Grate Furnace Research Articles

Study on the Similarity of the Parameters of Biomass and Solid Waste Fuel Combustion for the Needs of Thermal Power Engineering

The topic raised in this paper concerns an important aspect regarding the possibilities of the combustion and co-combustion of biomass substrates (energetic willow/sewage sludge), in light of the renewable energy source (RES) regulations, with fuel from waste defined here as RDF. Particular interest in these fuels and their mixtures results from a search by the thermal power engineering sector for fuels whose combustion will be associated with benefits, e.g., the acquisition of energy origin licenses, availability and lower cost of obtaining fuel (RDF), as well as moderate investment contributions using the existing base of coal grate boilers. This article indicates the possible variants of the combustion and co-combustion of the abovementioned fuels, referring to the use of technical and technological capabilities of coal grate furnaces. It was possible by comparing the mechanical and thermal load of grates, as well as so-called indicators for the quantitative assessment of combustion (i.e., ignition rate and mass loss rate). The result of the combustion of the fuel as above in a laboratory-scale furnace (samples weighing up to 1 kg), as well as their thermogravimetric analysis, indicates a great similarity of the mass loss curve, temperature profiles and combustion phases, as well as indicators of a quantitative assessment in the process of biomass and RDF oxidation. The obtained results of the mechanical and thermal load capacity of the grates constitute the basis for further analyses of fuel conversion, without the risk of thermal and/or mechanical grate overload. Relying on the research of the indicators for the quantitative assessment of combustion, it was found that the combustion process of the tested fuels could be carried out while maintaining the quality of burning the fuel layer. Based on the conducted research, it can be assumed that it is possible to replace coal fuels with biomass, RDF fuel, sewage sludge using coal furnaces for their combustion, which means a hybrid nature of the furnace due to the type of fuel.

Fuzzy model predictive control for small-scale biomass combustion furnaces

This work presents a fuzzy model predictive controller for small-scale grate furnaces based on a newly derived biomass combustion model. Several local linear controllers are designed for a selected number of operating points utilizing a gap metric. The resulting local predictive controllers are merged with membership functions to form a global nonlinear fuzzy control structure. The presented framework intends to improve the transient and steady state operation by applying an optimal control strategy with state estimation and to cover the entire operating range of the furnace. The open loop results of the introduced combustion model are parameterized and cross-validated with measured data from a test furnace. In order to find suitable parameters for the grey-box model, a local sensitivity analysis is conducted to contribute to an efficient parameter estimation process. Closed loop simulation results of the fuzzy model predictive controller, a linear model predictive controller and a PI control algorithm are presented and compared. Based on the performance of the proposed fuzzy controller, its application, advantages and disadvantages are discussed. Additionally, the impact of the different controllers on the formation of carbon monoxide is investigated based on estimation models from literature. The simulation results show that the fuzzy model predictive controller performs best in the considered categories.

Kinetics and fusion characteristics of municipal solid waste incineration fly ash during thermal treatment

During the thermal treatment of municipal solid waste incinerators (MSWI) fly ashes (FA), the fusion characteristics, kinetics, and phase transition are the theoretical basis on heavy metals solidification improvement and energy consumption reduction. In this study, the thermal behavior of MSWI FA from two typical furnaces in China, i.e., grate furnaces (GF) and circulating fluidized bed (CFB), are systematically investigated. The results show that FA melting can be divided into five stages but with different positions and areas of weightlessness peaks between two types of FA. We identify four kinetic models from which to obtain the apparent activation energy and pre-exponential factor during the melting transition. The apparent activation energy of GF FA ranges from 466 to 1016.2 kJ/mol, which is generally higher than that of CFB FA (380.1–636.6 kJ/mol). The weightlessness rate of GF FA is approximately three times that of CFB FA because of its high chloride load. Silicon dioxide (SiO2) in FA is prone to react with other compounds to form low-melting-temperature minerals at 1100 °C, such as Ca2MgSi2O7. Therefore, the fusion temperatures of CFB FA with a higher content of SiO2 are about 150 °C lower than that of GF FA, which is consistent with the theory that the fusion temperature of FA is at its lowest when the ratio of basic to acid oxides approaches 1. Finally, SiO2–CaO–Al2O3 ternary phase diagrams, ash fusion temperatures, and several fouling and slagging indices are verified to predict the ash fusion characteristics of MSWI FA.

