Climate change has led governments worldwide to switch to renewable energy sources. In this regard, the government of the Republic of Korea has recently revised its renewable energy certification (REC). The ratio for biomass/coal co-firing has been eliminated, whereas the weight for biomass-whole firing has been reduced by 50%. Meanwhile, a new weight for unused forestry byproducts has been added and set at 2.0, which translates to increased demand for biomass-based fuels (biofuels). Specifically, the demand for wood pellets, which are typically pulverized and molded with woody biomass for use as biofuel, has been increasing rapidly. However, the current domestic production of wood pellets is not sufficient to address this demand and, consequently, large amounts of wood pellets have to be imported. In this regard, according to the Forest Biomass Energy Association, domestic wood pellet production was estimated to be approximately 67,000 tons while its import amounted to 2,431,000 tons [1
] Consequently, there is an urgent need to find suitable replacements for wood pellets. In this context, several studies have examined the possibility of using agro-byproducts as fuel.
In Korea, the domestic potential of agro-byproducts was estimated to be approximately 4018 × 103 ton/year [2
]. Here, we note that among various agro-byproducts, chaff and rice straw, which amount to 61%, are used as compost or livestock feed. However, the other byproducts are left mostly unused or are subjected to direct combustion. Further, it is noteworthy that agro-pellets suffer from the disadvantage of a low calorific value relative to coal; consequently, it becomes necessary to increase the calorific value of agro-pellets for better combustion performance. The torrefaction process was introduced as one possible method to solve this problem.
Torrefaction is a thermochemical conversion process wherein biomass fuel is preheated to temperatures of 200–300 °C, over an interval of around 1 h. This process can improve fuel characteristics such as the calorific value and the hydrogen/carbon ratio (H/C ratio) to a level comparable with that of solid fossil fuels. However, excessive torrefaction can lead to energy loss due to excessive mass loss; thus, the determination of the optimum torrefaction conditions becomes important. For this determination, systematic analysis studies are required.
Kanwal et al. [3
] demonstrated the physicochemical effects of torrefaction via proximate and ultimate analysis and true density, grindability, and hydrophobicity studies of sugarcane bagasse. Garcia et al. [4
] compared the energy properties of torrefied wood and elephant grass pellets in relation to “un-torrefied” pine and elephant grass pellets. Azocar et al. [5
] fabricated brown pellets under moderate torrefaction conditions at a temperature of 145 °C. Spirchez et al. [6
] developed a mass reduction model of the torrefaction process for beech pellets and validated the model. Peng et al. [7
] studied the effects of the wood torrefaction process on the resulting energy density and hardness. Yang et al. [8
] developed a synchronized torrefaction and pelleting process at the laboratory scale and compared their results with previous processes such as torrefaction after pelletizing (TAP) or pelletizing after torrefaction (PAT). Oh et al. [9
] developed and validated a mass reduction model for agro-byproducts (pepper stem) and determined the optimized torrefaction conditions.
Against this backdrop, in this study, we consider the torrefaction process for effective utilization of unused agro-biomass as a possible fuel. The torrefaction process was conducted with select agro-byproduct pellets, and their mass reduction was measured. Subsequently, ultimate analysis and calorific value measurements were performed, and the properties of the torrefied agro-pellets were evaluated.
This study was conducted with the objective of examining the feasibility of using agro-byproduct biomass as a biofuel and improving its efficiency. We utilized torrefaction to examine the feasibility of agro-byproduct pellets as fuel and attempted to determine optimal torrefaction conditions. The examined agro-pellets exhibited high thermal degradation relative to the WP (wood) sample, which corresponded to a low mass yield. However, the agro-pellets exhibited a large increase in calorific value due to their large mass loss. The increase in the WP calorific value was approximately 15%p and the maximum increase among all the agro-pellets considered was 35%p. However, some CFP samples exhibited a decreasing tendency of the calorific value possibly due to loss of the bio-oil component of CFP. Nevertheless, the CFP calorific value increased under severe torrefaction conditions. The energy yield also exhibited a tendency similar to the mass yield for all samples. Hygroscopicity experiments indicated the torrefaction conditions increased the pellet hydrophobicity. We speculated that the torrefaction process “broke” the H–O bond, which hindered water absorption. The exergy of each sample was also calculated. Based on these parameters, we estimated the optimal torrefaction conditions. The conditions for the WP samples were selected as the optimal conditions. The optimal temperature and time conditions for PEP were determined as 230 °C and 40 min, respectively. The optimal temperature and time conditions for PRP were 230 °C, 40 min and 50 min. respectively. The CFP conditions were optimal in all cases except for the temperature and time values of 290 °C, 40 min and 50 min, respectively. However, CHP could not be practically considered as a fuel. In future studies, we plan to conduct our investigations at a pilot-scale facility.