AIR TREATMENT

Combustion gas and air are combined to provide the compressed mass which drives the turbine. A number of products and sub systems are utilized to optimize the use of the air.

Two recent developments in gas turbine air treatment are the expanded use of HEPA filters for gas turbine intake air and SCR with both CO and NO_x catalyst for the tail-end gas. Some dust that the inlet filter does not remove is going to deposit on the catalyst. A facility can use a very small catalyst pitch and save a lot of money, but the increase in pressure loss along with deposition will increase operating cost and decrease electricity output. Therefore, the choice of inlet filter needs to be viewed initially in terms of catalyst pitch selection and then in terms of catalyst life.

inlet filter selection decision tree   catalyst selection decision tree

First in California and now in other places, the stack gas emission limits are lower than the ambient air particulate matter concentration. In these cases, the inlet air filter becomes a key element in stack gas compliance. Gas turbine inlet ambient air undergoes a series of treatments. The initial treatment is to remove large weather-related contaminants e.g., snow, rain, etc. The humidity and temperature of the ambient air are also adjusted to increase the weight and therefore electrical output. This treatment can range from fogging nozzles to a full air conditioning system. Droplets are formed, coalesced and removed. Particulate filtration can be with a series of filters ranging from coarse to HEPA or it can take place with self-cleaning cartridges.

tempering air decision tree   catalyst decision tree

One alternative for NO_x control during combustion is the low NO_x burner. Another alternate is water injection. Once the air has been mixed with the gas and combusted it passes through other systems in the gas path. In a combined cycle process there is likely to be a duct burner to adjust HRSG steam temperature. This can add to the pollutants. CO and NO_x catalysts are also utilized. However the accompanying ammonia injection can create ammonia slip which is regulated and also tends to foul the catalyst.

With single cycle systems many of the air treatment challenges are more complex. If a low temperature catalyst is used, tempering air is needed. This creates a challenge in providing laminar flow to the catalyst. If a high temperature catalyst is used, higher catalyst costs are encountered and higher maintenance is possible.

conditioning decision tree   coalescer decision tree

The air treatment needs are not static. The higher performance turbines are more likely to be compromised by small particles. The use of gas turbines and certainly the use of SCR are expanding to applications that are more challenging. The seawater and salts found in marine applications including floating production, storage and offloading (FPSO) units are examples.

Application in refineries in South America where inlet air quality may be low and fuel includes less than pristine liquids is another example. The rapid cycling of turbines complementing wind and solar is another newer challenge.