PROBLEMI PEPELA I EKSPERIMENTALNO ODREĐIVANJE UTICAJA ADITIVA NA SAGOREVANJE BIOMASE

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1 PROBLEMI PEPELA I EKSPERIMENTALNO ODREĐIVANJE UTICAJA ADITIVA NA SAGOREVANJE BIOMASE B. S. Repić *, A. D. Marinković *, D. V. Dakić **, D. M. Đurović *, A. M. Erić *, G. S. Živković * * Univerzitet u Beogradu, Institut za nuklearne nauke Vinča, P.Fah 522, Beograd, Srbija ** Univerzitet u Beogradu, Inovacioni centar Mašinskog fakulteta, Kraljice Marije 16, Beograd, Srbija Apstrakt: Biomasa je najznačajniji potencijal obnovljivih izvora energije Srbije. Od toga 60% čini poljoprivredna a 40 % šumska biomasa. Šumska biomasa se već sada značajno koristi dok je poljoprivredna praktično neiskorišćena. Razlog toga leži u nepostojanju adekvatnih tehnologija za njeno sagorevanje. Ova biomasa zahteva posebne tehnologije korišćenja zbog nepovoljnih temperaturskih karakteristika mineralnog dela i karakteristika meljivosti. Sprovedena istraživanja su pokazala da se dodavanje aditiva u praksi pokazao kao praktičan i efikasan načina pomoću kojeg je moguće da se minimiziraju problemi sa sinterovanjem pepela. U radu su prikazani rezultati istraživanja potencijalnog uticaja aditiva na karakteristike pepela više vrsta poljoprivrednih biomasa. Istraživanjem su određivane temperature topivosti pepela ispitivanih biomasa uz dodavanje aditiva u količini 5% od ukupne mase uzorka. Istraživanje uticaja aditiva sprovedeno je sa 6 vrsta aditiva čije su osnove glina ili pesak. Ispitivanjima je pokazano da je primenom aditiva moguće sigurno i pouzdano sagorevati i one biomase koje imaju niske temperature topivosti pepela, kao što su slama pšenice i kukuruzovina. Ključne reči: biomasa, sagorevanje, pepeo, aditivi, topivost ASH PROBLEMS AND EXPERIMENTAL DETERMINATION OF THE EFFECTS OF ADDITIVES DURING BIOMASS COMBUSTION B. S. Repić *, A. D. Marinković *, D. V. Dakić **, D. M. Đurović *, A. M. Erić *, G. S. Živković * * University of Belgrade, Vinca Institute of nuclear sciences, P.O. Box 522, Belgrade, Serbia ** Innovation Center, Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16, Belgrade, Serbia

2 Abstract: Biomass is deemed to be the main source of renewable energy in Serbia. 60% of entire biomass potential is in agricultural and 40% is in forest biomass. Forest biomass is already having a significant uses while agricultural is practically unused. The reason for this lies in the lack of adequate technologies for its burning. This type of biomass requires special technologies for use due to temperature characteristics of the mineral part and grinding characteristics. Performed research has shown that the additive proved to be convenient and efficient tool by which it is possible to minimize the problems of ash sintering. This paper presents results of the potential impact of additives on the characteristics of several types of agricultural biomass ash. The research determined ash melting point of the tested biomass, adding an additive in the amount of 5% of the total weight of the samples. Investigation of the effects of additives was carried out with 6 types of additives whose base is clay or sand. The tests demonstrated that the application of additives make possible safely and reliably biomass combustion those type of biomass that have low melting point of ash, such as wheat and maize straw. Key words: biomass, combustion, ash, additives, solubility 1. INTRODUCTION In terms of sustainable energy development in Serbia, there is a growing need for using the alternative energy sources. Alternative energy sources are, in most cases, renewable: biomass, wind power, solar energy, hydro-power and geothermal energy. A need for the utilization of this kind of energy sources is dictated by the market, on one side, as well as by environmental protection, on the other. Prices of fossil fuels grow proportionally to the decreasing of fossil fuel reserves. Since available reserves of fossil fuels in Serbia, especially