Stoker-fired Boilers Research Articles

Evaluating the Potential for Combustion of Biofuels in Grate Furnaces

The paper assesses the impact of combustion of biofuels produced based on municipal sewage sludge in stoker-fired boilers on the amount of pollutant emissions and examines the tendency of ash deposition of biofuels formed during the combustion process. The combustion tests were performed in a laboratory system enabling simulation of a combustion process present in stoker-fired boilers. The study was conducted for three types of biofuels; i.e., fuel from sewage sludge and coal slime (PBS fuel), sewage sludge and meat and bone meal (PBM fuel) and fuel based on sewage sludge and sawdust (PBT) with particle size of 35 mm and 15 mm. This paper describes and compares the combustion process of biofuels with different granulation and composition and presents the results of changes in emission values of NOx, SO2, CO, and CO2. The emission results were compared with the corresponding results obtained during combustion of hard coal. The results showed that biofuels with lower particle sizes were ignited faster and the shortest ignition time is achieved for fuel based on sewage sludge and coal slime-PBS fuel. Also, the highest NO and SO2 emissions were obtained for PBS fuel. During the combustion of fuel based on sewage sludge and meat and bone meal (PBM), on the other hand, the highest CO2 emissions were observed for both granulations. Biofuels from sludge show a combustion process that is different compared to the one for hard coal. The problems of ash fouling, slagging, and deposition during biofuels combustion were also identified. The tendency for ash slagging and fouling is observed, especially for fuel from sewage sludge and meat and bone meal (PBM) and fuel based on sewage sludge and sawdust (PBT) ashes which consist of meat and bone meal and sawdust which is typical for biomass combustion.

Experimental Research on NOx Emission Characteristics During Coal Combustion in the Stoker-fired Boiler

In this paper, the combustion stability and nitrogen oxides (NOx) emissions test were made in two coal-fired chain grate boilers with hot water supplies of 29MW and 46MW. The investigations aim to clarify the key characteristics of NOx emission, e.g.instability of NOx emission, temperature and gaseous components profiles and the unsteady variation of NOx emission with power level changing. Main conclusions include: 1) In the process of stable combustion, NOx emissions still have±5% fluctuations, it is not stable. 2) The furnace temperature distribution show double temperature peaks. One peak (~1500°C) is located on the burning solid-fuel surface on the chain grate, and the other (~1050°C) is ~5.6m above the arch. 3) With increasing the thermal power, NOx emission shows a dramatic increase, higher around 1.25 times, as due to the rapid adjustment of coal feeding in grate furnace. Those above show the NOx control difficulty and the proper injector location of SNCR (selective non-catalytic reaction), which is key data collected for further NOx reduction retrofits.

CFD modelling of particle size effect on stoker coal­fired boilers combustion

In the previous study, CFD simulation had been developed to predict combustion characteristic on the Fluidized Bed Boiler and Pulverized Boiler. The high demand on coal used for stoker-fired boilers in Indonesia the power plants provide challenge due to low thermal efficiency problem. In this study, CFD simulation is observed to predict the temperature distribution, heat of reaction, CO and CO 2 mass fraction on the stoker-fired boiler. Boiler geometry is modelled as the combustion chamber until the area before economizer. The selection of boundary conditions is set according to the governing equations available in the ANSYS Fluent software. Parameter design of coal which are particle size and properties of coal is determined to investigate the effect of the observed values. Four models are set to provide a combination of particle size and properties of coal. The solution strategy is developed to reduce instability of the simulation process. Coal combustion modelling includes several physical processes that could result in numerical stability issue when all processes are solved at once. The three stages were used to run the solution of the model. Plot of temperature distribution, heat of reaction, CO 2 and CO mass fraction is generated. The maximum temperature in the 1st to 4th model is 1440.95, 1473.85, 1347.72 and 1617.17 [ o K]. The amount of CO produced from each model tends to increase; respectively from the 1 st to 4 th model is 2.314E-07, 5.878E-07, 5.678E-07 and 7.904E-07. Based on the simulation results, it can be seen that the particle size of coal affects the combustion characteristic in the Stoker Coal-Fire Boiler.

