A. LAPPAS and E. HERACLEOUS, CPERI — Chemical Process Engineering Research Institute, Greece

Abstract: The production of synthetic fuels from biomass via Fischer — Tropsch (FT), otherwise known as biomass-to-liquids (BTL) process, constitutes one of the most promising routes for tomorrow’s fuels. In this chapter, basic topics, as well as current advances in the production of FT biofuels, are discussed. Starting with a short discussion on biomass gasification and syngas conditioning, the main types of FT reactors and catalysts, along with the different technologies for upgrading FT liquids to premium fuels are thoroughly discussed. Closing, recent advances in the commercialization of the BTL process are presented, along with a discussion on the advantages and limitations of this process and its outlook in the future fuels market.

Key words: biomass-to-liquids (BTL) process, Fischer-Tropsch (FT) synthesis, biomass gasification, Fischer-Tropsch reactors, upgrading of BTL-FT products.

19.1 Introduction

Growing environmental and security of supply concerns are the main drivers that bring about changes to fuel products. European Union (EU) policies on local air quality, climate change and sustainability, applied via Fuel Directives or Emission Directives, have strongly influenced research efforts and advances in conventional fossil, synthetic and bio-origin fuels. These, in combination with the depletion of the crude oil reserves, have rendered the production of hydrocarbons via the Fischer-Tropsch (FT) synthesis as one of the most promising routes for tomorrow’s fuels. According to a recent study (Takeshita and Yamaji, 2008), ‘FT synfuels become a major alternative energy carrier and have a noticeable share in the global final energy mix regardless of CO2 policy.’

The production of fuels via FT involves the conversion of the feedstock to synthesis gas (carbon monoxide and hydrogen) and subsequent synthesis of hydrocarbons via the FT synthesis reaction:

CO + 2 H2 ^ ‘-CH2-’ + H2O [1.1]

where ‘-CH2-’ represents a product consisting mainly of paraffinic hydrocarbons of variable chain length.

Generally, the FT process is operated in the temperature range of 150-300°C to avoid high methane by-product formation. Increased pressure leads to higher conversion rates and also favours the formation of desired long-chain alkanes. Typical pressures are in the range of one to several tens of atmospheres. The FT hydrogenation reaction is catalyzed mainly by Fe and Co catalysts, while the size and distribution of the hydrocarbon products of the reaction is generally governed by the Anderson-Schulz-Flory (ASF) chain polymerization kinetics model (Bartholomew, 1990).

One of the most important advantages of FT is its versatility concerning both feedstock and products. The FT process can produce hydrocarbons of different lengths from syngas originating from any carbon-containing feedstock, such as coal, natural gas and biomass. Depending on the feedstock, the process is referred to as CTL (coal-to-liquids), GTL (gas-to-liquids) or BTL (biomass-to-liquids). Moreover, synthetic fuels have distinct environmental advantages over conventional crude-refined fuels since they are virtually free of sulphur, nitrogen and aromatics. At the same time, they are largely compatible with current vehicles and fully blendable with conventional fuels and can thus be handled by existing fuel infrastructure. However, both the high energy demands and the large capital cost of FT plants contribute to the high price of synthetic FT fuels, and as a consequence, the economic viability of the FT process largely depends on the price of crude oil.

The FT process is not a new concept. It was first developed in Germany in the 1930s, as Germany was very poor in oil resources and needed, during the Second World War, to develop an independent source of transportation fuels based on their abundant coal resources (Davis, 2002). The exploitation of the vast oil reserves of the Middle East after the Second World War made the FT process uneconomical and interest decreased, with the exception of South Africa. South Africa has vast coal deposits, and the high oil prices combined with the oil embargo during the 1970s led to the great development of the FT process from SASOL (South African Synthetic Oil Limited) (Overett et al, 2000). The technical advances in the FT process and the increasing crude oil prices in combination with the depletion of the crude reserves have led, in the last few decades, to a renowned worldwide interest in the FT process. The FT process has already been commercialized on a large scale. Sasol Synfuels currently operates two CTL plants, processing 45 million tonnes of coal per year and fulfilling about 28% of South Africa’s diesel and petrol needs (Dry, 2002). Since 1993, Shell in Malaysia (Bintulu) and PetroS A in South Africa (Mossel Bay) have been operating industrial FT synthesis facilities, which produce liquid fuels from synthesis gas that originally comes from natural gas (GTL). Shell is currently constructing a new GTL plant in Qatar, which will be the world’s largest plant converting natural gas into 140 000 barrels per day of clean-burning liquid transport fuel and other products (Shell, 2009). A similar plant is also being built by Sasol and Qatar Petroleum in Qatar in the Persian Gulf.

This renewed interest in the FT process during the 1980s and 1990s was initiated based on the depletion of crude reserves, the subsequent increase of the crude oil price and the worldwide existence of much larger reserves of natural gas and coal. Today, global warming and the universal efforts for CO2 emissions reduction rekindle the interest in FT technology, as high-quality clean biofuels, compatible with existing infrastructure and vehicle technology, can be produced via the FT process using a wide variety of biomass resources. Materials foreseen to be used in the BTL process include wood and forest residues, agricultural residues and by-products, bagasse, lignocellulosic feedstock from processing residues (paper slurry, black liquor, etc.) and energy crops, with wood being the most commonly considered biomass feed.

The use of renewable resources as feedstock, with all associated environmental advantages, undoubtedly gives synthetic fuels a new dynamic. The production of synthetic fuels from biomass comprises of the three basic steps of all FT processes: gasification of the feedstock (in this case biomass) for production of synthesis gas (CO and H2) and gas cleaning/conditioning, FT synthesis for middle distillates production and upgrading of the FT liquids to high-quality fuel products. However, the development of a commercial BTL process is currently hindered by the fact that, in contrast to GTL, for which industrial synthesis gas production processes have been well known and used for several decades, there is at present no industrial unit for biomass gasification in existence. Closest to commercialization is CHOREN, a German-based technology company that has operated a BTL demonstration plant since 2005 and is currently constructing the first commercial BTL plant, employing their patented biomass gasification process and the Shell SMDS (Shell Middle Distillate Synthesis) FT process.

Research is actively ongoing on all the three steps of the process in an effort to improve the overall efficiency, with special focus on the biomass gasification step and subsequent gas conditioning prior to the FT reactor in order to meet the strict FT gas purification requirements. Several different types of gasification technology [e. g. fixed bed, circulating fluidized bed (CFB), entrained flow gasifiers, etc.] and operation modes have been considered and assessed and will be discussed later in the chapter.

In the next paragraphs, an overview of the basic topics, including current up-to-date advances in the production of biofuels via FT synthesis, will be discussed. Starting with a short discussion on biomass gasification, including types of gasifiers and gas cleaning techniques, we will then thoroughly describe the main types of reactors and catalytic materials currently employed for FT, followed by a comprehensive discussion on the different processes and technologies for the upgrading of the FT liquids to premium fuel products. In Section 19.3, we will give a description of the final BTL fuel products and their properties. Closing, the most recent advances in the commercialization of the BTL process will be presented, along with a discussion on the advantages and limitations of this process and its outlook in the future fuels market.