those of high quality, are relatively limited, this problem becomes even more emphasized. On the other hand, it is necessary to harmonize the energy production legislation and practice in Serbia with the directives of the European Union, in the sense of intensifying the utilization of renewable energy sources and thus reducing pollution and greenhouse effect formation. Biomass is one of key renewable energy sources. This is the reason for the development of cheap thermal devices (boilers and furnaces) burning biomass from agricultural production as quite available and cheap energy source. These devices could be used primarily in villages, small towns and small businesses processing agricultural goods (greenhouses, dairy farms, slaughterhouses etc.). The devices could also be used for heating schools, hospitals, prisons and other institutions. According to the official date of Ministry of Infrastructure and Energy of Republic of Serbia [1], Serbia is in dispose of 4.3 Mtoe of renewable energy sources, while biomass is represented with

3 2.7 Mtoe. 60% out of registered biomass potential are residuals from agricultural production, and the rest is wood biomass. Currently, only a small portion of waste biomass is being used in energy production mostly for heating (not taking into account burning in the individual households, in small ovens), for several reasons: low electricity price and non-resolved problems in biomass gathering [2]. Also, there is no regulated biomass market, and no developed technologies for its utilization as fuel. Besides, small financial power of potential buyers have to be mentioned, as well as costly commercial credits and total absence of state subsidizing of biomass facilities. The best way for utilizing residual agricultural biomass for energy production in industrial or district heating is to be used close to place of its gathering - in large agricultural companies. That is the optimal solution, from energy, as well as economy point of view. Agricultural biomass is usually collected in form of bales, varying in size and shape, so it is most convenient to use it in that form. One of the most efficient ways, recommended by many institutions worldwide, is the combined heat and power (electricity) production CHP, which use residual biomass as fuel, and have least as possible power consumption [3]. 2. MATERIALS AND METHODS Production of heat and electricity by burning biomass can be very important for Serbia, since biomass is one of the most important renewable energy sources. On the process of thermal use of solid biofuels affects type of used solid biofuels, its physical characteristics (particle size, density, moisture content, calorific value) and chemical composition. Based on the made efforts, the technology for biomass combustion has reached a high level of development. The most important advantage of this fuel compared to fossil is that their thermal using is CO 2 neutral. In addition, emission of carbon monoxide (CO), nitrogen oxides (NOx) and total organic carbon (TOC) is considerably lower in comparison to the emission of these compounds resulting from the fossil fuels burning. However, disadvantages of using this type of fuels are the technical problems which are related to the ash sticking and formation of ash deposits on the plants walls during the combustion process, thus limiting use of biomass as a fuel [4]. Different types of agricultural biomass that are mostly used in our country and widely, contains similar concentrations of carbon, hydrogen and oxygen, but show significant differences in the concentrations of the major elements that form ash (Si, Ca, Mg, K, Na, P, S, Cl, Al, Fe, Mn), the concentrations of heavy metals (Cu, Zn, Co, Mo, As, Ni, Cr, Pb, Cd, V, Hg) and a concentration of N. The weight percentages on a dry basis for C, H and O are 30 to 60%, 5 to 6%, and 30 to 45%. At the same time, nitrogen, sulphur and chlorine can be found in the amounts less than 1% on dry matter [5].

