Biomass pyrolysis oils have potential to be used as a fuel oil substitute. Combustion tests have shown that the oils burn efficiently in standard or slightly modified boilers and engines with rates similar to those for commercial fuels. However, these tests also identified several challenges in bio-oils applications resulting from their properties. The oils have heating values of only 40−50% of that for hydrocarbon fuels. They have a high water content that is detrimental for ignition. Organic acids in the oils are corrosive to common construction materials. Solids (char) can block injectors or erode turbine blades. Over time, reactivity of some components in the oils leads to formation of larger molecules that results in high viscosity and in slower combustion. This paper discusses physical and chemical characteristics of bio-oils relevant to fuel applications as well as some low-cost methods for improvement of these properties. It also provides bio-oil specifications proposed by some industrial users and recommendations for storage and handling.

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.

The storage properties of fuels are critical in regard to the introduction of a new fuel into markets. The fuel must be homogeneous, and the properties of the fuel should not change significantly during the storage at the customer's facility. In this research, the storage stability of wood-based pyrolysis liquids was followed by analysis of the changes in the physical properties and chemical composition during storage. The main physicochemical changes took place during the first six months. The high-molecular-mass (HMM) fraction of water-insolubles, which were originally lignin-derived material, increased, because of polymerization and condensation reactions of carbohydrate constituents, aldehydes, and ketones. Therefore, the average molecular mass of pyrolysis liquids increased, which was also observed as an increase in viscosity. There was a clear correlation of the average molecular mass with the viscosity, water-insolubles, and the HMM fraction of water-insolubles. The chemical changes in the aging were similar to those which occurred during the accelerated aging test. The decrease in volatile aldehydes and ketones increased the flash point of the liquids. The increase in viscosity increased the pour point. Water was formed as a byproduct in various condensation reactions. Increases in the amount of water decreased the heating value. The density of the liquid increased, because the increase in HMM lignin fraction was more significant than the increase in water.