Syngas for use in integrated gasification combined cycle (IGCC) applications must be free of contaminants such as particulates and trace metals which could cause damage to the gas turbine. The ratio of hydrogen to carbon monoxide (CO) is not as important as in other applications which use syngas derived from gasification, at least if the turbine has been designed to handle increased hydrogen content. Some gasification technologies, such as catalytic gasification, may result in the syngas having relatively high hydrocarbon content, making it more similar to natural gas, which is ideal for utilization in combustion turbines. This may be explained by the simple fact that gas turbines developed for use in IGCC applications with syngas have invariably been based on natural gas combustion turbines. Despite the similarities between syngas and natural gas, there are differences which impact the design of the combustion turbines they fuel.
Gasification-derived syngas differs from natural gas in terms of calorific value, composition, flammability characteristics, and contaminants. Oxygen-blown, entrained flow IGCC plants typically produce syngas with a heating value range of 250 to 400 Btu/ft3 (HHV basis), which is much lower than the 1,000 Btu/ft3commonly associated with natural gas. The combustor requires a specified heat input to maintain performance, so a significantly higher flow rate is required for syngas than natural gas for a similar turbine size. Also, natural gas consists mainly of methane (CH4), whereas syngas consists mainly of CO and hydrogen (H2). The H2composition of the syngas results in a higher flame speed and broader flammability limits, meaning the syngas produces a stable flame at leaner conditions than natural gas and the combustion speed is much quicker than natural gas. This more rapid combustion speed limits the use of conventional natural gas combustor nitrogen oxide (NOx) control. Another complication is the relatively high concentrations of hydrogen sulfide (H2S) in syngas compared to natural gas.
To combat these issues, diluents such as nitrogen or steam are used to lower the flame temperature. The lower temperature limits the formation of NOx as the syngas is combusted. Nitrogen is an ideal solution in oxygen blown applications as it should be readily available as a by-product from the air separation unit.
Data on the composition of clean syngas being used to fire gas turbines at a range of IGCC facilities is presented in the table below.1
|1.||Key Combustion Issues Associated with Syngas and High-Hydrogen Fuels [PDF] (Dec 2006)
Vincent G. McDonell, University of California Irvine