Suggested guidelines for emergency treatment of medical waste during COVID-19: Chinese experience.

During the period of COVID-19, the medical waste disposal capacity is seriously inadequate. The main technical process of the municipal solid waste incineration system is the same as that of the medical waste incineration system. Under the conditions of optimizing the technological process, improving the supporting facilities, and controlling the co-processing ratio, the municipal solid waste incinerator (grate furnace) co-processing medical waste is feasible. Some suggested guidelines for emergency treatment of medical waste from COVID-19 have been provided by China.

Fuel briquettes from sludge (cake) of concentrating mill

Briquettes made of cake (sludge) with addition of woody sawdust in a volume from 8 to 14% are studied here. Introduction of a moisture-removing agent in a ratio that is sufficient for initial stable shaping and hardening without thermal drying satisfies the requirement for mechanical strength at transportation. Briquettes have a mechanical strength of more than 97%, which does not change during freezing and thawing, underburning after combustion is no higher than 4%, and ash content is not higher than 36%. Stability of briquettes burning can be defined as steady and long-lasting. Such fuel briquettes for the purpose of their use in grate and other furnaces can be produced according to the scheme of pressing in extruders or roller press.

Characteristics of ash and slag from four biomass-fired power plants: Ash/slag ratio, unburned carbon, leaching of major and trace elements

The utilization of biomass energy is attracting worldwide attention due to the worsening energy crisis and the concerns on carbon dioxide emission. However, the rapidly development of biomass-fired power plants generate enormous amount of slag and ash. At present, the main treatment method of biomass slag and ash is landfill, which not only requires high cost, but also causes a series of environmental issues. Aiming at this problem, the slag and fly ash from four biomass power plants were sampled and characterized. The major/trace element composition and leaching characteristics of slag and fly ash are analyzed, and the effect of volatile mineral in these solid residues on furnace efficiency is also evaluated. The results indicate that for biomass-fired grate furnaces, in the generated solid residues, the slag accounts 60 ~ 70% while the fly ash accounts for 30 ~ 40%. The volatile elements in slag are much lower than those in fly ash, and the unburned carbon content of fly ash is generally lower than that of slag. Due to the enrichment of volatile minerals in fly ash, instead of 815 °C, 550 °C is suggested to measure the unburned carbon content in fly ash. When the measuring temperature of unburned carbon decreases from 815 °C to 550 °C, the measured energy loss from the incomplete combustion of solid fuels decreases by one third, which affords a more reasonable evaluation on combustion efficiency. The contents of cadmium and lead in certain fly ash samples exceed the standard, however, all fly ash samples have a high leaching rate of potassium but low leaching rates of copper, zinc, cadmium, lead, chromium and arsenic. It indicates that the fly ash from biomass-fired grate furnace is suitable for producing potassium fertilizer by leaching instead of direct soil fertilization or landfill. The leaching rate of most minerals in slag is much lower than that in fly ash and the contents of heavy metals are far below the prescribed upper limit in standard, which indicates that the slag from biomass-fired grate boiler is more suitable for direct use in soil improvement due to the low contents of hazardous elements.

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.

Emission characteristics of particulate matters from a 30 MW biomass-fired power plant in China

The emission characteristics of particulate matters from a full-scale biomass-fired power plant equipped with bag filters were investigated. Results show that particle size distribution at the inlet of bag filter from the combustion of blended feedstocks is bimodal, while that of dry bark feedstocks is essentially unimodal with negligible emission of coarse particles with diameter in the range of 1.0–10 μm. The combustion of blended fuels generates higher yields of submicro particles than that of dry bark feedstocks. Elemental analysis shows that submicro particles in all cases mainly consist of potassium, chlorine, and sulfur. Higher chlorine but lower sulfur contents are observed in submicro particles from blended fuels compared with that from dry bark feedstocks. The vibrating operation of grate furnace reduces submicro particles emission as well as the sulfur content in all particles. Ion chromatography results show that sulfate ion and chloridion are the two most abundant water-soluble anions in particulate matters, while water-soluble cations are richest in potassium with considerable content of calcium, magnesium, sodium and ammonia. The contribution of organic and element carbon in particulate matters is in the range of 1–3%, indicating high combustion efficiencies and low organic and element carbon emissions of biomass-fired grate furnaces.

