Fast-pyrolysis oil 'PyOil' is a crude mixture of liquid carbohydrates and hydrocarbon fractions which is made by heating biomass in the absence of oxygen. PyOil can be upgraded to make synthetic gasoline, low sulfur diesel fuel, and even synthetic jet fuels, using techniques familiar to the existing petroleum refining industry! However, the acidic PyOils are difficult to store in their raw state and can polymerize to highly viscous liquids over time. The purpose of this project, led by Frontline BioEnergy in collaboration with Iowa State University's Center for Sustainable Energy Technologies, is to develop high quality, time-stable and storable fast-pyrolysis oils (PyOil). OBJECTIVES: The general objective of this Phase-1 project is to explore 'feasibility' of hot gas filtration in a pyrolysis process to produce a high quality bio-oil. Technical feasibility objectives: (1) Produce high-quality bio-oil as characterized by the char and alkali content. Phase I goal is <25 ppm potassium; alternatively a stretch goal <10 ppm total alkali would replicate the bench-top results of Agblevor and Besler (1996)). (2)Compare the viscosity and storage stability properties of bio-oils condensed after either a cyclone or baghouse filter. (This comparison has never been presented in the literature). (3)Explore the benefits or potential benefits of rapid latent cooling with water or with dilute methanol as a temperature control strategy prior to filtration. Determine if there is any advantage shown by improved dust cake management in the hot gas filter-defined as an ability to control pressure drop across the filter tube sheet. APPROACH: While it is recognized that there can be a small yield penalty from increased vapor cracking reactions in the baghouse, the advantages for filtering the vapor stream prior to condensation are quite compelling [Scahill, Diebold and Feik (1996)]: (1) A free flowing char and low alkali bio-oil are discretely collected which enables marketing as separate high quality products. (2) Eliminates the detrimental influence of char associated alkali on long-term bio-oil stability, pumping performance, and downstream appliance contamination. (3) Eliminates the monetary and environmental costs of disposing the liquid/solid sludge produced by filtering char from the condensed oil. The forerunning work by NREL showed that hot gas filtration of bio-oil vapors is not trivial and more research is needed. To our knowledge, after more than ten years, no other research group has taken up the challenge to continue the efforts of Agblevor, Scahill, Diebold, Feik and Czernik. Their groundbreaking work provides an excellent foundation for the present study. Frontline has studied their work in detail and has had many discussions with John Scahill (now with DOE) to form an appreciation for the challenges and to help us innovate a new strategy for overcoming their observed difficulties. Oasmaa et al (of VTT, Finland) notes that secondary cracking reactions are slow below about 662F (350C), so filtration below this temperature may reduce yield losses. We must first demonstrate that bio-oil vapor filtration can be performed reliably at higher temperatures. Later we would experiment with reducing the filter temperature. Oasmaa also indicated that residence times as high as 2 seconds at 750F (400C) may be acceptable for fuel applications. [Oasma and Peakocke (2001), pp 11] Frontline will operate the fluid bed pyrolyzer at 932F (500C), giving an apparent residence time ~ 0.4 to 0.5 s, and will immediately cool the generated vapors to achieve the filtration temperature of approximately 734F (390C). Lower temperatures may be attempted in future work (but due to limited funds, that exploration is generally outside the proposed scope of work). The available filter is slightly oversized, but it is acceptable for research and feasibility testing. NREL scientists were making progress using the hot gas filtration technique to make high quality bio-oils in the mid 1990's. Frontline's proposal is an opportunity to reduce several of the possible difficulties identified in their early research
