Tagiev Sanan Mehman oglu

Kuzbass State Technical University, Kemerovo

Technologies for the Conversion of Biomass to Liquid Transportation Fuels

Cellulosic biofuels have been proposed as part of the solution to climate change and the dependence on fossil fuels, not only because they can be produced domestically from a variety of renewable biomass feedstocks, but also because they can reduce the greenhouse gas emissions, since the carbon dioxide captured when the feedstock crops are grown and cultivated balances the carbon dioxide released when the fuels are burned. According to the Energy Information Administration, biofuel consumption in primary markets is expected to reach 20% of renewable energy sources by 2030. Although corn ethanol has led the adoption of renewable biofuels in the transportation industry, the Renewable Fuels Standard (RFS), part of the Energy Independence and Security Act of 2007establishes a target of 36 billion gallons of renewable fuels by 2022 with cellulosic biofuels contributing more (16 billion gallons) than corn ethanol (15 billion gallons). Biomass-to-liquids (BTL) technology, which converts cellulosic biomass to liquid transportation fuels, has been considered a promising approach for overcoming the market barrier resulting from the current vehicle technology and fuel distribution infrastructure [1].

The two major technologies reviewed here are gasification followed by Fischer-Tropsch synthesis and fast pyrolysis followed by hydroprocessing.

Gasification followed by FT synthesis

The gasification technology produces transportation fuels through Fischer-Tropsch synthesis with electricity as byproduct. There exist two techniques of gasification: an oxygen-fed, low-temperature (870°C), nonslagging, fluidized bed gasifier and an oxygen-fed, high-temperature (1300°C), slagging, entrained flow gasifier. Both gasifiers are followed by Fischer-Tropsch synthesis, which involves converting carbon monoxide and hydrogen into liquid hydrocarbons. The major operational steps of this conversion technology are preprocessing, gasification, syngas cleaning, fuel synthesis, hydroprocessing, power generation, and air separation.

Biomass feedstocks are first dried to reduce the particle sizes during pretreatment. Each gasifier requires a specific particle size. The low-temperature (LT) option can handle larger feedstock size of 6 mm, whereas the high temperature (HT) option requires a smaller feedstock size of 1 mm. Low-temperature gasification also has the advantages of lower capital cost and high heat transfer rates within fluidized bed; but it has lower thermal and carbon efficiency. High-temperature gasification has advantages of higher carbon conversion, low tar, and methane content [2].

Fast pyrolysis followed by hydroprocessing

Fast pyrolysis is a process of heating biomass without oxygen that converts feedstock into gaseous, liquid, and solid products. Its processing steps include biomass pretreatment, fast pyrolysis, solids removal, oil recovery, char combustion, and hydroprocessing. Biomass feedstocks are dried to around 7% moisture content, and particle size is reduced to diameter of 3 mm during pretreatment. The dried biomass is then fed into a fluid bed pyrolyzer operating at 480°C and atmospheric pressure. Pyrolysis vapors enter a cyclone, which separates solids and vapors. The vapors are then condensed in an indirect heat exchanger that yields bio-oil. Noncondensable gases and solids from the pyrolysis reaction are sent to a combustor to produce the required heat for the drying and pyrolysis processes. Bio-oil is collected in a storage tank that acts as a buffer to the upgrading process [3].

The hydroprocessing step involves hydrotreating and hydrocracking. Hydrotreating is an exothermic process that removes undesired compounds such as oxygen in bio-oil. Hydrocracking is a process that breaks down larger molecules into naphtha and diesel. During hydrotreating, hydrogen can be provided from outside source or can be extracted from the bio-oil. Around one-third of the bio-oil is needed to produce the required amount of hydrogen in the hydrogen production scenario. Separator, reformer, and pressure swing adsorption are needed before hydroprocessing. A gravity separator separates pyrolysis lignin from the water-soluble bio-oil. Aqueous bio-oil and steam are sent to a high-temperature reformer, which produces syngas. This syngas is fed with methane into a pressure swing adsorption reactor to produce hydrogen. In the end, bio-oil is converted into transportation fuels via hydroprocessing [4].

The results of the implementation report show the potential of BTL in detail. A considerable amount of fuel can be produced with feasible technologies, thus making an important contribution to the security of supply. BTL also has a high potential to reduce carbon dioxide emissions. To exploit these possibilities, further steps by industry and the state are required. The company is responsible for the economic risk when assessing its decisions on how far to go in the development and production of BTL. Today, the cost of BTL production, which is higher than that of conventional fuels, is of great importance here. If large-scale BTL production is to be made economically competitive with first-generation biofuels, the optimization potential identified needs to be exploited. Industry and investors will have to provide the necessary capital; government investment grants and guarantees are also required for the first plants. Reliable legal and political parameters are also of prime importance. Cooperation and networking between all of the players involved - agriculture and forest management, investors, operators, and the petroleum and automotive industries - will be both beneficial and expedient. Reliance is placed on market mechanisms for the long-term and cheap provision of biomass. The producers must be shown the perspectives related to this and provided with incentives to achieve attractive yields with suitable plant crops.

Bibliography:

1. Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply; U.S. Department of Agriculture and U.S. Department of Energy: 2005.

2. Biomass Program Multi-Year Program Plan 2010; U.S. Department of Energy: Office of Energy Efficiency and Renewable Energy: 2010.

3. Roadmap for Biomass Technologies in the United States; U.S. Department of Energy: Biomass Research and Development Initiative: 2007.

4. National Biofuels Action Plan; Biomass Research and Development Board: U.S. Department of Agriculture and U.S. Department of Energy: 2010.