Effect of Sludge Pellets Addition on Combustion Characteristics and Ash Behaviour of Municipal Solid Waste

Co-combustion of sewage sludge and municipal solid waste (MSW) in grate furnaces (GF) is a promising method to meet strict environmental requirements and reduce construction costs given its high market share in China. However, considering the high ash content and low heat value of sludge, co-combustion of sludge and MSW in GF may lead to combustion instability and an increase of fly ash (FA) production, which further aggravates ash deposition and corrosion of heating surfaces. Besides, semi-dried sludge powder with tiny particle size may cause dust explosion and increase the clinker ignition loss of slag ash due to the leakage in grates. In this study, semi-dried sludge powder is compressed into pellets through a rotary ring die granulator to investigate the effect of sludge pellets addition on gaseous pollutant emissions, combustion characteristics and ash behavior (e.g., ash fusion temperature and the clinker ignition loss of slag ash). The results show that the ratio of FA produced from sludge pellets incineration is lower than that of sludge powder. The clinker ignition loss of slag ash of sludge pellets increases with the increase of its compressive strength. The highest clinker ignition loss under the low combustion temperature is 11.9% which is far beyond the limit (5%) for GF. The addition of desulfurizer (CaO, mass ratio: 1%) in sludge pellets is found useful to reduce sulfur dioxide emission with the maximum 84% desulphurization rate at the combustion section. The decrease of ash flow temperature from co-combustion of sludge and MSW is less than 10 °C compared to that from mono-combustion of MSW, which suggests that co-combustion of sludge with the mixing ratio of 10% in municipal solid waste incinerators (MSWI) has little impact on fly ash slagging characteristics. The ash characteristics and gaseous pollutant emissions during co-combustion of sludge pellets and municipal solid waste are investigated.

Investigation of Ash Deposition Dynamic Process in an Industrial Biomass CFB Boiler Burning High-Alkali and Low-Chlorine Fuel

The circulating fluidized bed (CFB) boiler is a mainstream technology of biomass combustion generation in China. The high flue gas flow rate and relatively low combustion temperature of CFB make the deposition process different from that of a grate furnace. The dynamic deposition process of biomass ash needs further research, especially in industrial CFB boilers. In this study, a temperature-controlled ash deposit probe was used to sample the deposits in a 12 MW CFB boiler. Through the analysis of multiple deposit samples with different deposition times, the changes in micromorphology and chemical composition of the deposits in each deposition stage can be observed more distinctively. The initial deposits mainly consist of particles smaller than 2 μm, caused by thermophoretic deposition. The second stage is the condensation of alkali metal. Different from the condensation of KCl reported by most previous literatures, KOH is found in deposits in place of KCl. Then, it reacts with SO2, O2 and H2O to form K2SO4. In the third stage, the higher outer layer temperature of deposits reduces the condensation rate of KOH significantly. Meanwhile, the rougher surface of deposits allowed more calcium salts in fly ash to deposit through inertial impact. Thus, the elemental composition of deposits surface shows an overall trend of K decreasing and Ca increasing.

Environmental performances of a modern waste-to-energy unit in the light of the 2019 BREF document

The study compares the environmental performances of a new-generation, large scale, combustion-based waste-to-energy unit, active since 2010, with those of different “virtual” units, defined in the light of the Best Available Techniques REFerence document (BREF) for Waste Incineration published by the European Community on December 2019. The average performances of these units have been evaluated in terms of air emissions, material consumptions and energy recovery, based on data related to 355 “existing” European waste incineration lines and those established for the future “new” plants. An attributional Life Cycle Assessment has been used to compare and quantify the environmental performances of the selected units, all equipped with a moving grate furnace and similar air pollution control systems. A sensitivity analysis quantifies how even more severe requests for emission and energy performances as well as the evolution of the European electricity mix until the year 2030 can affect the comparative assessment. The results indicate that the considered large scale waste-to-energy plant has good environmental performances, even in an electricity mix characterised by 45% of renewable sources. This allows an easy compliance with the Best Available Techniques Associated Emission Levels of the new waste incineration BREF document. Possible further improvements of its performances should be focused mainly on a further increase of the energy efficiency, provided that it is economically viable.

