Low-temperature FT synthesis produces long-chain waxy hydrocarbon molecules (C₂₁₊) with a characteristic distribution of individual hydrocarbon compounds, analogous to polymerization. Upgrading of primary FT products uses hydroprocessing to produce a maximum yield of high-quality fuels.

Because there is no practical experience with large-scale BTL plants thus far, overall mass balance and achievable product yields have to be estimated on the basis of extrapolations from coal or gas as feedstock. Various studies have recently been published with estimated yield and efficiency values of the overall process based on flowsheet simulations. These studies indicate that efficiencies in terms of chemical energy and carbon recovered in the hydrocarbon product may be expected in the range of 30-50 and 25-45%, respectively.

Given the shortage of arable land and water in many countries, there is a need to achieve maximum fuel yields per unit of land. In addition to high biomass yields in agriculture and forestry, high conversion efficiencies in biomass processing are required. Similar to biomass feedstocks, overall yields of synfuel products from coal or natural gas are significantly lower than the fuel yields from petroleum refining.

—Unruh et al.

Unruh et al. attribute the limited carbon or chemical energy efficiency of the overall BTL process to four factors:

1. Overall stoichiometry;
2. Different temperature levels between the heat required for the endothermic gasification step and the exothermic synthesis reaction;
3. Energy requirements for continuous operation; and
4. Product selectivity—i.e., which fraction of the reactants is converted to the desired products (e.g., to C_5-20 hydrocarbons during FT synthesis and hydrocracking).

To improve the carbon conversion efficiency of the BTL process, Unruh et al. highlight three strategies:

1. Minimize O₂ consumption in gasification and exothermicity of the overall reaction;
2. Improve hydrocarbon selectivity during synthesis and hydrocracking; and
3. Add hydrogen (as CH₄ to gasification or as H₂ to FT synthesis) and allow for CO₂ hydrogenation.

Strategies 1 and 2 are being followed in the development of improved gasification processes or catalysts (e.g., for modifying Anderson-Schulz-Flory (ASF) product selectivities in FT synthesis). For strategy 3, conceptual reflections and a reactor concept for integrated CO₂ hydrogenation in FT synthesis will be presented in the following. A novel membrane reactor, allowing for in situ H₂O removal, should help to increase CO₂ conversion during FT synthesis with a Fe-based catalyst.

—Unruh et al.

On the basis of simulated case studies, the KIT team carried out experiments in a lab-scale fixed-bed membrane reactor at conditions where the strongest effects of H₂O removal could be expected.

The combined theoretical and experimental study showed measurable effects of membrane integration in a lab-scale fixed-bed FT reactor, and provided directions for further membrane development—i.e., benchmarks for membrane permeances and permselectivities. Accordingly, the KIT team is developing new types of membranes.

They note, however, that the use of membranes as a strategy to maximize carbon efficiency will affect plant economics in a complex way. Most critical are the large membrane areas required, because of low membrane permeabilities, which will compete with heat-transfer areas inside the reactor.

A quantitative assessment of the concept of in situ H₂O removal during FT synthesis and the potential beneficial effects on reactor performance will result from an ongoing study. Here, the potentials are being addressed with in situ H₂O removal via alternative methods (selective permeation through membranes, adsorption, and chemical conversion) and also after an external CO₂ (reverse) shift reactor combined with a FT reactor with a cobalt catalyst. Results regarding reactor analysis and practical process implications will be presented in a subsequent publication.

—Unruh et al.

Resources

• Dominik Unruh, Kyra Pabst and Georg Schaub (2010) Fischer-Tropsch Synfuels from Biomass: Maximizing Carbon Efficiency and Hydrocarbon Yield. Energy Fuels, Article ASAP doi: 10.1021/ef9009185