Forward Osmosis for the Water Removal From the Aqueous Phase of Pyrolysis Oil

Forward Osmosis for the Water Removal from the Aqueous Phase of Pyrolysis Oil

Divina Kaombe, May-Britt Hägg

Department of Chemical Engineering

Norwegian University of Science and Technology

Trondheim, Norway

Renewable energy consumption has increased due to the fast depletion of finite fossil fuel reserves, increase of energy demand and environmental concerns. The high energy production cost for conventional fuels and political instability of many oil producing countries, calls for the development of renewable energy resources. Wood and other forms of biomass including energy crops, agricultural wastes, forestry wastes and nut shells are some of the main renewable energy resources available. These can be converted into liquid, gaseous and solid fuels or chemical feedstock at low cost in environmentally friendly manner [1, 2]. “Energy from biomass based on short rotation forestry and other energy crops can contribute significantly towards the objectives of the Kyoto Agreement in reducing the greenhouse gases emissions and to the problems related to climate change” [2]. So far, there is no commercially viable way to offset the carbon dioxide added to the atmosphere produced by fossil fuel combustion. Two-thirds of global anthropogenic carbon dioxide emissions are contributed by combustion of fossil fuels. Carbon dioxide is a major greenhouse gas (GHG); therefore it contributes significantly to climate change with its associated environmental problems [3]. Considering environmental perspective, there is a need for the promotion of using biomass derived fuels.

Pyrolysis oil is the biomass based oil that can be used as an alternative fuel. However, recently there is an increased interest of using the lignins of the pyrolysis oil for production of various chemicals such as food flavourings, specialities, resins, agri-chemicals, fertilizers, emissions control agents, etc.; that poses a challenge in using the pyrolysis oil as the fuel [4]. Due to the demand for lignins; pyrolysis oil is separated into lignin rich phase and carbohydrates rich aqueous phase. Removal of lignin rich phase from pyrolysis oil leaves the oil with less lignin content, higher water content and less energy density; consequently, the use of this part of oil as a fuel is limited. It is well known that higher water content with lower energy density limits application of aqueous phase of the pyrolysis oil as a fuel. It is reported that the water content of bulk pyrolysis oil without phase separation ranges 15 wt % - 30 wt % with energy density ~ 19.5 MJ/L [5]. Therefore, reduction of the water content can increase the energy density of the aqueous phase of the pyrolysis oil; subsequently, suitable to be used as a combustion fuel. In this study the FO process is used to remove water from the pyrolysis oil aqueous phase. Cellulose acetate membranes in flat sheet mode are used in the process. Three different draw solutions are used: NaCl, MgCl₂ and AlCl₃ solutions; the aim is to investigate which draw solution generates high transmembrane flux.  The highest flux was found when pyrolysis oil feed was treated with MgCl₂ solution. This was confirmed by analysis of the energy density and water content which showed that the oil samples treated with the MgCl₂ solution had relatively low water content and high energy density. The results from the investigations with different draw solutions will be presented.

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4.    A. V. Bridgwater, Renewable Fuels and Chemicals by Thermal Processing of Biomass, Chemical Engineering Journal, 91 (2003) 87-102.

5.    D. Mohan, C. U. Pittman, P. H. Steele, Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review, Energy & Fuels, 20 (2006) 848-889.