Parameters influencing wet biofuel drying during combustion in grate furnaces

Mainly grate-fired furnaces are used for heat production, as these systems are designed for high thermal efficiency with low emission levels of gaseous pollutants when firing good quality biomass with 30–45% moisture content. However, high demand for biomass during the winter makes it necessary to necessitates use wet biofuel of lower quality with varying moisture content that can reach up to 60% wt. causing the burning instability and incomplete combustion. These negative effects could be avoided by optimizing the furnace control to intensify biomass drying on the grates, but the parameters affecting wet biofuel drying must be known. In order to expand the knowledge of the wet biofuel drying process and establish the parameters that influence the drying process most significantly, a special rig based on characteristics of a 6 MW grate furnace was produced and drying experiments simulating the primary air flow in the furnace, the recirculation of the flue gases and the primary air flow together with exposure to thermal radiation were performed. The results revealed that the radiation from surfaces has the biggest impact on biomass drying when preheated air of the temperature of 200 °C is supplied and the moisture content of the biomass is reduced to 0.66 of its initial value. It was also determined that the drying intensity depends on the mixing period and the drying intensity can be increased by up to 15% by additional mixing. Influence of the drying agent temperature depending on the particle size was established as well.

The effect of the ratio of excess air and the process time on CO and NOx emissions from the combustion of alternative fuel in a grate furnace

One of the basic problems of environmental protection is the problem of municipal waste management. As the experience of the richest developed countries shows, it is possible to achieve a zero-landfill level of economy. However, such a level can only be achieved if several dozen percent of municipal waste generated is subjected to processes of energy utilisation. At the same time, at present, the EU countries are moving away from waste incineration processes towards the processes of energetic use of fuels based on them. In the first part of the article, the problem of thermal utilisation of municipal waste in Poland is highlighted. The results of analyses of selected fuel parameters of municipal waste and formed fuels have been presented. The second part of the paper outlines the issue of fuel combustion in grate furnaces. In the further part of the article the methodology and results of laboratory tests of combustion of formed fuel are presented. The research has been carried out on a test stand enabling simulation of processes occurring in furnaces with mechanical grates. Within the framework of the presented studies, the influence of total process duration and excess air ratio on the amount of CO and NOx emissions was taken into account. The paper presents both the changes in the emissions of the abovementioned gases over time and the total emissions related to the unit of the formed fuel burnt.

Sewage Sludge Bottom Ash Characteristics and Potential Application in Road Embankment

The paper focuses on sustainability-related applications in civil engineering by using environmentally friendly, alternative construction materials. The paper presents geotechnical properties of thermally converted, municipal sewage sludge in a grate furnace in an incineration plant. Bottom ash and its mixture with sand have been tested to show that they can be considered as a substitute for natural soil built-in road embankments. The product of sewage sludge combustion and its combination with sand meet all code requirements for material suitable for road embankments. As a result of the 30% reduction of resistance to failure values after waste soaking, which causes relatively low California Bearing Ratio (CBR) values of soaked waste, waste should be built into places isolated from groundwater and precipitation. That is also indicated by the possibility of heavy metals leaching from the waste because such content is much higher than in uncontaminated soils, although leaching does not exceed the limits commonly quoted for natural soil solutions. Tested bottom ash as a product of combustion in a grate furnace is a more preferred material for earthworks than fly ash generated during incineration in a furnace with fluidized bed due to the particle size and heavy metal concentrations.

Microbial induced solidification and stabilization of municipal solid waste incineration fly ash with high alkalinity and heavy metal toxicity.

