Gasifier: Gasifiers for Special Applications
Oil and Gas Partial Oxidation
The gasification of liquid and gaseous feedstocks, like oil and natural gas, was developed by Texaco and Shell in the late-1940s/early-1950s according to Higman (2003). These companies have been in the market ever since with entrained-flow, refractory-lined reactors, top-mounted downflow burners and operation temperatures around 1,250-1,450°C. Partial oxidation of oil and gas involves reaction of the feedstocks with less oxygen than required for complete combustion, a process that is very similar to solid fuel gasification, but because of the fluid state of the oil or gas some gasification stages like pyrolysis and char gasification either do not take place or take place at a much less significant level.
When processing liquid feedstocks like oil, a small amount of residual carbon remains (0.5-1% by mass), which is necessary to sequester ash from the reactor. Other significant differences from solid feedstock gasification include burner design, methods for synthesis gas (syngas) cooling (like water quenches or heat exchangers), and soot handling procedures.
For the most part, processes that are suitable for liquids can be used for partial oxidation of natural gas or other gaseous feedstock; however the feed-preheat and burner systems must typically be adjusted. Unlike liquid feed, virtually no residual carbon is formed (few hundred ppm mass) and what is formed is free of metals which simplifies soot management.
Natural gas partial oxidation is performed primarily to produce hydrogen. Another method to generate hydrogen from natural gas is through steam methane reformation, but partial oxidation is typically much faster and requires a smaller reactor. Initially, however, partial oxidation produces less hydrogen than steam methane reformation for a given amount of fuel input.
One interesting difference is the chemical composition of the feedstock. Natural gas is mostly methane, which has a hydrogen:carbon ratio of 4. Oil is a complex mixture of hydrocarbons. In general, as the length or size of the hydrocarbon increases the hydrogen:carbon ratio decreases (by weight petroleum is approximately 83-87% carbon and 10-14% hydrogen, or around 1.67 hydrogen:carbon). Coal is composed primarily of carbon. These compositional differences in the feedstock affect the final syngas composition.
Partial Oxidation Gas Turbine (POGT)
The Gas Turbine Handbook (developed by NETL, 2006) describes a specific type of turbine that has been developed, incorporating partial oxidation, as opposed to complete combustion, of gaseous fuels like natural gas for power and potentially other product co-production. This turbine is referred to as a Partial Oxidation Gas Turbine (POGT) [PDF] (2006). Some differences from conventional gas turbines include the use of a partial oxidation reactor (POR) instead of a combustor and the use of a much smaller compressor (typically less than half the air flow is required). The gases from the POR have a much higher specific heat than complete combustion gases and, therefore, more energy can be extracted by the expander (which allows for the smaller compressor mentioned previously).
Virtually any hydrocarbon fuel can be combusted in the POR as long as it is operated in fuel rich conditions (equivalence ratios of around 2.5). The POGT produces two products: power and a secondary fuel, usually a hydrogen rich gas. The secondary fuel gas can then be burned in a bottoming cycle or cleaned and used for synthesis applications. In a two-stage power set-up, POGT can approach 70% overall efficiency. The secondary fuel gas, depending on initial fuel type and whether air or oxygen-enriched air is used, is typically a low to medium heating value fuel with high hydrogen content. This hydrogen concentration can be increased with steam injection to the POR, promoting steam reformation.
A history of POGT development can be found in the Partial Oxidation Gas Turbines [PDF] (2006) section of the Gas Turbine Handbook. This document also examines POGT applications:
- Power Generation—Provides fundamentally higher energy conversion efficiencies, and inherently lower nitrogen oxide (NOx) emissions (compared to conventional gas turbine); however, more complicated control systems are required for start-up and shut-down. In addition to integration with a standard bottoming cycle, POGT secondary fuel gas could feed a fuel cell.
- Power and Hydrogen—POGT can co-produce hydrogen (or syngas) and electric power. Hydrogen production could use steam injection to promote the steam reformation reaction inside the POR.
- Integration with Industrial Furnaces, Boilers, or Chillers—POGT could be used in applications like steel annealing and reheat, glass melting, or aluminum reclamation that require both power and the high temperatures of the produced secondary fuel gases.
- Gasification, by Christopher Higman and Maarten van der Burgt, contains a section on Oil Gasification and Partial Oxidation of Natural Gas. As of 2008, it is in its second edition.
- Gas Turbine Handbook, has a section on Partial Oxidation Gas Turbines [PDF] (2006) and a substantial bibliography, 2006.
- The Gas Technology Institute has been investigating POGT since 1995.