4 In comparison with coal, biomass generally contains less carbon, aluminium, and sulphur, but more oxygen, chlorine, potassium and calcium [6]. These inorganic elements influence on the processes of biomass combustion, as well as the melting of the ash, and therefore on the corrosion processes and ash slugging on the plant walls. It is therefore very important to know the quantity of inorganic elements in various biofuels and reactions effects that they cause during combustion in order to design facility properly, or in order to choose the right fuel for facility that already exists. Also, this is very important information for the sector, which is engaged in the production of biomass, as to the quality of biofuel produced may affect to some extent Ash problems during biomass combustion To reduce problems with slagging and sintering of ash generated by burning agricultural biomass the several treatments was investigated. Among all the tested operations the methods of adding additive proved to be a practical and efficient minimizing problem with the sintering of ash. Additives are substances that can decrease the tendency of sintering, the formation of deposits and corrosion in the plants [7]. Numerous studies in the laboratory and pilot plant have shown that additives can react with alkali metal chlorides, converting them into less harmful compounds. The aim of this paper is to analyse the possibilities of applying different potential additives to change the characteristics of the ash generated by agricultural biomass combustion in order to increase its melting point and avoid the problems associated with corrosion and ash slagging in the boiler. The chemical composition of biofuels, in particular content of elements that form ash effects on the choice of combustion technology and process control. The ash is inorganic, incombustible part of the fuel that lags after complete fuel combustion, which contains most of the mineral fractions derived from biomass. The main elements that form ash (Al, Ca, F, K, Mg, Na, P, Si, Ti) determined its solubility and influenced on the formation of deposits. The elements that make volatile compounds in the ash as Cl, S, Na, K, As, Cd, Hg, Pb, Zn play a major role with regard to gas and aerosols emissions, as well as the formation of deposits, corrosion processes and the use/disposal of ash [5, 6, 8]. Mentioned elements of biofuel together with Cl and S form the various compounds in the ash. Cl, S, K and Na in biofuels play a major role in the formation of deposits and influence on the corrosion process. One of the most important elements in view of the caused problems is the Cl. Cl concentration dictates the amount of alkali metal which vaporizes during combustion, as well as transport of alkali metals from the fuel to the heat surfaces where alkali form sulphates. KCl 2 is among the most stable high temperature, gas-phase compound containing Cl. In the absence of chlorine, hydroxides of alkali metals are the most stable gas-phase compound in the flue gases. Chlorine has a strong corrosive effect on metal surfaces in furnaces, and HCl emissions. When the

5 concentration of Cl in the fuel is higher than 0.1% of the total weight of the fuel can be expected to cause problems with corrosion [9]. Straw of some biofuels (wheat, rape seed, etc.) has a concentration of chlorine of 0.4% and 0.5%, so it can be assumed that there may cause damage of the furnace surface by corrosion. Also, the high content of Cl means the possibility of the formation of different chlorine hydrocarbons. Thus a high content of Cl and alkaline elements in some biomass can cause damage in furnace and on the heating surfaces. The agricultural biomass ash (straw, grasses, and grains) contains a lower concentration of Ca and higher concentrations of Si, K, and Cl in relation to the forest biomass. A high percentage of Si with K and Cl causes problems of ash deposit formation at high temperatures of combustion. The main sources of these problems are: a) reacting of alkali metal with the Si, which causes the formation of alkali silicate, which melt at low temperatures, b) reacting of alkali metal with S where alkali metal sulphates are formed on the heating surfaces. As it can be seen alkali compounds play a role in both processes. K is the alkali metal dominant in the majority of the biomass [10]. A high percentage of K leads to trouble of ash slagging. The melting points of pure K, Cl and K 2 SO 4 were 776 C and 1067 C. Typical ash formed by straw burning starts to melt and become sticky in the range of C. When the flue gas reaches this temperature, potassium compounds begin to condense. The formation of deposits due to sticking of particles of fly ash can be accelerated with a mixture of alkali and heavy metal salts (a mixture of alkali metal chlorides and sulphates of zinc chlorides). The importance of sulphur during biomass combustion is not reflected on SO 2 emissions, but it has significant role in the corrosion process. Higher concentrations of SO 2 in the flue gas can cause sulphatization of alkali and alkaline earth chloride which reduces temperature of the flue gases, and comes to the release of chlorine. If these reactions occur in the ash particles precipitated on the surface of the tubes of the heat exchanger, this release of chlorine may cause corrosion by the formation of FeCl 2 or ZnCl 2 on the metal surface. The efficiency of fixation of the sulphur in the ash depends on the concentration of the alkali and alkaline-earth metal, especially calcium in the fuel. The presence of Ca and Mg usually increases the melting point of the ash. Ca forms chlorides and sulphates, but its compounds are less volatile than potassium, and generally increase the melting point of the ash. The same conclusion is for magnesium. The main compounds, which are formed by Ca and Mg in the combustion process, are oxides and carbonates of a smaller scale. Ca, Mg, K and P, are the nutrients matter and agents needed to improve the land, which is of great interest for use of biomass ash in forests or agricultural land.

