Wet Torrefaction of Biomass – Production and Conversion of Hydrochar

Abstract

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. It is because native solid biomass fuels are highly bulky and inhomogeneous. They normally have higher moisture content, inferior heating value, and poorer grindability, compared to coal. These drawbacks limit the use of biomass as fuel. Pretreatment of biomass via chipping and/or pelletizing for example is therefore a common practice in order to overcome the drawbacks. This operation adds more costs to biomass fuels, but improvements in the fuel properties are limited. Wet torrefaction (WT) is a promising method for pretreatment of biomass for use as fuel. The method involves the use of hot compressed water, within 180–260 °C approximately, as reaction medium. Like dry torrefaction (DT), which may be defined as mild thermal treatment of biomass within 200–300 °C, WT improves significantly the fuel properties of biomass. In addition, due to the use of water as reaction medium, WT is highly suitable for low cost biomass sources such as forest residues, agricultural wastes, and aquatic energy crops, which normally have very high moisture content. This PhD was carried out to technically assess the WT process as a pretreatment method for production of advanced solid biofuel, hydrochar, from forest residues, a low cost biomass resource in Norway. As the first step, stem woods from Norway spruce (softwood) and birch (hardwood) were tested as feedstocks. This choice made it possible to compare with the results from previous studies on DT of biomass using identical feedstocks. WT experiments were carried out using a bench‐top autoclave reactor of 250 ml in volume from Parr Instrument, with nitrogen as purge gas. Effects of various WT process parameters on the yield and the fuel properties of hydrochar (solid fuel obtained from biomass WT) were examined. The pyrolysis and combustion reactivity of hydrochar, produced under various WT conditions, was studied thermogravimetrically by means of a Mettler Toledo TGA/SDTA 815e. Multipseudo‐ component models with different reaction orders were adopted for kinetic modelling and extraction of the kinetic parameters from these thermochemical conversion processes of hydrochars. Effects of WT on the kinetics were also discussed. In the second step, forest residues were used as feedstock, employing similar approaches as in the first step. In addition, carbon dioxide was tested as purge gas and compared with nitrogen for evaluating the possibility to use and recover heat of the flue gas from combustion plants. Finally, the pelletability of hydrochar from forest residues was investigated and compared with that of untreated feedstock. The pelletization was performed using a single pellet press. Different compressing pressures (20, 40, 80, 160, 240 MPa) and temperatures (120, 180 °C) were applied to produce pellets. The pellet strength was then tested via diametric compression test, employing a 60 mm diameter probe connected to a Lloyd LR 5K texture analyzer. Effects of WT on the mass density, energy density and mechanical strength of the pellet were investigated. The major findings from the studies reported in this PhD are: · Both reaction temperature and holding time have significant effects on the mass yield, energy yield, and fuel properties of the hydrochar. · Pressure also enhances the torrefaction rate; however, the effect becomes marginal above a certain pressure. · Feedstock particle size slightly affects the yield and fuel properties of the hydrochar. · Ash content of biomass fuel is significantly reduced by WT. · Given the same solid yields, WT requires significantly lower torrefaction temperatures and shorter holding times than DT. · Given the same solid yields, solid biomass fuels upgraded via WT have greater heating values than via DT. · Hardwood is more reactive and produces less hydrochar than softwood in identical WT conditions. · Forest residues are more reactive than stem woods in identical WT conditions. · WT in CO2 enhances the torrefaction process, but reduces the heating value of hydrochar, compare to WT in N2. · The pellets made from wet‐torrefied forest residues are more compressible and mechanically stronger than the pellets made from raw forest residues. · Overall, WT has positive effects on the fuel properties of biomass.