Biomass and Municipal Solid Waste (MSW) continue to be great feedstocks for conversion to fuels and chemicals through the use of gasification, followed by the Fischer Tropsch process.
However, three issues hamper further development in gasification technology. These issues are:
- Low operating pressure in the gasifier
- Oxygen supply
- FT products upgrading
Low Pressure Gasification
Low pressure gasification is the most commonly used gasifier for MSW. However, the low pressure gasification ultimately requires four (4) stages of compression. The first stage is required to get through the syngas cleanup section of the plant. Three (3) additional stages are required to get the syngas up to the operating pressure of the Fischer-Tropsch process. Furthermore, the MSW solids addition to a low pressure gasifier requires a double slide gate valve system to add solids to the gasifier in small batches. This valve will add the solids while at the same time sealing in the hot combustions gases.
An improvement to the system is made when the gasifier can be operated at higher pressures.
Use of a solids pump similar to one Rocketdyne has manufactured, will allow the gasifier to easily operate at 50 psig without sliding gate valves. See figure above. This elevated pressure provides a means to get through the gas cleanup system.
Source of Oxygen
The source of oxygen for gasification is typically an air separation unit (ASU) which adds additional installed costs to any gasification project. These can be purchased or leased from the major industrial gas producers.
As an alternative, water, which is 88.9 wt. % oxygen, can be used rather than pure oxygen. This saves the cost of the ASU. In addition, water is easily pumped up to an elevated gasifier operating pressure.
Products Produced from Syngas
The Fischer-Tropsch process produces broad range mixture of hydrocarbon liquids. Typically, the heaviest cut is a wax product, but it will require further upgrading to be a quality product. There are two (2) basic options for upgrading:
- Hydrocracking – the wax cut can be cracked with a typical hydrocracker unit, and then fractionally distilled. The cracking process will maximize the production of diesel fuel from the FT process.
- Hydrotreating – the wax can be hydrotreated to improve quality. This is followed by fractionating the wax into various commercial grades.
Both of these unit operations require extensive capital investment at the plant site, but are typically paid out based upon the increased product revenue. .
Rather than following the gasifier with an FT Process, a newer option is the production of aromatic chemicals such as benzene, toluene, xylenes and trimethyl benzenes from the syngas gas. This technology is proprietary to Epurga, Inc. of Benton, Louisiana. The syngas is reacted over a proprietary catalyst bed, with conditions set to maximize aromatics. These higher value chemicals can be used as octane enhancers and are considered green gasoline thus they are available for RINS credits. They can also be further refined into their individual components.
MSW is an abundant feedstock, but the traditional processes used for gasification have proven problematic. These problems can be addressed and the overall process economics improved with the following modifications.
- Operating the gasifier at higher operating pressure – Pressurized solid gasification lowers installed cost by eliminating at least one and possibly two stages of gas compression.
- Using water as a source of oxygen – The use of water as a source for oxygen, rather than pure oxygen eliminates the requirement for an ASU which in turn lowers installed cost.
- Product improvement – Upgrading the FT products will increase the capital cost but will provide a payout from improved product yield and quality. A different technology such as the Epurga Process instead of Fischer-Tropsch can provide additional income by producing high priced aromatic compounds.