Fast Pyrolysis of Woody Biomass in a Pyroprobe Reactor

Abstract

The future depletion of conventional fossil fuel reserves, the ever increasing need for energy self – reliance and the global concern around environmental change caused by their use, have made heat and power generation from alternative and sustainable sources a primary research focus worldwide. Biomass is such a source, constituting a clean and renewable fuel, while being the third fuel resource in the world after coal and oil in terms of abundance. Through thermochemical processes biomass can be employed for energy, chemicals and transport fuel production. Pyrolysis is the thermochemical process of biomass decomposition into various useful products in the absence of an oxidation medium. Torrefaction is a mild pyrolysis process, typically carried out in a temperature range between 230oC and 300oC. This pretreatment process offers benefits with respect to biomasses energy density, while reducing the oxygen and hydrogen to carbon ratios and its hygroscopic nature. In this work untreated and torrefied biomass species were pyrolysed under different final temperatures in a Pyroprobe 5200 reactor. In particular, raw and torrefied (at 250oC and 265oC) wood ash (Fraxinus excelsior) and raw and torrefied (at 300oC) Torrcoal, which consists of mixed wood residues, were investigated. The feed was reduced to a particle size less than 75 μm and the experiments were performed with a sample size of 30 mg in the temperature range of 600oC to 1000oC at a heating rate of 600oC/s, in order to achieve fast pyrolysis conditions. The main purpose of these experiments was to determine the effect of final pyrolysis temperature and torrefaction to the yield and nature of pyrolysis final products. In terms of individual gases, CO, CO2, CH4 and H2 were identified using a micro GC. Tar compounds (phenol and PAHs) were identified and quantified using a HPLC system. The aforementioned experiments produced mass closure values between 65% and 84%, which can be considered as satisfactory due to the difficulties in measuring gravimetrically pyrolytic water, higher hydrocarbon gases and light tar compounds. Increasing pyrolysis temperatures had a negative effect on char yield, however above 800oC it appeared to stabilize. Maximum values of the liquid yield were obtained at 600oC and 700oC before attaining a decreasing trend for the ash materials and stabilizing for Torrcoal materials at 900oC. Gas yields increased until 900oC for Torrcoal species and ash torrefied at 265, but for the rest species the increase continued until 1000oC. CO was the major gas produced above 800oC overtaking CO2 at that temperature, while CH4 and H2 yields became significant above 700oC and 800oC respectively. Torrefaction and increased torrefaction severity led to an increase of the char yield of the pyrolysis process. Their effect on the liquid yield was the opposite. In terms of total gases production the differences were minimal and only noticeable above 800oC. However, the quality of the pyrolysis gas was higher for the torrefied species. Regarding the tar compounds analysis, phenol yield decreased until 800oC before reaching a plateau thereafter. PAH species yield increased steeply between 800oC and 900oC. Their production still increased above that temperature but at a slower rate. Naphthalene was the major PAH produced. Torrefaction seemed to favour phenol production while its effect on PAHs was not significant. Between the two biomass species studied, the higher phenol yield of Torrcoal pyrolysis along with its higher char and lower liquid yield are indicative of higher lignin content for this biomass.