Environmental impact of emissions from incineration plants in comparison to typical heating systems

In recent years, five modern municipal waste incineration plants have been built in Poland. Next ones are being constructed and at the same time building of several others is being considered. Despite positive experience with the operation of the existing installations, each project of building a new incinerator raises a lot of emotions and social protests. The main argument against construction of an incineration plant is the emission of pollutants. The work compares emissions from municipal waste incineration plants with those from typical heating plants: in the first part, for comparison large heating plants equipped with pulverized coal-fired boilers (OP-140), stoker-fired boilers (three OR-32 boilers) or gas blocks with heat output of about 100 MW have been selected, while the second part compares WR-10 and WR-25 stoker-fired boilers most popular in our heating industry with thermal treatment systems for municipal waste or refuse-derived-fuel (RDF) with similar heat output. Both absolute emission and impact - immission of pollutants in vicinity of the plant were analyzed.

Protective Coatings in the Power Boilers which are Used to Combust Waste – Surfacing Anticorrosive Layers

Abstract General requirements and technological solutions of incinerating plants were presented. Heat processing of waste in fluidized and stoker-fired boilers were described. A problem of corrosion and the methods of preventing it in boilers used to combust waste were shown. Materials of increased resistance to corrosion used in power industry were characterised.

Approach of using Corn Residue as Alternative Energy Source for Power Production: A Case Study of the Northern Plain Area of Thailand

As Thai's energy security heavily relies on imported crude oil products, the Thai government has promoted the use of alternative energy sources as a substitute for fossil fuel through the Renewable Energy Development Plan. According to this plan, 3,630 MW of electricity should be derived from biomass or agricultural residue in 2021. Although some agricultural residue has been commercially used as fuel in biomass power plants, corn residue is still leftover and has a high energy potential in some regions, e.g. in the northern part of Thailand. This study focuses on the supply chain management in using corn residue as an alternative energy source for power production in Nakorn Sawan and Petchchaboon Provinces. In 2011, almost two million tons of corn residue were generated; about 200,000 tons of which were corncob (HHV of 15-16MJ/kg) and the rest was corn trash/skin and stems (HHV of 13-15MJ/kg). Because of the uniform and compact shape of corncobs, they can be completely utilized as fuel in a conventional stoker-fired boiler for heat production. In contrast, corn trash/skin and stems are leftover in the field or burnt on the field without any energy recovery because they are bulky and non-uniform fuel, with high moisture content, which leads to difficulty in collection and transportation, as well as for thermal conversion. According to the results of this study, a processing unit is necessary to improve the properties of corn trash/skin and stems to be used as solid fuel. This processing unit can be established as a centralized plant or on the field where the residue is generated. Regarding the economic analysis, the overall cost of bio-pellet from corn trash/skin/stems is estimated to be 49.4-56.8 US$ per ton, depending on the pelletizing capacity. The bio-pellets from corn residue can easily be transported to be used as feedstock for heating or power generation by direct combustion, co-firing, or gasification technology.

Iron-containing phases in fly ashes from different combustion systems

Abstract The investigations of iron-containing phases existing in fly ashes were performed using transmission Mössbauer spectrometry. The examined samples of fly ashes were collected from different coal combustion systems, that is, stoker-fired boiler in municipal heating plant and pulverized coal boiler in power plant. Several phases are identified in the samples: iron oxides, paramagnetic aluminosilicate glass with Fe3+ ions and Al2O4-type spinel with Fe2+ ions. It was pointed out that proportions of contents of phases strongly depend not only on the combustion temperature but also on the way of ash collection.

Mixing of Biomass and Coal in a Static Mixer as an Example of Technological Solutions Involving Implementation of Renewable Energy Sources

Abstract The purpose of this article is to demonstrate possibilities of mixing coal with biomass using static mixers. The paper present simple and inexpensive methods for producing fuel mixes for heat-generating plants equipped with stoker-fired boilers of power ranging from 3 up to 50 MW. Static mixer proposed in this article may constitute an alternative for currently used solutions in production facility. Based on statistical analysis, the article demonstrates that five mixing elements of the mixer is optimal number to ensure best biomass (chips and pellets) concentration distribution in a mixture with fine hard coal.

