Fuel oil quality and combustion of fast pyrolysis bio-oils

Fast pyrolysis bio-oils are supposed to replace fuel oils in many stationary applications including boilers and furnaces. However, these bio-oils are completely different from petroleum fuels and other bio-oils in the market, like biodiesels, as regards both their physical properties and chemical composition. When the unusual properties of these bio-oils are carefully taken into account, their combustion without a pilot flame or support fuel is possible on an industrial scale. Even blending of these oils with alcohols in order to improve combustion is not necessarily required. In the recent industrial scale bio-oil combustion tests, bio-oil has been found to be technically suitable for replacing heavy fuel oil in district heating applications. This kind of replacement, however, needs some modifications to be made to the existing units, which need to be engineered carefully. For example, all the parts in contact with bio-oil should be replaced with parts made of stainless steel or better, and the suitability of all gaskets and instruments needs to be checked. In general, the emissions in the bio-oil combustion are very dependent on the original levels of solids, water and nitrogen in the oil being combusted. Typically, the emissions levels are between those of light fuel oil and the lightest heavy fuel oil, but particulate emission may be higher. On the other hand, there are practically no SOx-emissions generated in the bio-oil combustion. The NOx-emission in bio-oil combustion mainly originates from fuel-bound nitrogen. Staged combustion for NOx-reduction may be recommended, as successful air staging in natural gas, heavy and light fuel oil combustion has already been done. The recent bio-oil combustion tests have also shown that bio-oil combustion technology works well, and there are not many possibilities of further lowering particulate emissions, since the majority of the particulates are typically incombustible matter. Therefore, it is recommended to reduce the solids content of the bio-oil to <0.1 wt% if possible, and to ensure that inorganics in the form of ash and sand are present at as low a concentration as possible. Current burner designs are quite sensitive to the changes in the quality of the bio-oil, which may cause problems in ignition, flame detection and flame stabilization. Therefore, in order to be able to create reliable bio-oil combustion systems that operate at high efficiency, bio-oil grades should be standardized for combustion applications. Consequently, international standards, norms, specifications and guidelines should be defined and created urgently. ASTM standardisation is already going on and CEN standardisation should be initiated 2013. Careful quality control, combined with standards and specifications, all the way from feedstock harvesting through production to end-use is recommended in order to make sure that emission targets and limits in combustion applications are achieved. The authors would like to indicate that there are possibilities for all the burner technologies and models described in this publication to be further developed to meet the challenges generally caused by the nature, quality and characteristics of the bio-oils. So far, relatively few burner manufacturers have developed commercially available burner models for fast pyrolysis bio-oils. Environmental requirements affect the commercialization of the burner technologies and the quality of the oil required for the combustion applications. Naturally, the end-user of the oil is interested in the total costs of the combustion concept compared to those of fossil fuels. Therefore, the cost-effectiveness of the total package is extremely important. The authors are involved in developing further cost-efficient fast pyrolysis bio-oil combustion and flue gas handling applications in the future.