Electricity Cogeneration by Combined Cycle

Unconverted synthesis gas that remains after the methanol production section can still contain a significant amount of chemical energy. These gas streams may be combusted in a gas turbine, although they generally have a much lower heating value (4-10 MJ/m3NTP) than natural gas or distillate fuel (35-40 MJ/m3NTP) for which most gas turbine combustors have been designed. When considering com­mercially available gas turbines for low calorific gas firing, the following items deserve special attention (Consonni et al. 1994; Rodrigues de Souza et al. 2000; van Ree et al. 1995): the combustion stability, the pressure loss through the fuel injection system, and the limits to the increasing mass flow through the turbine.

Different industrial and aeroderivative gas turbines have been operated suc­cessfully with low LHV gas, but on the condition that the hydrogen concentration in the gas is high enough to stabilize the flame. Up to 20% H2 is required at 2.9 MJ/m3NTP. Hydrogen has a high flame-propagation speed and thus decreases the risk of extinguishing the flame (Consonni et al. 1994).

Injecting a larger fuel volume into the combustor through a nozzle originally designed for a fuel with much higher energy density can lead to pressure losses, and thus to a decreased overall cycle efficiency. Minor modifications are sufficient for most existing turbines. In the longer term, new turbines optimised for low heating value gas might include a complete nozzle combustor redesign (Consonni et al. 1994).

The larger fuel flow rate also implies an increase in mass flow through the turbine expander, relative to natural gas firing. This can be accommodated partly by increasing the turbine inlet pressure, but this is limited by the compressor power available. At a certain moment, the compressor cannot match this increased pressure any more and goes into stall: the compressor blocks. To prevent stall, decreasing the combustion temperature is necessary; this is called derating. This will lower the efficiency of the turbine, though (Consonni et al. 1994; van Ree et al. 1995). Higher turbine capacity would normally give a higher efficiency, but as the derating penalty is also stronger, the efficiency gain is small (Rodrigues de Souza et al. 2000).

Due to the setup of the engine the compressor delivers a specific amount of air. However, to burn one m3NTP of fuel gas less compressed air is needed com­pared to firing natural gas. The surplus air can be bled from the compressor at different pressures and used elsewhere in the plant, e. g., for oxygen production (van Ree et al. 1995). If not, efficiency losses occur.

All the possible problems mentioned for the currently available gas turbines can be overcome when designing future gas turbines. Ongoing developments in gas turbine technology increase efficiency and lower the costs per installed kW over time (van Ree et al. 1995). Cooled interstages at the compressor will lower compressor work and produce heat, which can be used elsewhere in the system. Also gas turbine and steam turbine could be put on one axis, which saves out one generator and gives a somewhat higher efficiency.

Turbines set limits to the gas quality. The gas cleaning system needs to match particles and alkali requirements of the gas turbines. When these standards are exceeded, wearing becomes more severe and lifetime and efficiency will drop (van Ree et al. 1995). However, the synthesis gas that passed various catalysts prior to the gas turbine has to meet stricter demands. It is therefore expected that contaminants are not a real problem in gas turbines running on flue gas from methanol production.