Impact of solid fuel combustion technology on valence speciation of chromium in fly ash

Fly ash (FA) generated in real furnaces was used to evaluate the impact of the kind of the solid fuel burnt and combustion technology on chromium speciation, particularly the presence of Cr(III) and Cr(VI) forms as well as readily and hardly leachable chromium(VI) species in FAs. The FAs originated from a pulverized coal combustion boiler (PCC boiler), a fluidized bed combustion boiler (FBC boiler), a stoker-fired boiler (SF boiler), a municipal solid waste incinerator (MSWI), a cement rotary kiln (CRK) and a modern domestic boiler (DB). The speciation analysis of chromium was carried out by means of extraction followed by catalytic cathodic stripping voltammetry with adsorption of Cr(III)-DTPA complexes (CCSV-DTPA) for determination of Cr(VI) and AAS was used for determination of Cr content.It has been revealed that the antagonistic action of alkali metal compounds and iron oxides plays a crucial role in shaping valence speciation of chromium. According to the proposed transformation path of oxidation of Cr(III) to Cr(VI), hard coal combustion in an SF boiler, an FBC boiler or a domestic boiler will generate FAs with a low Cr(VI) level. Replacing fuel with biomass should create favorable conditions for generating FA enriched with Cr(VI). Relatively high concentrations of Cr(VI) can also be expected in FA generated in the process of high-temperature combustion of coal in PCC boilers.

Iron speciation in coal fly ashes—chemical and Mössbauer analysis

Iron speciation analysis of fly coal ashes was performed using transmision Mossbauer spectrometry as well as chemical methods. The investigated samples of ashes came from stoker-fired boiler and pulverized-fuel boiler (of significantly higher combustion temperature). The Mossbauer spectra show noticeable differences in iron-bearing phases content. In the first case aluminosilicate glass dominates, whereas in the second one—a spinel-type phase.

The impact of coal characteristics on the fouling of stoker-fired boiler convection surfaces

This paper presents a comparison of the degree of fouling in a hot water stoker-fired boiler after it has been fired with two types of coal with different characteristics for an extended period of time. The extent at which indices that characterise the tendency to cause slagging and fouling on boiler heating surfaces found in the literature correspond to the actual state of ash deposits in the boiler is discussed. New ranges of fouling indices values are proposed that correspond to the results from observing fouling in the boiler.

Effect of secondary air on soot nucleus production in stoker-fired boilers

Black smoke emission and soot nucleus production in a stoker-fired boiler by secondary air are studied numerically and experimentally. Temperature field, flow field, and soot nucleus concentration are predicted numerically. It reveals that the secondary air gives about 10% reduction of the soot nucleus production, and the black smoke emission is controlled evidently.

Hydrogen Chloride Bonding with Calcium Hydroxide in Combustion and Two-Stage Combustion of Fuels from Waste

The present study deals with the problem of limiting HCl mobility in processes of combustion of two solid recovered fuels (SRF1 and SRF2) having significant but different contents of chlorine. Large-scale experiments were carried out at a temperature of 1050 °C with the use of a stoker-fired flat grate boiler with the heat output of 1 MW, and with a two-stage combustion system for medical waste incineration (MWI) with the same heat output in which first the process of gasification in the temperature of 650 °C occurs, and subsequently the previously obtained syngas is fired in the temperature of 1000 °C in the reburning chamber of the gas generator. In such a way, two different technologies of combustion have been compared, with regard to HCl capture efficiency using widely available calcium hydroxide sorbent Ca(OH)2. In order to simulate the increased concentration of chlorine in SRF1, the fuel was being supplemented with three various additions of poly(vinyl chloride) (PVC) recyclate. Before combustion, SRF1 and SR2 were also blended with Ca(OH)2 whose weight amount was being changed: from 0 to 3 wt % with a step equal to 0.5 wt % (combustion) and from 0 to 1.5 wt % with a step equal to 0.5 wt % (two-stage combustion). For both of the methods of SRFs combustion, series of experimental findings have been presented, showing trends in the changes of HCl concentration in flue gas, depending on (i) the amount of PVC recyclate added to SRF1 and amount of Ca(OH)2 added to both fuels; and (ii) the molar ratio Ca/Cl2. Finally, the ratio of HCl concentrations in flue gas after combustion and two-stage combustion, depending on the amount of added PVC and Ca(OH)2 to the fuels was analyzed. The achieved results indicate that in a two-stage process (gasification and recombustion), HCl can be much more effectively bonded than during combustion in stoker-fired boiler.

