Determination of the effect of steam conditioning and torrefaction process on pellet quality parmeters in olive pruning residues and greenhouse tomato residues

In recent years, the production and consumption of wood pellets has increased worldwide in order to meet domestic and industrial thermal needs. Increasing demand brings along the disruptions in the sustainable supply of forest industry residues, which is the primary raw material of wood pellets. This situation has led the world major pellet producer countries to import raw materials or directly wood pellets. The use of domestic energy resources is in an extremely strategic position today. The use of existing domestic agricultural residues as pellets is a solution to the pellet raw material supply problems. In order to evaluate agricultural residues as pellets, there are sufficient experimental studies on optimizing the pelletizing variables that are different from forest products and investigating their compatibility with fuel improvement technologies. There are not enough experimental studies on optimizing the pelletizing variables of agricultural residues that are different from forest products and investigating their compatibility with fuel enhancement technologies. The variable physical and chemical structure of agricultural residues on a product basis confirms the necessity of determining the pelletization characteristics of each agricultural residue and its reactions to thermochemical conversion methods on a product basis. It is expected to make significant contributions to pellet production worldwide through the sustainable supply of agricultural residues, optimizing pelletizing variables, and adapting fuel enhancement technologies to pellets. In this study, greenhouse tomato stems (SD) and olive pruning residues (ZB) were used as alternative pellet raw materials. The pelletizing with steam conditioning method, which is widely used in wood pellet production, has been adapted to agricultural residues. Thus, the effect of the hot steam level on the pelletizing parameters and the physical properties of the pellets were investigated. Since agricultural residues have relatively low quality fuel properties compared to wood pellets and coal, the fuel properties of the pellets have been improved by the torrefaction method. The obtained pellets were burned in an automatic domestic pellet stove and their combustion performance and flue gas emission values were determined. According to the results obtained, while the hot steam pelletizing process increased the pellet production capacity by 112% for SD pellets and 50% for ZB pellets. Hot steam conditioning reduced the specific energy consumption during pelletizing, and values of 114.07 kWh/ton in SD and 152.80 kWh/ton in ZB were calculated. Steam conditioning affected the pellet production parameters and pellet physical properties for both agricultural residues at different levels according to the raw material physical properties and lignocellulosic structure. In general, the increase in the level of hot steam strengthened the bond between the ground particles forming the pellet, allowing denser and durable pellets to be produced. Thus, the highest particle density and bulk density values in SD pellets were calculated as 1395 kg/m3 and 833 kg/m3, respectively. At the most efficient steam level for ZB pellets, the particle density and bulk density values were 1289 kg/m3 and 736 kg/m3, respectively. The improvement effect of steam conditioning on pellet production parameters and pellet physical properties is directly related to the particle size and lignocellulosic content of the ground raw material. The torrefaction of the pellets was carried out in nitrogen environment at 220, 250 and 280 °C temperatures for 15 minutes. While the increased temperature caused an improvement in the fuel properties of the pellets, there was an increase in the physical fragility of the pellets. In addition, the increase in the level of steam conditioning enabled the production of pellets in denser form compared to pelletizing without hot steam. Thus, dense pellets showed resistance to torrefaction. The highest enhancement factors obtained by the torrefaction process were calculated as 1.20 and 1.18 for SD and ZB pellets, respectively. The calorific value of the untreated SD pellets, which was 16.91 MJ/kg, increased to 20.29 MJ/kg with the torrefaction process, and the calorific value increment observed from 17.52 MJ/kg to 20.73 MJ/kg for untreated and torrefied ZB pellets. A high level of correlation (R2≥0.90) was found between the difference in color parameters of the pellets darkening with increasing torrefaction temperature and their torrefaction efficiency. In the combustion of the pellets, the nylon based hanging twine and plastic clip residues contained in the tomato stem residues caused slaging and clogging in the combustion chamber. In this respect, insufficient air flow caused CO and NOx emissions to remain at 2363-11616 ppm and 123-296 ppm, respectively. CO and NOx emissions in ZB pellets were in the range of 764-2353 ppm and 134-206 ppm, respectively. The sulfur content of greenhouse tomato stalks at the level of 0.26-0.44% caused SO2 release at the level of 35-203 ppm during combustion. During the combustion of ZB pellets, ash accumulation and slagging did not occur in the combustion chamber, and their combustion efficiency is quite superior to SD pellets. Average flue gas temperatures of SD and ZB pellets were 110-213 °C and 198-269 °C, respectively. The pelletizing characteristics of agricultural residues show differences according to the residue type, ground raw material particle size and the level of hot steam conditioning. In this regard, pelletizing variables should be considered separately according to the residue type for efficient pelletizing of agricultural residues. The torrefaction process, which is used to improve fuel properties, is a promising conversion technology that can be used to improve the fuel quality of agricultural residue pellets. The nitrogen and sulfur contents of fertilizers used in the cultivation of agricultural products directly and indirectly affect the elemental contents and combustion characteristics of biomass fuels obtained from agricultural residues. In addition, foreign materials such as hanging rope and plastic clips, which are obligatory to be used by cultivation methods in greenhouses, have the potential to adversely affect the combustion quality of pellets to be produced from greenhouse residues. In general, the use of agricultural residues as pellet raw materials has a strategic importance in the production of national and local energy resources. In order to maximize the thermal efficiency of agricultural pellets, studies should be carried out on specific combustion systems and combustion methods.