Abstract
Auto-thermal operation of biomass torrefaction can help avoid additional heat investment and the associated costs to the system. This work provides a general method for relating the feedstock-specific parameters to the energy balance and pre-diagnosing the potential of auto-thermal for different biomass torrefaction and pyrolysis systems. Both solid and gas thermal properties under various torrefaction conditions and their influences to the torrefaction system energy balances are considered. Key parameters that influence the process auto-thermal operation are analyzed, which include torrefaction reaction heat, torrefaction conditions, drying method, biomass species, and inert N2 flowrate. Equations of torgas and biomass higher heating values (HHVs), as well as the torrefaction reaction heat at different operating conditions are developed. It is found that torgas and biomass HHVs increase with torrefaction temperature and biomass weight loss. Torrefaction reaction heat has a linear relationship with the biomass weight loss, with a positive slope at 250–260°C, and a negative slope at 270–300°C, which indicates that torrefaction shifts from endothermic to exothermic at ∼270°C. Applying advanced drying technology and avoiding the use of N2 can help the system achieve auto-thermal operation at lower torrefaction temperature and residence time, thus leading to a higher process energy efficiency and product yield. This is the first work to relate the micro level element changes of biomass to the macro level process energy balances of the torrefaction system. This work is important in design and operation of the torrefaction system in both pilot and industrial scales to improve process efficiency and predict product quality in a reliable and economic manner.
Highlights
The utilization of raw biomass faces several challenges, e.g., low bulk energy density, hydrophilicity, and high transportation energy requirement
Qcom = mdb · α · HHVtor · ξcom where mdb (t/hr) is the mass flow rate of the dry biomass entering the torrefier, α is the fractional biomass weight loss in torrefaction, and HHVtor (GJ/t) is the higher heating value of the torgas, and ξcom is the thermal efficiency of the combustion process
For a preliminary analysis, we have considered two types of drying technologies for evaluating the system with the auto-thermal operation: conventional drying technology with drying heat of 3.0 MJ/kg water evaporated, and advanced drying technology with 1.0 MJ/kg water evaporated, respectively
Summary
The utilization of raw biomass faces several challenges, e.g., low bulk energy density, hydrophilicity, and high transportation energy requirement. Biomass is decomposed, and condensable and noncondensable volatiles are released Those volatiles, called torgas, can be combusted to provide heat for the torrefaction and the drying processes. The heat requirement of a torrefier under given operating conditions was evaluated to explore the feasibilities of integrating torrefaction with other processes or units, e.g., pellet production process (Mobini et al, 2014), other thermochemical conversion processes (Anuar et al, 2017; Atienza-Martínez et al, 2018), piston engine unit (Director and Sinelshchikov, 2019), and power generation plant (Sermyagina et al, 2016; Haseli, 2019). With adequate knowledge from laboratory scale torrefaction tests, it avoids engineering trails and predicts product properties in a reliable and economic manner To our knowledge, this is the first work to relate the micro level element changes of biomass to the macro level process energy balances of the torrefaction system. Impacts of using different drying technologies and inert N2 in the torrefaction auto-thermal operation are analyzed
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