Torrefaction Process and Technology

1. Torrefaction Process

Torrefaction is a pre-treatment technology that helps generate biomass with strong compressibility that avoids disintegration during handling and storage which results to a low cost and reduce investment for co-firing application. Torrefaction is an emerging technology which will accelerate the development of biomass utilization technology. The two recent biomass technologies of torrefaction include dry and wet torrefaction. The torrefied biomass of Dry torrefaction is hydrophobic, contains low moisture content, high heating value, and energy density which is a good characteristics for combustion, pelletization, and gasification. This type of torrefaction technology is conventional and easy to be commercialized. Still, in order to utilize it effectively, further intensive work is needed specifically the customized torrefaction which is more practical for application (Chen ZhiWen et., 2018). The most recognized advantage of torrefied biomass include compatible properties to coal mainly heating value, grindability, bulk energy density, and hydrophobicity. According to a study conducted by Bharat Regmi (2017), biomass undergoes greater alteration in fuel properties during torrefaction. After the temperature, gas flow rate, and residence time of poplar wood was analyzed, the outcome showed promising improvement in physicochemical properties by torrefaction. Saima Sohni, et., al (2017) concluded that underutilized palm kernel shell (PKS) can be an interesting raw material for conversion technologies based on combustion and gasification processes because of the maximum FC and lignin content after studying the physicochemical characterization of Malaysian crop and agro-industrial biomass residues as renewable energy source. The oil palm frond, oil palm trunk, and kenaf are found to possessed high polysaccharide quantity together with lower lignin content making it a valuable feedstock for biochemical conversion. A Brazilian experimental study of torrefaction process for energy valorization of woody biomass was studied by Clarissa Gusmao Figuero et., al (2019). Eucalyptus urophylla wood and prototype screw reactor were used to evaluate energy valorization of Brazilian woody biomass subjected to dry torrefaction treatment. The temperature and residence time were observed and analytical methods such as proximate and ultimate analyses, and T-test were assessed. Hence, torrefaction process increases the carbon, energy, and ashes contents of wood chips. Also, reduced volatile matter content, equilibrium moisture, and average size is evident along with bulk and energy densities that exhibit an initial increase with later reduction. As a whole, torrified wood chips have better indices of quality for production of heat and energy than raw wood chips. Maria Gonzales Martinez et., al (2016) used the Thermogravimetric Analysis Device to study the regime controlling torrefaction which involves a characteristic time study of thermal and chemical phenomena involved. For the experiment process, biomass samples were loaded in a three-plate crucible of 10mm diameter and suspended in the TGA oven. Volumetric weight, mass heat capacity, thermal conductivity, emissivity, and porosity were the parameters investigated. Results show that the characteristic time of external convection and internal conduction are much lower than the characteristic time of chemical reaction of torrefaction which indicates that heat transfer phenomena is are faster than the chemical reaction, which is the limiting factor of the transformation.

2. Torrefaction Technologies

There are differents types of torrefaction technologies that are used for generating biomass. The most common technologies used are dry and wet torrefaction which differs in products and process Chen ZhiWen, et., al (2018). Important data from several sources and studies were obtained to present the results of the studies of the dry torrefaction process of various types of fluidized bed biomass in an inert gas (nitrogen) medium, a study of Rafail Isemin, et., al (2020). It is shown that applying torrefaction process in fluidized bed allows a significant acceleration in the process of producing bio-coal with the same characteristics. Also, technological problems that arise during the process and ways to overcome it are considered. Paola Brachi (2016) determined the suitable process parameters for the proper operation of the new lab-scale fluidized bed torrifier. Tomato peel, olive husk, Thermogravimetric analyzer, calorimeter, and CHN 2000 LECO analyzer were used. Analytical methods include TGA, proximate and ultimate analyses, and hygroscopicity. The temperature, residence time, moisture content, proximate composition, ultimate composition, HHV, and LHV are the parameters investigated. The behavior of fluidization process is essential to achieve simple and reliable thermal control of the torrefaction process. Also, it is important because it avoids the hot spots into the reactor, potentially deriving from highly exothermic reaction involved in torrefaction process, and to acquire a uniform quality of the torrified product. In a study “Quantification of the Toreffaction Effects on the Grindability and the Hygroscopicity of Wood Chips” conducted by Baptiste Colin, et., al (2017), the influence of operating parameters on torrefaction performed in pilot rotary kiln is observed. Pilot-scale rotary kiln made of a rotating cylinder electrically heated was used for the torrefaction process of wood chips. As the biomass torrefaction progresses, changes in ultimate and proximate compositions result to a higher carbon content of the material, thus increasing its energy content. A method for the evaluation of fundamental grindabilty of biomass simultaneously taking into account the milling energy consumption and the particle size of the ground products showed that the apparent specific surface grounding energy is divided by 6.3 between chips with a moisture content of 41% and dry chips. After the torrefaction with mass loss of 25%, this energy measure in turn is reduced by a factor of 8.1. Organic Waste Torrefaction- A Review: Reactor of Systems and Biochar Properties is a study conducted by Pawel Stepien et., al (2017) that presented an organic waste torrefaction technology and reviewed its reactor system. The study came up with two main conclusions: (1) by adaption of pyrolysis reactors, the torrefaction of some biomass can be easily applied. The problem may be related with torrefaction energy balance because of low calorific value of torrgas or problem with mechanical movement of the feedstock trough the reactor caused by friction and/or melting of materials (ex. plastics) and; (2) consider the biomass/waste type and properties, the components of energy balance, pollution degree of the torrgas, desired biocarbon properties, energy demand, economy, and the current situation of the biomass/waste, and biocarbon utilization market in selecting the torrefaction reactor.