The following is an analysis of the potential to generate liquid hydrocarbons from two of them, wood and bagasse/trash from ethanol production.

Biomass to Liquid process(Fischer-Tropsch synthesis)

The synthesis of hydrocarbons from carbon monoxide, CO hydrogenation over transition metal catalysts, was discovered in 1902. Collectively, the process of converting CO and H2 mixtures to liquid hydrocarbons over a transition metal catalyst has become known as the Fischer-Tropsch synthesis. Two main characteristics of FTS are the unavoidable production of a wide range of hydrocarbon products and the liberation of a large amount of heat from the highly exothermic synthesis reactions. Consequently, reactor design and process development has focused heavily on heat removal and temperature control. The focus of catalyst development is on improved catalyst lifetimes, activity, and selectivity. Product distributions are influenced by temperature, feed gas composition (H2/CO), pressure, and catalyst type and composition. There are four main steps to producing FT products: syngas generation, gas purification, FT synthesis, and product upgrading. When the feedstock is biomass, its conversion to a suitable feed gas for FTS, containing H2 and CO, takes place through gasification. But in this case, a pre-treatment prior to gasification is required, and generally consists of screening, size reduction, magnetic separation, “wet” storage, drying, and “dry” storage. Gasification can take place at different pressures, either directly or indirectly heated (lower temperatures), and with oxygen or air. Direct heating occurs by partial oxidation of the feedstock, while indirect heating occurs through a heat exchange mechanism. Upgrading usually means a combination of hydrotreating, hydrocracking, and hydroisomerization in addition to product separation. Unlike conventional fuels, FT fuels contain no sulfur and low aromatics. These properties, along with a high cetane number, result in superior combustion characteristics.

Discussions

From the information available in the literature, our studies suggest the use of a process, based on the FTS, aiming at the best use of wood and sugarcane byproducts (bagasse/trash) for the production of high-quality liquid byproducts, such as diesel, naphtha, base oils, and paraffin, and also the concomitant generation of electricity. The scheme of the chosen process involves the following steps: biomass pre-treatment section, generation of syngas through the gasification process (atmospheric fluidized bed air blown gasifier), and adjustment of the ratio of the H2/CO to be fed to the Fischer-Tropsch reactor (cobalt-based catalyst) through a shift reactor. In Tables 1 and 2, the elementary and immediate analyses of biomass can be found in percentage weight adopted in this study.

Table 3 shows the results of biomass consumption for their two feedstocks, aiming at the production of high-quality liquid byproducts as well as electric power generation.

The results listed in Table 3 show a decrease of around 13 percent in the consumption of biomass when wood is used to supply the process. The advantages and disadvantages of this scheme need to be studied further, considering, for example, the availability and cost of the raw materials. Using entirely the syngas generated in the gasification stage, this study also estimated (Table 4) the potential for electric power generation considering a Combined Cycle – CC – and a Condensing Extraction Steam Turbine – CEST.

It is important to point out that FTS produces a residual gas stream that may be used to generate electric power through a combined cycle. This allows the sugarcane bagasse to be directed towards plant BTL, thus increasing the production of liquid byproducts and keeping the electric power generation for use by the plant and/or neighborhoods. Various studies are currently trying to perfect the generation of electric power and the production of liquid byproducts with or without power generation via FTS. All start from the most diversified generators of biomass, aiming to increase the use of this kind of raw material for the world’s energy sources, thus decreasing dependence on non-renewables. Currently, most efforts are concentrated on the development of adequate gasification processes for each type of biomass. In the case of Brazil, some studies have already demonstrated the viability of bagasse and trash from sugarcane processing as a feedstock in ethanol fuel mills.

In short, the GTL process is linked not only to the use of natural gas as a raw material, but also to any source of carbon that may generate a synthesis gas in the adequate ratios for the Fischer-Tropsch process (H2/CO between 0.5 and 1.2 for iron-based catalysts, and 2.0 for cobalt-based catalysts). We think the GTL process as “synthesis gas for liquid byproducts” should be encouraged, since in the near future, the use of this technology could be vital as the world’s energy mix gradually shifts from non-renewable fossil sources to renewable ones. This is a new and promising process route for the production of high-quality liquid byproducts concomitant with ethanol production from sugarcane and its byproducts. It will allow for the increased production of “green” fuels while decreasing the pollution caused by the low energy efficiency of Brazil’s current ethanol production process.

Fernando B. de Oliveira is a process engineer and Sirlei S. A. de Sousa is a GTL senior consultant at the Petrobras R&D Center in Rio de Janeiro.