6 Based on the known chemical composition of the ash can be applied empirical indices (index of alkali, acid-base ratio and agglomeration index) to predict the behaviour of ash matter [11]. 3. RESULTS AND DISCUSION Additives for combustion are used in power plants to ensure efficient combustion, i.e. to reduce the emission of CO, hydrocarbons, particulates, NOx and SO 2, as well as to control the processes of agglomeration and corrosion [12]. These are solid, liquid and gaseous substances, which may change the physical and/or chemical properties of the fly ash in such a manner that the deposits become less problematic, and this is achieved by increasing the melting point of the fly ash. They can increase the melting point of the ash that is formed during the combustion of the biomass of agricultural residues. This is accomplished by forming a mixture of ash/additive, which has a higher melting point and thus can make ash less sticky. Additives are substances that have a capacity to lower sintering tendency of ash deposits and reduce corrosion problems caused by alkaline ash components. This is achieved by converting the alkaline substances into less harmful compounds [7]. A change in the ash characteristics achieved with the use of additives is accomplished through an increase in the ash melting temperature i.e. through higher melting temperature of ash/additive mixture. In addition, additives can make ash less sticky. Additives react with KCl and NaCl, forming K-Na compounds with relatively high melting points and HCl which is released into the flue gas [10]. HCl remains in the gaseous phase all the way through the boiler and is ultimately released to the stack and into to the environment, together with the flue gas. In this way chlorine is removed from the sediments, hence preventing the corrosion causing temperature distribution. Additives are therefore added in order to bind the gaseous alkaline compounds and form less harmful compounds. The resultant compounds are generally less volatile and are usually formed as coarse ash particles which remain at the bottom section of the boiler throughout the entire combustion process. From environmental point of view, such situation is much more favourable then the formation of fly ash particles which are carried by flue gas and released into the atmosphere. In this manner, additives also contribute to particulate emission reduction [10]. Effectiveness of additives depends on several factors [12]: a) additive particle size distribution smaller additive particles provide larger surface area available for the process reactions; b) reaction temperature and time; c) additive composition (active additive component); d) stoichiometry (sufficient amount of additives).

7 Additives can be divided into several groups [13]: additives containing calcium, phosphorus, sulphur, aluminium or aluminium silicate. Additives are selected from substances which are easily handled, produce non-toxic residues and are deemed effective. There are various kinds of mineral additives, such as sand ( mm), chalk, kaolin (clay), rolovite (clay fraction), bentonite (clay fraction), aluminium sulphate, mono calcium phosphate, dicalcium phosphate, farm manure, brewery sludge, dolomite, calcite, bauxite, emalthite, gibbsite etc. All these additives have been used as alkali binding agents or for preventing chemical reactions with elements causing the formation of eutectic mixtures [12]. The effectiveness of the additives has been determined by comparing the combustion of the fuel and the mixture of fuel and additive. After that was measured total emission of particles, their size distribution and chemical composition in flue gas. As support for a better understanding of the whole reaction, it was determined chemical composition of ash from the bottom of the boiler, as well as the analysis of the gaseous compounds of HCl and SO 2, as well as a control of the combustion process are measured concentration of O 2, CO 2, CO, TOC (Total Organic Carbon) and NOx emissions. Two additives that are often applied and meet these requirements are the limestone (calcium carbonate) and kaolin (clay). Limestone which was added to the boiler during combustion of biomass reacts with HCl. This reaction is most effective between 550 C and 700 