In this second article on fuel oil, use of forestry residue pyrolysis liquids, their physicochemical properties, and the behavior of these liquids are described. Understanding of the chemical composition of forestry residue liquids enables the selection of correct handling and storage conditions. Forestry residue is one of the most viable biomass feedstocks for liquid production in Northern softwood forest zone. A 10-25 wt % top phase with a high heating value is produced from forestry residue due to the high content of extractives and low water content. However, it has high solid and ash contents. The main product, bottom phase, is similar to bark-free wood pyrolysis liquid: volatile acids 8-10 wt %; aldehydes and ketones 10-15 wt %; water 25-30 wt %; "sugar constituents" 30-35 wt %; water-insoluble, mainly lignin-based constituents 15-20 wt %; and extractives (2-6 wt %). Its physical properties (water 28 wt %, pH 3.0, viscosity at 40 °C 15 cSt, LHV 14 MJ/kg, solids < 0.05 wt %), making it suitable for fuel oil use. The solids content is typically lower than in pine liquids. Needles and bark in forestry residue, especially in fresh green feedstock, yields high alkali metal (400-1000 mg/kg), ash (0.1-0.2 wt %), and nitrogen (0.1-0.4 wt %) contents of the liquid compared to pine (50 mg/kg, 0.02-0.03 wt %, < 0.1 wt %, respectively) liquids. This results in higher NO x and particulates emissions in combustion. In pyrolysis of forestry residue attention should be paid on solids/alkali removal and decrease in nitrogen content.

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Fast pyrolysis bio-oil (pyrolysis liquid) from plant residues is one alternative to replace fossil fuels and feedstocks. Fast pyrolysis liquid is a potential source of revenues for companies who have biomass residues at their disposal. Once produced, bio-oils may be shipped, stored, and utilized much like conventional liquid fuels once their specific fuel properties are taken into account. First encouraging large scale bio-oil utilization tests with published results were carried out in Stockholm in the 1990s in a heating boiler designed for heavy fuel oil. Industrial ovens are also potential users of bio-oil. Bio-oil would also be an interesting fuel for small scale distributed heat or power production. However, introducing a new fuel into the markets is not going to happen easily. Bio-oil is quite different from conventional liquid fuels, and many challenges remains to be overcome. A stepwise market introduction is proposed: bio-oil would first replace fuel oil in boilers, where its properties would not be prohibitive. Once the overall utilization chain has been proven, more demanding uses may be introduced. VTT has been developing an integrated concept, in which fast pyrolysis is integrated to a fluidized-bed boiler. The experimental work on fast pyrolysis has been focused in supporting this concept. In Scandinavia, forest residues are the most feasible feedstocks for pyrolysis. These residues contain extractive matter that yield a second liquid phase. This is both an opportunity (for recovering byproduct) and a challence (for using both phases as fuel). Agrobiomasses are more challenging feedstocks for energy use due to the high amount of alkali metals and nitrogen in the oil. In addition, they produce more water during pyrolysis, causing phase instability. In this paper, fast pyrolysis is discussed including experimental results from pyrolysis of wood and agricultural residues as well as results from a techno-economic evaluation.

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Although high liquid yields of a single phase product can be obtained from bark free “white” wood feedstocks by fast pyrolysis processes, lower yields and a two phase product are obtained from feedstocks containing bark and needles as is commonly found with forestry residues. The liquid yield is thus reduced from levels of 70−75 wt % to those of 60−65 wt %. This will have a significant impact on the economic viability of pyrolysis projects in Scandanavia as forestry residues are a major source of raw materials. The forestry residue product is composed of an extractive rich upper phase which varies from 10 to 20% of the total product and a bottom phase closely resembling the normal bark free wood product. Phase separation occurs due to the higher extractive content of the residues which due to their much lower oxygen phase separate. Extractives are composed of components such as fatty acids, fatty alcohols, terpenes, resin acids, and terpenoids which have lower oxygen content than pyrolysis liquid compounds in general and which phase separate forming an upper phase that has a higher viscosity and heating value than the bottom phase. The phase separation was found to be enhanced by an increase in temperature and/or in storage time.