace efficiency. Measurements of temperature and gas species concentrations inside the fuel bed are necessary to improve our understanding of the highly complex processes involved in biomass combustion. There have been few experimental studies of the fuel bed of industrial scale grate furnaces. The present study measured temperature and gas species concentrations within a thick burning bed of wet woody biomass, in a 4MW reciprocating grate boiler. Measurements were carried out under three different operating conditions through ports located in the wall of the furnace using a stainless steel probe incorporating a thermocouple. Temperatures of about 1000°C were measured close to the grate, indicating intense combustion at the bottom of the fuel bed. The temperature distribution along the bed height showed that different stages of the combustion process take place in horizontally adjacent layers along the grate. Higher flow rates of the primary air resulted in relatively higher CO and lower CO2 and NO concentrations in the fuel bed.

Development of Territories Used as Landfills for Municipal Solid Waste

Multiple methods of development of landfills are available: recreation-based, forestry-based, landscaping etc. But due to the environmental profile of waste-affected areas, forests and parks can hardly be made there. The alternative to those common methods is the construction-based development, i.e. building such facilities as waste sorting stations, ballast and junk fraction briquetting facilities, as well as plants for recycling of construction waste into recycled crushed aggregates and facilities for the decontamination of contaminated soil by grate firing or dumping. This material offers principal theses for feasibility of the constructionbased development of MSW-affected areas, also groups of indicators for comprehensive assessment of the landfill body state are presented.

Modeling of the biomass combustion on a forward acting grate using XDEM

The grate firing system is one of the most common ways for the combustion of biomass because it is able to burn a broad range of fuels with only little or even no requirement for fuel preparation. In order to improve the fuel combustion efficiency, it is important to understand the details of the thermochemical process in such furnaces. However, the process is very complex due to many involved physical and chemical phenomena such as drying, pyrolysis, char combustion, gas phase reaction, two phase flow and many more. The main objective of this work is to study precisely the involved processes in biomass combustion on a forward acting grate and provide a detailed insight into the local and global conversion phenomena. For this purpose, XDEM as an Euler–Lagrange model is used, in which the fluid phase is a continuous phase and each particle is tracked with a Lagrangian approach. The model has been compared with experimental data. Very good agreements between simulation and measurement have been achieved, proving the ability of the model to predict the biomass combustion under study on the grate.

NOx Emission Model for the Grate Firing of Biomass Fuel

Establishing an appropriate NOX production and decomposition model is of practical significance to the grate firing of biomass fuel. This study used the CHEMKIN software package to simulate the combustion process of biomass pyrolysis and char combustion. Through rate of production (ROP) analysis and simplification based on GRI-Mech3.0, the 15-step reaction mechanism of volatile-N converting into NOX and the 12-step reaction mechanism of char-N converting into NOX were specified. It was found that in the NOX generated from the fuel, N was mainly in the form of NO and N2O. HCN and NH3 were the important intermediate products. NH3 was mainly converted into NO and some converted into N2O, while HCN mainly consumed NO and produced N2O. According to the transfer characteristics of the biomass fuel nitrogen, the NOX production and decomposition model of the biomass fuel nitrogen in grate furnace firing was established. A simulation computation on the NOX production was implemented for an actual furnace. The established model was confirmed reliable through the comparison of field test data and simulation results.

APPLICATION OF NUMERICAL MODELLING TO BIOMASS GRATE FURNACES

The direct combustion of the biomass is the most advanced and mature technology in the field of energetic biomass utilisation. The legislations on the amount of emitted pollutants and the plant efficiency of biomass combustion systems are continually being restricted. Therefore constant improvement of the plant efficiency and emission reduction is required Numerical modelling is gaining increasing importance for the development of biomass combustion technologies. In this paper an overview about the numerical modelling efforts deal with the most relevant phenomena in biomass grate firing systems is given. The numerical modelling results in a deeper understanding of the underlying processes in biomass combustion plants. Therefore, it leads to a faster and safer procedure of development of a new technology.