C, which results in increasing the amount of Cl in the larger particles of fly ash. This leads to a corresponding decrease in the share of gaseous HCl in the flue gases. Addition of lime causes translation of alkali chloride in alkaline sulphate in fine particles of fly ash. During some investigation of highly alkaline biomass combustion have been used some additives on the basis of Ca, including the limestone. It is obtained that the gaseous emissions of SO 2 could be reduced up to 25%. The reactions for the reduction of chlorine and sulphur with lime are as the follows [14]: CaO (s) + 2HCl (g) CaCl 2(s) + H 2 O (g) CaO (s) + 1/2O 2(g) + SO 2(g) CaSO 4(S) Limestone can prevent slagging of ash particles in furnaces. Adding CaO during the agricultural biomass (straw, grain, grass) combustion was observed decreasing of ash particles slagging, as it comes to the incorporation of potassium into calcium compounds, which caused the formation of calcium/magnesium potassium phosphate which has a higher melting point. Kaolin is a clay mineral, which is mainly composed of kaolinite Al 2 Si 2 O 5 (OH) 4, which can eliminate gaseous alkaline compounds which are produced by combustion, by binding of alkali elements in the mineral, which has a higher melting point. The reaction between potassium chloride and the kaolin at high temperatures, for the ash straw, has been presented by expressions [15]:

8 Al 2 Si 2 O 5 (OH) 4 + 2KCl 2KalSiO 4 + H 2 O + 2HCl Al 2 Si 2 O 5 (OH) 4 + 2KCl + SiO 2 2KalSi 2 O 6 + H 2 O +2HCl Addition of limestone or clay during oat grains burning showed a significantly lower carbon monoxide and TOC emission. Addition of limestone is drive down emissions of HCl, but had no effect on SO 2 and NOx emission. The lack of reduction of SO 2 can be explained by the reaction between calcium and phosphorus, which has been shown to inhibit the capture of the sulphur during combustion of fuel rich in phosphorus. Also, it is not observed emission reduction of particles. Addition of clay increases HCl emissions, and does not significantly reduce emissions of SO 2 and NOx, but reduces the emission of fly ash particles into the environment. During the straw combustion were investigated four different additives: sand, dicalcium phosphate (DCP), chalk and bentonite. Each additive was added to the straw before the milling for approximately 5% of the biomass weight. The next relations show how additive reacts, i.e. with which ash components, and in which ratio [12]: a) Sand: 2KCl + SiO 2 +H 2 O K 2 O SiO 2 + H 2 O, ratio K : Si = 2 b) DCP: (CaHPO 4 2H 2 O): KCl + CaHPO 4 2H 2 O CaKPO 4 + 2H 2 O + HCl, ratio K : P = 1 c) Chalk: K 2 O SiO 2 + 2CaCO 3 2CaO K 2 O SiO 2 + 2CO 2, ratio K : Ca = 1 d) Bentonit: 2KCl + Al 2 O 3 2SiO 2 + H 2 O K 2 O Al 2 O 3 2SiO 2 + 2HCl, ratio K : Al = 1 Experiments have shown that the chalk and DCP are unsuitable as additives, primarily because of their physical characteristics. Sand and bentonite are additives that promise, but it is necessary to establish further testing to finally gauge their effect on formed deposits. Bentonite is an expensive product, so it should find a cheaper product with similar characteristics. The main challenge is to select the most suitable additive, which must be effective, inexpensive and does not cause problems with the material handling or environmental problems. Neither one so far-used additive cannot be said to fulfil all these requirements. Kaolin (clay) is known as a highly reactive but expensive. The sand, on the other side, is not expensive, but it is less reactive. To prevent problems of slagging and ash deposition during combustion of pitted olives was added following additives: kaolinite, klinohlor (clinochlore) and ankerite. The percentage of the additive in relation to biomass was 5%. When such additives are added kaolinite or klinohlor, the concentration of alkali metal element (K, Na), chloride, calcium and iron in the fly ash were reduced. Alkaline compounds are retained in the particles of fly ash from the bottom of the boiler, or are evaporated with the flue gas, while iron and calcium compounds stayed in the ash particles from the bottom of the boiler. When used as an additive ankerite, the concentration of the compound of the alkali (Na, K), chlorine, and iron were reduced, while the concentration of Ca and

