Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
pubs.acs.org/EF
Comparative Characterization of a Torrefied Wood Pellet under Steam and Nitrogen Atmospheres Yongwoon Lee,† Won Yang,*,† Taeyoung Chae,† Byeol Kang,† Jinje Park,‡ and Changkook Ryu*,‡ †
Thermochemical Energy Systems R&D Group, Korea Institute of Industrial Technology, Cheonan 330-825, South Korea School of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, South Korea
‡
ABSTRACT: Torrefaction is a pretreatment technology for biomass to enhance its fuel quality for thermal conversion. Torrefied biomass has many advantages for handling and storage including high energy density and hydrophobicity. It also improves the grindability, which is beneficial for cofiring with pulverized coal in a power plant. This study aims to investigate the optimum operating conditions for a biomass torrefaction process using steam as a purge gas. Wood pellets were torrefied using steam heated to between 230 and 300 °C in a lab-scale packed-bed reactor. The characteristics of the torrefied wood pellets (TWPs) were compared to those produced under nitrogen. The results showed that when the temperature increased from 230 to 300 °C, the TWP yield using steam decreased from 82 to 46 wt %, but the carbon content and heating value increased from 54 to 75 wt % and 21.14 to 28.85 MJ/kg, respectively. On the other hand, the TWPs using nitrogen had a slightly larger mass yield but with a lower carbon content and heating value. A significant increase in the grindability was achieved by torrefaction at 270 °C, compared to the wood pellets. However, the TWPs produced even at 300 °C had grindability lower than that of a subbituminous coal. Steam can be a good medium for torrefaction, which is also stable and free from the risk of self-ignition.
1. INTRODUCTION Torrefaction is an important pretreatment technology that thermochemically improves the fuel quality of solid biomass in a mild temperature typically below 300 °C. It produces upgraded solid fuels with a mass yield of about 70 wt % containing about 90% of the chemical energy in the original biomass, leading to high energy density.1,2 Moreover, torrefied biomass has hydrophobic characteristics with a low moisture content, reducing the decomposability for microbes and increasing the energy density by about 1.3−2 MW/m3.3,4 Therefore, it has the advantage of better storage and handling with reduced transportation costs. During torrefaction, the biomass undergoes thermochemical changes.5 First, the time at an initial temperature at 50−120 °C causes water to evaporate. Second, the treatment at 120 to 150 °C lessens the binding effect of lignin in the biomass. Third, hydrogen and carbon bonds in the biomass start to decompose at 150 to 200 °C, and the biomass becomes hydrophobic. Fourth, the decomposition of hemicellulose starts at the temperature range between 200 and 250 °C. Finally, most of the hemicellulose decomposes, and the decomposition of the lignin and cellulose begins at the same time at 250−300 °C.6 The biomass structure becomes brittle and nonfibrous because of the thermal deposition in this zone. Above 300 °C, the biomass is excessively devolatilized and carbonized, which is not desirable because of significant energy loss to tar and volatile gases.7 Therefore, the typical torrefaction process maintains the temperature range between 200 and 300 °C with a low particle heating rate (