Nitrous oxide and methane emissions and nitrous oxide isotopic composition from waste incineration in Switzerland

Solid waste incineration accounts for a growing proportion of waste disposal in both developed and developing countries, therefore it is important to constrain emissions of greenhouse gases from these facilities. At five Swiss waste incineration facilities with grate firing, emission factors for N2O and CH4 were determined based on measurements of representative flue gas samples, which were collected in Tedlar bags over a one year period (September 2010–August 2011) and analysed with FTIR spectroscopy. All five plants burn a mixture of household and industrial waste, and two of the plants employ NOx removal through selective non-catalytic reduction (SNCR) while three plants use selective catalytic reduction (SCR) for NOx removal.N2O emissions from incineration plants with NOx removal through selective catalytic reduction were 4.3±4.0g N2O tonne−1 waste (wet) (hereafter abbreviated as t−1) (0.4±0.4g N2O GJ−1), ten times lower than from plants with selective non-catalytic reduction (51.5±10.6g N2O t−1; 4.5±0.9g N2O GJ−1). These emission factors, which are much lower than the value of 120g N2O t−1 (10.4g N2O GJ−1) used in the 2013 Swiss national greenhouse gas emission inventory, have been implemented in the most recent Swiss emission inventory. In addition, the isotopic composition of N2O emitted from the two plants with SNCR, which had considerable N2O emissions, was measured using quantum cascade laser spectroscopy. The isotopic site preference of N2O – the enrichment of 14N15NO relative to 15N14NO – was found to be 17.6±0.8‰, with no significant difference between the two plants. Comparison to previous studies suggests SP of 17–19‰ may be characteristic for N2O produced from SNCR. Methane emissions were found to be insignificant, with a maximum emission factor of 2.5±5.6g CH4 t−1 (0.2±0.5g CH4 GJ−1), which is expected due to high incinerator temperatures and efficient combustion.

Euler-Euler simulation of wood chip combustion on a grate - effect of fuel moisture content and full scale application

Nowadays, it is common practice to perform CFD calculations for optimisation purposes of technical biomass combustion applications. A numerical model for wood chip combustion on grate firing arrangements has been developed. The model is based on an Euler-Euler approach, enabling a detailed multiphase description of the combustion chamber in terms of flow, turbulence and heat transfer. The model explicitly accounts for interactions between bed and freeboard region and comprises a global description of the whole incineration process associated with wood combustion. For validation purposes, the effect of fuel moisture content in a 240 kWth test facility has been observed experimentally and the results are opposed to the model predictions. Additionally, measurements within a 58 MWth full scale grate firing system have been conducted and the scalability of the numerical model towards industrial applications is investigated.

Ash transformation and deposit build-up during biomass suspension and grate firing: Full-scale experimental studies

An attractive option for reducing the net CO2 emissions is to substitute coal with biomass in large power plant boilers. However, the presence of chlorine (Cl) and alkali metals (K, Na) in biomass may induce large operational problems due to ash deposit formation on the superheater tubes. The aim of this study was to investigate ash transformation and deposition behavior in two biomass-fired boilers, firing wheat straw and/or wood. The influence of straw firing technology (grate and suspension) on the ash transformation, deposit formation rate and deposit characteristics has been investigated. Bulk elemental analysis of fly ashes revealed that fly ash from suspension firing of straw has high contents of Si, K and Ca, while fly ash from straw firing on grate was rich in the volatile elements K, Cl and S. Investigations of deposit formation rates were made in the superheater and convective pass regions of the boilers by use of an advanced online deposit probe. During straw firing on grate, the measured deposit formation rate was close to 38g/m2/h. Data from straw suspension firing showed a deposit formation rate of 41g/m2/h. The deposit formation rates during straw suspension firing and straw grate firing were on similar levels. This was observed even though the concentration of fly ash in the flue gas was significantly higher during straw suspension firing. The influence of co-combustion of wood with straw on deposit formation rate, probe heat uptake and deposit characteristics was also investigated during suspension firing conditions. Data from 35% straw suspension firing with wood showed a deposit formation rate of 33g/m2/h for the first 12h. The deposit formation rate increased to 41g/m2/h with 100% straw firing. The probe heat uptake reduction up to 40h of exposure time was 3.0, 7.3, 8.4 and 16.5kW/m2 during 35, 65, 80 and 100% straw firing, respectively.