9 Mg were significantly higher. In all three cases added of additives reduced slagging and deposition caused by alkali. The overall conclusion of the performed experimental investigations is that the amount of the additive that is needed to effectively reduced slagging of ash particles and the deposition on the combustion plant walls depends on the type of additive and biomass, which is applied Experimental results In the Agricultural Corporation PKB Belgrade, the boiler with thermal power of 1.5 MW was built. The boiler is planned for burning large bales of agricultural biomass with cigarette combustion principle. In this plant is over a long period of time burned soybean straw and straw of rapeseed [16-18]. Using these two types of biomass during operation have not noticed problems with corrosion and solubility of ash. However, the combustion of large bales of wheat straw caused the ash slagging and melting because their ash solubility has very low sintering temperature of ash generated by combustion at 880 o C. To solve the problem of using biomass with low solubility temperatures of the ash was carried out testing on a real facility in PKB. For this purposes has been used a substances that were added to the biomass and examine its impact on the ashes. It is a liquid multipurpose additive for solid fuels PAC-KK-S MgNit [19], which has a feature that increases melting temperature of the ash, improves combustion, reduce slow-temperature and high-temperature corrosion, and reduces the emission of harmful products of combustion into the atmosphere. The additive is added by spraying bales prior to insertion into the furnace, and then determined the point of solubility of ash generated by combustion of wheat straw with and without additives. The sintering point remains the same 880 o C, while only an increase of softening point of 920 o C to 1020 o C. Potential additive impact on the biomass combustion process was examined in order to determine ash melting temperatures of biomass samples containing 5% by weight of selected additive. Biomass sample was mixed with the indicated amount of additive and the mixture produced was heated to 550 C in order to obtain examined ash. Investigation was carried out with six different clay and sand additives: 1) bentonite, 2) clay "K.M.Krusik", 3) clay "Cirinac", 4) kaolin "Bz", 5) sandy clay "Plocnik" and 6) clay "K.M.Cirinac". The obtained results are presented in Tables 1-3. With respect to the maize sample analysed (Table 1) it was observed that clay additives (no. 3 and 6) had caused a significant increase in the related ash melting temperatures. With respect to the analysed sample of wheat straw, which is generally characterized by low ash melting temperature, practically all additives were found to have caused an increase in the ash melting temperature of the sample. However, some of the additives used, such as kaolin and sandy

10 clay additive, were associated with particularly significant ash melting temperature increase (Table 2). In the case of rapeseed ash, characterized by high ash melting temperature, the use of each additive analysed resulted in decreased ash melting temperature (Table 3). Performed experimental investigation has demonstrated that the use of additives allows for safe and reliable biomass combustion, even of biomass varieties characterized by low ash melting temperature, such as wheat straw and maize. Table 1. Ash melting behaviour for maize straw Additives Value Without Shrinkage starting temp. o C Deformation temperature o C Hemisphere temperature o C Flow temperature o C Ash content at 550 o C % Table 2. Ash melting behaviour for wheat straw Additives Value Without Shrinkage starting temp. o C Deformation temperature o C Hemisphere temperature o C Flow temperature o C Ash content at 550 o C % Table 3. Ash melting behaviour for rape seed straw Additives Value Without Shrinkage starting temp. o C Deformation temperature o C Hemisphere temperature o C Flow temperature o C Ash content at 550 o C % CONCLUSIONS The chemical properties of different types of solid biofuels affect the processes of combustion and flue gases cleaning technologies. Agricultural biomass generally contains relatively high concentrations of chlorine and alkali metals, which affect the formation of deposits and cause corrosion in combustion facility. During combustion this kind of biomass has been obtained ash witch usually contains lower concentrations of calcium, and the higher concentrations of potassium and silicon. Therefore the ash begins to sinter and melts at lower temperature and to be deposited on