This paper presents an experimental study on the applicability of microbial induced carbonate precipitation (MICP) to treat municipal solid waste incineration (MSWI) fly ash with high alkalinity and heavy metal toxicity. The experiments were carried out on fly ashes A and B produced from incineration processes of mechanical grate furnace and circulating fluidized bed, respectively. The results showed that both types of fly ashes contained high CaO content, which could supply sufficient endogenous Ca for MICP treatment. Moreover, S. pasteurii can survive from high alkalinity and heavy metal toxicity of fly ash solution. Further, the unconfined compressive strength (UCS) of MICP treated fly ashes A and B reached 0.385MPa and 0.709 MPa, respectively. The MICP treatment also resulted in a reduction in the leaching toxicity of heavy metals, especially for Cu, Pb and Hg. MICP had a higher solidification and stabilization effect on fly ash B, which has finer particle size and higher Ca content. These findings shone a light on the possibility of using MICP technique as a suitable and efficient tool to treat the MSWI fly ash.

Comparing Fly Ash Samples from Different Types of Incinerators for Their Potential as Storage Materials for Thermochemical Energy and CO2.

This study aims to investigate the physical and chemical characterization of six fly ash samples obtained from different municipal solid waste incinerators (MSWIs), namely grate furnaces, rotary kiln, and fluidized bed reactor, to determine their potential for CO2 and thermochemical energy storage (TCES). Representative samples were characterized via simultaneous thermal analysis (STA) in different atmospheres, i.e., N2, air, H2O, CO2, and H2O/CO2, to identify fly ash samples that can meet the minimum requirements, i.e., charging, discharging, and cycling stability, for its consideration as TCES and CO2-storage materials and to determine their energy contents. Furthermore, other techniques, such as inductively coupled plasma optical emission spectroscopy, X-ray fluorescence (XRF) spectrometry, X-ray diffraction (XRD), scanning electron microscopy, leachability tests, specific surface area measurement based on the Brunauer–Emmett–Teller method, and particle-size distribution measurement, were performed. XRF analysis showed that calcium oxide is one of the main components in fly ash, which is a potentially suitable component for TCES systems. XRD results revealed information regarding the crystal structure and phases of various elements, including that of Ca. The STA measurements showed that the samples can store thermal heat with energy contents of 50–394 kJ/kg (charging step). For one fly ash sample obtained from a grate furnace, the release of the stored thermal heat under the selected experimental conditions (discharging step) was demonstrated. The cycling stability tests were conducted thrice, and they were successful for the selected sample. One fly ash sample could store CO2 with a storage capacity of 27 kg CO2/ton based on results obtained under the selected experimental conditions in STA. Samples from rotary kiln and fluidized bed were heated up to 1150 °C in an N2 atmosphere, resulting in complete melting of samples in crucibles; however, other samples obtained from grate furnaces formed compacted powders after undergoing the same thermal treatment in STA. Samples from different grate furnaces showed similarities in their chemical and physical characterization. The leachability test according to the standard (EN 12457-4 (2002)) using water in a ratio of 10 L/S and showed that the leachate of heavy metals is below the maximum permissible values for nonhazardous materials (except for Pb), excluding the fly ash sample obtained using fluidized bed technology. The leachate contents of Cd and Mn in the fly ash samples obtained from the rotary kiln were higher than those in other samples. Characterization performed herein helped in determining the suitable fly ash samples that can be considered as potential CO2-storage and TCES materials.

Determining operating characteristics of co-firing processes in grate furnaces

The combustion process in grate furnaces is determined by a number of independent factors (control parameters). The results of this process are dependent factors (effects).The knowledge of the simultaneous impact of all of the aforementioned independent factors on all of the analyzed dependent factors is required for the analytical use of grate furnaces. The functions that define this relationship are identified as process characteristics.In this study, the analytical experimental methods of specifying characteristics will be discussed. This consists of two stages. The first stage is based on defining the numerical shape of the functions of the aforementioned characteristics. Worth noting is that constant coefficients can and do coexist with both independent and dependent factors. The second stage is based on specifying the values of the measured coefficients by means of statistical methods, as well as evaluating their relevance. Moreover, the second stage also consists of evaluating how adequate the specified characteristics will be in relation to the results. The aforementioned characteristics are input-output dependencies.The main introductory element of this paper is to define the use of more modern methods in designing scientific experiments. Another significant element is the use of a new and innovative function of distributing the air stream across the length of the grate. The suggested method of specifying characteristics has been exemplified in the test grate furnace by characteristics of NO emissions. Its efficiency was at the level of 100 kW during the co-combustion of coal and sewage sludge.The process characteristics of both fuel combustion and co-combustion in grate furnaces have a vital impact due to the number of existing facilities. Knowing the characteristics enables a dynamic regulation (online), as well as technical, ecological and economic optimization of the conditions of exploiting grate furnaces.

