An investigation of residual fuel oil ash deposit formation and removal in cooled gas turbine nozzles

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Virginia Polytechnic Institute and State University


Results are reported from a series of experiments simulating the combustion and expansion processes of a heavy-duty combustion turbine engine burning a heavy residual fuel oil. The tests were carried out in a turbine simulator device, consisting of a combustion chamber and a turbine first-stage nozzle cascade sector. Both film, air-cooled and closed-circuit, water-cooled nozzle sectors were tested. These sectors were four-vane, three-throat sections with throat cross-sectional areas of approximately 50 (10⁻⁴) m². The test fuel was simulated by adding the appropriate contaminants to no. 2 fuel oil.

A series of seven full-length tests were performed, ranging in length from 22.5 to 88.2 hours. Four of the tests involved the watercooled nozzle sector and the remaining three used the air-cooled nozzle. The principle objectives of the tests were to assess the rate at which ash accumulates in the turbine nozzle and the relative difficulty in removing these deposits. The variable used to evaluate the extent of the ash deposit on the nozzle was the effective throat area, determined using the calculated gas flow rates, turbine nozzle inlet temperature, and the measured combustion chamber pressure. The parameters varied in the test program, other than the nozzle sectors, were the gas temperature and the gas pressure. The gas pressure variations served to vary the gas path surface temperatures at constant gas temperature.

The test conditions were nominal turbine firing (nozzle exit) temperatures of 1283 and 1394 K and combustor pressures of 3 and 6 atmospheres. A 2-to-l pressure ratio was maintained across the nozzle to insure sonic conditions at the throat sections. With the exception of one test, the data show that the deposit rates in the water-cooled turbine nozzle were lower than in the air-cooled nozzle. The effect of increasing the gas temperature was to dramatically increase the ash deposition rates. Decreased gas pressures (and hence surface temperatures) resulted in reduced deposition rates. Ash cleanability was enhanced by water-cooling.

Heat transfer data were analyzed from the water-cooled tests and gave significant insight into the ash deposit formation and removal phenomena. One of the more significant conclusions drawn from these data was that the major portion of the effective area decrease observed in a turbine nozzle because of ash deposits is due to the pressure face deposits.

A computer simulation of a combustion turbine engine was developed to aid in the evaluation of the turbine simulator test data. Results from field tests of full-sized production engines burning residual oil were used in the simulation to determine the relationship between the extent of ash deposition (throat area reduction) and turbine efficiency. This result was then combined with data from the turbine simulator tests to produce a real-time computer simulation of full-sized combustion turbine engines having air- and water-cooled first-stage turbine nozzles. It was found that water-cooling of the turbine nozzle would result in an increase in engine availability of 27 per cent when operating on heavy residual fuel oil.