Influence of particle diameter and material properties on mixing of monodisperse spheres on a grate: Experiments and discrete element simulation

The Discrete Element Method (DEM) can help to obtain detailed information on the mixing process within a bed of fuel particles on grate firing systems. In a first study it was shown by comparing experiments on a generic grate with DEM simulations that for a granulate consisting of monodisperse spheres the influence of grate stroke length and bar velocity can be reproduced. Prior to tackling particle assemblies which are polydisperse and consist of particles of complex shape it has to be shown that the influence of particle diameter and material properties of the particles is described correctly by DEM simulations. Thus experiments with monodisperse spheres of three different diameters (5, 10, 20mm) and two materials (plastic, wood) are compared to DEM simulations. The mixing process is measured at the front side wall and at the top surface of the particle bed and quantified by image analysis. Both visual observation and mixing parameter results attest a very good agreement between experiments and simulations. The influence of particle diameter and material properties measured in the experiments can be reproduced by DEM simulations.

Leachate formation and characteristics from gasification and grate incineration bottom ash under landfill conditions

Characteristics and formation of leachates from waste gasification and grate firing bottom ash were studied using continuous field measurements from 112m3 lysimeters embedded into landfill body for three years. In addition, the total element concentrations of the fresh ash were analysed and laboratory batch tests were performed to study leachate composition. The three-year continuous flow measurement showed that about one fifth of the leachates were formed, when the flow rate was >200l/d, covering <3.5% of the study time. After three years, the liquid/solid-ratio for the quenched grate ash was 1 (l/kg (d.m.)) and for the initially dry gasification ash 0.4 (l/kg (d.m.)). The low initial water and residual carbon content of the gasification ash kept the leachate pH at a high level (>13) major part of the study. In the grate ash leachate pH was lower (<8) due to the presence of organic carbon and biodegradation indicated by biological oxygen demand and redox potential measurements. In the gasification ash the high pH probably delayed leaching of major elements such as Ca, therefore, raising the need for a longer after-care period. The high pH also explains the higher leaching of As from the gasification ash compared to the grate ash both in the batch test and under landfill conditions.

CFD simulation of wood chip combustion on a grate using an Euler–Euler approach

Due to the increase of computational power, it is nowadays common practice to use CFD calculations for various kinds of firing systems in order to understand the internal physical phenomena and to optimise the overall process. Within the last years, biomass combustion for energy purposes has gained rising popularity. On an industrial scale, mainly grate firing systems are used for this purpose. Generally, such systems consist of a dense-packed fuel bed on the grate and the freeboard region above, where in the field of numerical modelling, it is common practice to use different sub-models for both zones. To avoid this, the objective of this paper is the presentation of a numerical model including a detailed three-dimensional description of the fuel bed and the freeboard region within the same CFD code. Because of the implementation as an Eulerian multiphase model, both zones are fully coupled in terms of flow and heat transfer, and appropriate models for the treatment of turbulence, radiation, and global reactions are presented. The model results are validated against detailed measurements of temperature and gaseous species close to the bed surface and within the radiative section of a 240 kW grate firing test facility.

