# Investigation of Pellet Properties Produced from a Mix of Straw and Paper Sludge
## 1 Introduction  
The advantage of straw is in its wide availability in almost all parts of the world, which is related to its low price. Along with its rapid renewability, these factors are the reason for the high potential of straw. The main problems of plant biomass combustion include low energy density, potentially high moisture contents, problems in terms of availability, and in combustion the low melting point of biomass can pose problems with corrosion. Nowadays, due to the growing awareness of greenhouse gas emissions, plant biomass as part of biomass is expected to be a very important element in achieving Europe's 20-20-20 climate and energy targets [1]. Therefore, the combustion of plant biomass has enormous potential in terms of sustainable electricity and heat production. Its disadvantage compared to dendromass is the relatively high content of alkali metals, potassium, nitrogen, sulfur, silicon, chlorine and other ash-forming elements. The higher content of these elements generally leads to increased concentration of emissions and particulate matter, as well as operational problems related to ash behavior during combustion [2]. The already mentioned low melting temperature of ash causes problems during the operation of burners. This includes slagging, fouling and in some cases corrosionof heat exchangers. Among other things, these problems reduce the efficiency of combustion systems, incur additional cleaning and maintenance costs for boilers, and in some cases even prevent the use of certain types of plant biomass for combustion [3]. Various methods have been investigated to prevent or reduce ash sintering during phytomass combustion. These approaches include: (1) application of additives; (2) fuel mixing; (3) decreasing the problematic compounds by leaching the fuel [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. The use of additives and blending of plant biomass with another type of fuel has proven to be an effective solution to these difficulties. The various waste materials are of particular interest as additives, especially if they contain active chemical compositions for economically attractive costs. In the studies of [PERSON] et al. the low cost additives (sewage sludge and marble sludge) were analyzed and proved able to abate biomass ash sintering and slagging [7; 9]. Paper sludge is a by-product of the paper recycling process, which can be considered as either a biofuel of poor quality due to its high water content or a waste product. On the other hand, the sludge was used as an effective additive to wheat straw due to the high content of calcite and kaolin [16]. According to the literature review, kaolin was successfully tested to increase the ash melting point during the combustion of wheat pellets with kaolin [5]. The reduction in slagging was noticed by blending kaolin and calcite in wood pellets [17]. The sintering temperatures of barley straw and husk ashes were considerably increased by adding kaolin and calcite [18]. The agricultural residues are usually pelletized in order to obtain higher energy and bulk densities. Application of additives into plant biomass before pelletization is an effective option to include the additives into the combustion process. In addition, the entire contact of fuel and additives can be achieved during the mixing and pelletization process. It may positively support the effects of the additives on plant biomass' ash chemistry [18; 19; 20; 21; 22; 23]. To better understand the process of obtaining energy from plant biomass, it is crucial to know the chemical composition of this energy source. This composition is a unique base code that characterizes and determines the characteristics, quality, potential applications, as well as the environmental problems of a fuel. The ultimate analysis of the individual plant biomass species reference is presented in Table 1.  
The carbon content of plant biomass is up to 50% of this element. A higher carbon content leads to a higher calorific value of the fuel.