Design and key heating power parameters of a newly-developed household biomass briquette heating boiler

Using biomass briquette fuel for rural household heating is in line with the current Chinese policy and helpful to alleviate the air pollution in winter in northern China. However, the existing biomass briquette boilers have several defects on structure design, fuel feeding, and combustion control, while the design code for the biomass briquette heating boiler has not been proposed thus far. In this study, a newly-developed household biomass briquette heating boiler is designed. Performance test shows that under the optimal operating condition, the emissions of flue dust, NOx, CO, and SO2 are 33.20, 62.48, 426, and 0 mg/m3, respectively; the Ringelman emittance ≤1; the heating power is 12 kW; the thermal efficiency is 84.69%. The tested pollutant emissions and thermal efficiency are in compliance with the limits from the standard. The two key heating power parameters, the grate plan heat release rate and furnace volume heat release rate of this boiler are also determined to provide theoretical basis for the structure design of biomass briquette heating boilers. Results show that the optimal values of these two parameters are 181.98 kW/m2 and 292.04 kW/m3. For the design of household biomass briquette heating boilers, the ranges of 150–210 kW/m2 and 250–330 kW/m3 are considered as the reasonable intervals of these two parameters.

Comparison of the Characteristics of Fly Ash Generated from Bio and Municipal Waste: Fluidized Bed Incinerators.

European solid waste incinerator plants still primarily use grate furnace technology, although circulating fluidized bed (CFB) technology is steadily expanding. Therefore, few investigations have reported on the environmental assessment of fly ash from fluidized incinerators. This research project aims to integrate information on fly ash derived from the combustion of municipal solid waste (FA1) and biomass (FA2) in fluidized bed incinerator facilities. Fly ash samples were comparatively analyzed by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM), and inductively coupled plasma optical emission spectroscopy (ICP-OES) to study the mineralogy, morphology, total heavy metal content, and leaching behavior, respectively. The analysis revealed that the two types of fly ash differ in their characteristics and leaching behavior. The concentration of most of the heavy metals in both is low compared to the literature values, but higher than the regulatory limits for use as a soil conditioner, whereas the high contents of Fe, Cu, and Al suggest good potential for metal recovery. The leaching ability of most elements is within the inert waste category, except for Hg, which is slightly above the non-hazardous waste limit.

Parameters affecting biomass drying during combustion in moving grate furnaces

The most efficient way so far to extract energy from renewable sources is combustion of solid fuel. Solid fuel furnaces of moderate capacity (5–10 MW) equipped with reciprocating grates are most popular. Grate combustion is a well-developed technology; however, to burn biofuel in this type of furnaces in the optimal and safe way, the fuel must be of high quality and have at least constant moisture content. However, increasing demand for biofuel results in increasing prices. To remain in the market and to stay competitive, heat producers choose to utilise moist biofuel of lower quality, whose moisture content can vary and reach up to 60% wt. The burning on the grate of such biofuel is complicated as the drying process occupies most of the space in the furnace. The purpose of this work was to analyse processes taking place in a furnace, such as: primary air supply, influence of flue gas recirculation and radiation from hot surfaces of the furnace to biofuel drying. Analysis of the data obtained would provide technical decisions facilitating optimal fuel combustion in a furnace without additional investments. Analysis of biofuel drying was performed in an experimental setup with a fixed fuel bed. The experiments were performed with wood chips and four different drying fluid temperatures. The results of experimental studies have shown that the drying rate of biofuels upper layers is strongly influenced by radiation from hot surfaces and the moisture content of the sample decreases by 18% wt.