High Carbon Ash from Stoker Fired Boiler of a Thermal Power Station as Support in the Reduction of Ketones: A Novel Application

Reduction of a few substituted cyclohexanones adsorbed on size classified high carbon ash fractions from a stoker fired boiler of a thermal power station using NaBH4 as the reducing agent with ethanol as the medium resulted in the formation of two epimeric alcohols. However, trans-4-t-butyl and trans-4-methylcyclohexanols are exclusively formed when the corresponding ketones adsorbed on one particular size classified ash fraction are reduced with NaBH4. In all other cases, a fairly high degree of stereoselectivity has been obtained. It has been observed that low carbon ash does not activate the ketones towards similar stereoselectivity, clearly establishing that the high carbon ash has a definite role to play. The stereochemical outcome of the reductions has been rationalized on the basis of a probable transition state involving Lewis/Br⊘nsted acid sites present in the ash. Mesopores and macropores present in the ash are most likely to act as pockets for adsorption of ketones. Product analysis has been carried out using 1HNMR and 13CNMR.

Quantifying the Benefits of Using Flue Gas Desulfurization Gypsum in Sustainable Wallboard Production

A suite of high-As, high-C fly ashes from a university-based stoker-fired coal boiler were analyzed by a number of techniques, including high-resolution transmission electron microscopy (HR-TEM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), and field-emission scanning electron microscopy (FE-SEM). The sooty carbon is in the form of nano balls with the major fullerenes at C~60~^+^, C~70~^+^, and C~80~^+^, with species at C~2~ increments from C~56~^+^ to C~78~^+^. Arsenic and Hg, among other metals, are found in association with the fullerenes, but, with our techniques, it is not possible to determine if the metals are encapsulated by the fullerenes or attached to the side of the structure. TOF-SIMS studies suggest an association of As with the Al-Si glass; an association of Pb with oxides, sulfates, and carbon; Hg with carbon; Se in elemental form with carbon; and Cr in a variety of forms, including nano carbons, Fe sulfates and oxides, glass, and Cr-oxyhydroxides.

Emission Characteristics of Granulated Fuel Produced from Sewage Sludge and Coal Slime

The neutralization of wastewater treatment residues is an issue for many countries. The European Union (EU) legal regulations have limited the use of the residues in agriculture and implemented a ban for their disposal. Therefore, urgent action should be taken to find solutions for the safe disposal of sewage sludge. The problem refers in particular to the new EU member countries, including Poland, where one can now observe an intensive development of sewage system networks and new sewage treatment plants. At the same time, these countries have few installations for thermal sewage sludge utilization (e.g., there is only one installation of that type in Poland). Simultaneously, there are many coal-fired mechanical stoker-fired boilers in some of these countries. This paper presents suggestions for the production of granulated fuel from sewage sludge and coal slime. Additionally, among others, lime was added to the fuel to decrease the sulfur compounds emission. Results are presented of research on fuel with two average grain diameters (∼15 and 35 mm). The fuel with such diameters is adapted to the requirements of the combustion process taking place in a stokerfired boiler. The research was aimed at identifying the behavior of the burning fuel, with special attention paid to its emission properties (e.g., to the emissions of oxides of nitrogen [NOx], sulfur dioxide [SO2], and carbon mon-oxide [CO], among others). The concentration and emission values were compared with similar results obtained while burning hard coal. The combustion process was carried out in a laboratory stand where realization of the large-scale tests is possible. The laboratory stand used made simulation possible for a wide range of burning processes in mechanical stoker-fired boilers.

