Article pubs.acs.org/EF
Comparative Assessment of Wet Torrefaction Quang-Vu Bach,*,† Khanh-Quang Tran,† Roger Antoine Khalil,‡ Øyvind Skreiberg,‡ and Gulaim Seisenbaeva§ †
Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway ‡ Department of Thermal Energy, SINTEF Energy Research, NO-7465 Trondheim, Norway § Department of Chemistry, Swedish University of Agricultural Sciences, SE-75007 Uppsala, Sweden ABSTRACT: Wet torrefaction of typical Norwegian biomass fuels was studied within the temperature window of 175−225 °C, using a benchtop autoclave reactor of 250 mL in volume from Parr Instrument. Two types of local biomass fuels were employed as feedstock, Norway spruce (softwood) and birch (hardwood). Effects of process parameters including pressure, reaction temperature, holding time, and feedstock particle size on the yield and properties of the solid products were investigated. It appears that birch wood is more reactive and produces less solid products than spruce wood in the same wet torrefaction conditions. Increasing pressure above the saturated vapor pressure of water enhances the torrefaction rate. Both reaction temperature and holding time have significant effects on solid product yield and fuel properties of wet torrefied biomass. The yield of solid products is slightly reduced with decreasing feedstock particle size. The ash content of biomass fuel is significantly reduced by wet torrefaction. In addition, a comparison between wet and dry torrefaction supported by regression analyses and numerical predictions shows that wet torrefaction can produce solid fuels with greater heating values at much lower temperatures and shorter holding times.
1. INTRODUCTION
of the raw biomass to make the process energetically viable.14−16 There are two torrefaction techniques, dry and wet torrefaction. Dry torrefaction (DT) is thermal treatment of biomass in an inert environment at atmospheric pressure and temperatures within the range of 200−300 °C.17,18 Wet torrefaction (WT) may be defined as treatment of biomass in a hydrothermal media (HM), or hot compressed water (HCW), at temperatures within 180−260 °C.19−21 During the past decade, research and development activities on DT for energy applications including combustion, gasification, and pyrolysis have been very active.16,22−33 It has been reported that, during combustion, torrefied biomass behaves more coal-like with more stable burning characteristics, compared to its untreated biomass.28,29 The efficiency of gasification and the quality of syngas are improved by torrefaction.16,30,31 Moreover, for fast-pyrolysis, torrefaction appears to decrease the yield of byproducts and to improve the quality of bio-oil.32,33 The technology of DT has been rapidly developed to the stage of market introduction and commercial operation. Several torrefaction installations have recently been built in Europe and North America, with a total capacity of several hundred thousand tons per year.34 However, it has been claimed that no clear winner in this area can be identified so far. This is partly due to the fact that optimal process conditions have not been well established for the various concepts and feedstocks. The majority of research and development in this area have been carried out for clean wood feedstocks, and it is
Biomass is a renewable and carbon neutral energy resource which has a high potential for replacing fossil fuels. However, the use of biomass for energy applications is not straightforward. Typical disadvantages of using biomass as fuel, compared to coal, include the lower bulk density, higher moisture content, inferior heating value, and poorer grindability. Although biomass resources are distributed over the world more evenly than the world proven coal reserves, an additional substantial disadvantage of biomass is its relatively less concentrated occurrence compared to coal which normally occurs highly concentrated in coal mines. These drawbacks increase the cost for handling, transport, and storage of biomass fuels, limiting the use of biomass for bioenergy applications. In addition, ash forming elements especially alkali metals may cause technical and performance problems for the downstream equipment in thermal energy conversion processes such as gasification and combustion.1−3 One way to overcome the aforementioned disadvantages of using biomass as fuel is to preprocess the fuel via torrefaction, which may be defined as mild pyrolysis of biomass. This is due to the fact that the main product of the torrefaction process is a hydrophobic solid fuel,4−6 which may be referred to as “biochar”,7,8 with much better grindability9,10 and superior heating value.11−13 The handling, transport, storage, and use of the biochar as fuel become easier and less expensive compared to the native biomass fuel. In addition to the solid product and depending on the treatment conditions, torrefaction produces byproducts in liquid and gas phases. However, their fractions are normally considered to be small, being less than 30% by weight on dry basis and containing less than 10% of the energy © 2013 American Chemical Society
Received: July 10, 2013 Revised: October 7, 2013 Published: October 8, 2013 6743
dx.doi.org/10.1021/ef401295w | Energy Fuels 2013, 27, 6743−6753
Energy & Fuels
Article
Table 1. Proximate and Ultimate Analyses for the Feedstock (Dry Basis) proximate analysis
a
ultimate analysis
type of biomass
asha
VMa
fixed Ca
Ca
Ha
Oa
Na
Sa
HHVb
Norway spruce Norway birch
0.23 0.28
86.50 89.46
13.27 10.26
50.31 48.94
6.24 6.35
43.38 44.60
0.07 0.11