11 the furnace walls. Additives are substances added during combustion and that change the characteristics of the ash by increasing the melting point of the ash. In order to partially solve the problem of using biomass from agricultural production influence of several kinds of additives based on clay or sand, on the melting point of the ash has been performed. Investigation of the potential impact of additives was carried out in such a manner as determined temperature of solubility of ash for investigated biomass (straw of wheat, rape seed and maize) with the addition of additives in the amount of 5% of the total weight of the sample. All analyses were conducted in accordance with appropriate standards. The obtained results demonstrated that with the application of the additive it can be surely and reliably combusted and those biomass that have a low temperature solubility of the ash, such as wheat straw or maize. ACKNOWLEDGEMENTS The paper has been realized in scope of Ministry of Education and Science of Republic of Serbia s project Development and improvement of technologies for energy efficient and environmentally sound use of several types of agricultural and forest biomass and possible utilization for cogeneration, Record number III REFERENCES [1] [2] Repic, B. et al., Development of the technology for combustion of large bales using local biomass, in: Sustainable Energy-Recent Studies, (Ed. A. Gebremedhin), InTech, Rijeka, 2012, pp [3] Quaak, P. et al., Energy from biomass, World bank technical paper no. 422, Washington, 1999 [4] Biedermann, F., Obernberger, I., Ash-related problems during biomass combustion and possibilities for a sustainable ash utilisation, Proceedings, World Renewable Energy Congress (WREC), Aberdeen, Scotland, May, 2005, pp. 1-8 [5] Obernberger, I. et al., Concentracions of inorganic elements in biomass fuels and recovery in the different ash fractions, Biomass and Bioenergy, 12 (1997), 3, pp [6] Vassilev, S. at all., An overview of the chemical composition of biomass, Fuel, 89 (2010), 5, pp [7] Bafver, L. et al., Particle emission from combustion of oat grain and its potential reduction by addition of limestone or kaolin, Fuel Processing Technology, 90 (2009), 3, pp [8] Becidan, M. et al., Ash related behaviour in staged and non-staged combustion of biomass fuels and fuels mixture, Biomass and bioenergy, 41 (2012), 6, pp [9] Nielsen, H.P. et al., Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot scale study, Fuel, 79 (2000), 2, pp [10] Khan, A.A.et al., Biomass combustion in fluidized bed boilers: Potential problems and remedie, Fuel Processing Technology, 90 (2009), 1, pp [11] Vamvuka, D. et al., Control methods for migrating biomass ash-related problems in fluidized beds, Bioresource Technology, 99 (2008), 9, pp

12 [12] Tobiasen, L. et al., Deposit characteristic after injection of additives to a Danish straw - fired suspension boiler, Fuel Processing Technology, 88 (2007), 11-12, pp [13] Wang, L. et al., A critical review on additives to reduced ash related operation problems in biomass combustion applications, Energy Procedia, 20 (2012), pp [14] Radojević, A. et al., Analysis and testing of agricultural biomass ash and potential additives, Contemporary Agricultural Engineering, 36 (2010), 4, pp [15] Bostrom, D. et al., Ash transformation chemistry during combustion of biomass, Energy and fuels, 26 (2012), 1, pp [16] Repić, B. et al., Investigation of the cigar burner combustion system for baled biomass, Biomass and Bioenergy, 58 (2013), 11, pp [17] Repić, B.S. et al., Soya straw bales combustion in high efficient boiler, Thermal Science,12 (2008), 4, pp [18] Mladenović, R. et al., The boiler concept for combustion of large soya straw bales, Enery, 34 (2009), 5, pp [19] Marinković, A. et al., Experimental determination of the effects of additives on agricultural biomass ash characteristics, Contemporary Agricultural Engineering, 37 (2011), 2, pp