Experimental Investigation of Hydrogen Chlorine Bonding with Limestone and Dolomite in the Furnace of a Stoker-Fired Boiler

The paper presents results of experimental semi-technical investigations to limit the mobility of chlorine released in the form of hydrogen chloride from the fuel in the stoker-fired boiler furnace. In the combustion process, limestone and dolomite were used as the sorbents. The background for the research was a fuel containing a low gram fraction of chlorine, namely, extracted rapeseed meal supplemented with polyvinyl chloride (PVC) recyclate granules, to diversify the quantity of the quick-release chlorine. The research results indicate the tendency ofchanges of the hydrogen chloride concentration in the flue gas in relation to the amount of PVC added to the fuel, i.e., gram fraction of chlorine, amount of limestone and dolomite added to the fuel, as well as the molar ratio Ca/Cl 2 or (Ca + Mg)/Cl 2 , respectively. It has been proven that the greatest dechlorination capacity is exhibited by hydrated lime, lesser by limestone, and the least by dolomite, which, nonetheless, can be used under certain conditions for HCl bonding in boiler furnaces.

Experimental Investigation of Hydrogen Chloride Bonding with Calcium Hydroxide in the Furnace of a Stoker-Fired Boiler

The paper presents the results of experimental technical investigations to limit the mobility of chlorine released in the form of hydrogen chloride from the fuel in a stoker-fired boiler furnace. In the combustion process, hydrated lime was used as the bonding material. The background for the research was a low-chlorine-gram-fraction fuel, namely, extracted rapeseed meal supplemented with PVC recyclate granules to diversify the quantity of the quick-release chlorine. The research results indicate the tendency of changes of the hydrogen chloride concentration in the flue gas in relation to the amount of PVC added to the fuel, i.e., the gram fraction of chlorine and the amount of calcium hydroxide added to the fuel, as well as the Ca/Cl2 molar ratio.

The black smoke control in a chain-grate stoker-fired boiler at the time of ignition and restart of the combustion

The black smoke is always emitted from the chimney in the chain-grate stoker-fired boiler at the time of ignition at ambient temperature and restart of the combustion after temporary flameout in China. The purpose of the work is to reduce the black smoke emission. A laboratory fixed-grate model has been used to simulate the combustion of coal in chain-grate stoker-fired boiler. The CO and O2 concentration in the flue gas have been measured with a flue gas analyser, and the black smoke emitted from the chimney has been screened with Charge Couple Device (CCD) video camera. Power 2# coal, sized at 5–25 mm, has been fired in the fixed-grate model. The secondary air has been used to enhance the turbulence in the furnace after the numerical simulation. The results of experiments show that the emission of the black smoke at the time of ignition of the coal at ambient temperature is more serious than that of restart of the combustion after the temporary flameout for the case of the temperature; the secondary air is helpful for reducing the black smoke emission for enhancing the intensity of the turbulence in the furnace; selection of coal particle size is necessary, the smaller the size of the coal is, the more serious the black smoke emission is, and the effect of reducing the black smoke emission with the secondary air is more evident with smaller-size coal. The industrial test has been employed to study the effect of the air demand. It indicates that rational stoichiometric air/fuel ratio is helpful for reducing the black smoke emission in the restart of the combustion after the temporary flameout. Copyright © 2006 John Wiley & Sons, Ltd.

Effect of coal and air staging on the ignition plane movement and emissions during rapid large load changes on chain-grate stokers

A major concern when operating stoker-fired boiler plant during rapid large load changes is maintaining stable ignition because 'burn-back' may occur or, at the other extreme, burning coal can be discharged from the end of the grate if the coal and air settings are not appropriately controlled. A suitable coal/air staging practice has been identified for chain-grate, stoker-fired boilers under rapid load-following conditions. The flame-front migration and carbon monoxide and oxygen concentrations in the flue gases were recorded during experiments on a test facility at Casella CRE. The findings were confirmed in tests on a full-scale industrial boiler and indicate that the coal feed should lead the air flow on load increase and vice versa on turndown in order to improve transient ignition stability and reduce carbon monoxide and smoke emissions.