Discrete element analysis of experiments on mixing and stoking of monodisperse spheres on a grate

Understanding the details of the mixing and stoking process on grate firing systems is crucial for the optimization of the combustion process in waste or biomass incineration plants. The Discrete Element Method (DEM) can help to obtain further information on the mixing process within a bed of fuel particles. Especially the influence of a change in operational parameters can be examined avoiding large experimental effort. In the current paper five simulations for a generic grate are compared with the corresponding experiments. The experiments were carried out throughout an anterior parameter study on mixing and stoking on a grate [Sudbrock F.; Simsek E.; Wirtz S.; Scherer V.: “An experimental analysis of the influence of operational parameters on mixing and stoking of a monodisperse granulate on a grate”, Powder Technology 198, Issue 1, 29–37, 2010] [19]. The system considered is equipped with vertically moving bars which induce stoking. In a first approach monodisperse plastic spheres are used. The grate is encased by a transparent polycarbonate housing which provides optical access to the movement of the particles in the wall planes. The mixing process is measured and quantified by image analysis of the front wall of the grate. The mixing behaviour of the particle assembly observed in experiments and simulation appears to be very similar indicating that DEM is able to predict the particle mixing in the bed. In order to quantify the visual observations the mixing behaviour has been evaluated by different mixing parameters. They are compared in dependence of the number of strokes of the grate bars. A good agreement between measurements and simulations could be observed.

Improvement of a Combustion Unit Based on a Grate Furnace for Granular Dry Solid Biofuels Using CFD Methods

The steadily increasing domestic and industrial energy demand can only be satisfied by optimal utilization of all available resources. The aim of this project is the design and construction of an improved small-scale combustion unit for biofuels like wood, straw pellets, and especially grain. Using computational fluid dynamics (CFD) methods and measurement data from a pilot unit, this study contributes to the continuous enhancement of the biomass firing technology by addressing the commonly known problems regarding emissions and ash melting. Based on the calculated results improvements for the existing prototype, new geometry has been suggested and was planned to be included in the design of a new 1.5-MW pilot-scale grate firing unit that was to start operation in early 2009.

An estimation of the energy potential of agro-industrial residues in Spain

In this paper, we assess the potential for the generation of electricity in Spain from agro-industrial residues. The industries considered are olive-oil mills, rice mills, wineries, dairy plants, breweries and wood, meat and nut processing plants. The methodology used is based on statistical data, and is integrated into a Geographical Information System (GIS) from which geo-referenced results are obtained. The waste-to-electricity technologies analyzed are: grate firing followed by steam turbine, co-firing in coal power plants and anaerobic digestion plus internal combustion engine. The combined potential for the agro-industrial residues in Spain is estimated at 2625 ktoe/year of primary energy (1.85% of the primary-energy consumption in Spain in 2008). Olive mill and wood processing residues have the largest energy potentials. Comparisons are presented with (partial) results from other studies. Considering only profitable plants, grate-firing followed by steam-turbine cycle is the conversion option with the largest potential, totaling 653 MW e and an electrical generation of 4.57 TWh (1.44% of the gross electrical generation in Spain in 2008). A complete sensitivity analysis is done to investigate the influence of the different economic parameters. A reduction by 50% of the investment costs of grate firing would increase profitable power to 1102 MW e (and production to 7.70 TWh).

An experimental analysis on mixing and stoking of monodisperse spheres on a grate

A better understanding of the mixing and stoking process is crucial for an optimization of the combustion process on grate firing systems. Thus experimental studies were carried out to analyse the response of a particle assembly on varying grate operational parameters. To reduce the number of variables which affect the system a generic grate design was chosen and a material of monodisperse spheres was selected. The grate system applied uses vertically moving parallel bars to induce mixing. Different patterns of bar motion were created by linking the bars in groups of uniform movement. A transparent polycarbonate side wall gives optical access to front layer of spheres. The mixing process was measured and quantified by image analysis of this visible layer. When applying a constant number of bar strokes it is found that the mixing performance is independent of the bar velocity. However, mixing performance increases nearly linearly with the stroke length. It turned out that specific “movement patterns” could be identified which show improved mixing behaviour. The results provided here may also be used for comparison with simulations of the particle mixing with the Discrete Element Method (DEM).