Plants with combustion modification

In practical open circuit gas turbine plants with combustion, real gas effects are present (in particular the changes in specific heats, and their ratio, with temperature), together with combustion and duct pressure losses. We now develop some modifications of the a/s analyses and their graphical presentations for such open gas turbine plants, with and without heat exchangers, as an introduction to more complex computational approaches. 

Japan was the first country to do this, starting with SCR for the removal of NOx from gas, coal and oil-fired power plants [63]. Utility companies in Japan introduced the catalytic SCR process as early as 1972. At present, many countries utilize SCR with Germany and Japan being the leading countries. Although priority is currently given to combustion modifications for NOx control in European countries, extended operation of catalytic SCR is expected in the near future. 

The results of different combustion measures varies with the specific conditions. In general 25-40% NOx reduction has been reached by combustion modification measures in the FRG, reaching 600-800 mg NOx/m at 6% O2 on dry bottom boilers and 900-1200 mg N02/m at 6% O2 on wet bottom boilers, using low NOx burners in combination with air staging in the furnace. Advanced low NOx burners have recently been installed at some plants, where about 50% NOx reduction has been achieved. NOx emissions in the range of 300-600 mg N02/m are reported from Japan with a combination of combustion measures. 

Plants (B) with modification of the fuel in combustion-turbine (CRGT) cycles... 

We consider first Cycles A of Table 8.1 A and the a.ssociated Figs. 8.6-8.8. These are cycles in which the major objective is to separate or sequestrate some or all of the carbon dioxide produced, and to store or dispose it. This can be achieved either by direct removal of the CO2 from the combustion ga.ses with little or no modification to the existing plant or by modest restructuring or alteration of the conventional power cycle so that the carbon dioxide can be removed more easily. 

In particular, the cycles involving fuel or oxidant modification do not look sufficiently attractive for their development to be undertaken, with the possible exception of the multiple PO combustion plant proposed by Harvey et al. [14]. The Matiant plant has the advantage of relatively simple CO2 removal and high efficiency and may prove to be attractive, but it again looks complex and expensive. 

In addition to collection and transportation costs, there are major expenses associated with the preparation of the tyres for combustion or with modifications to the fuel feeding systems of power plants (e.g., Goddard 1992 Lamarre... 

One advantage of the post-combustion C02 capture route is that it does not require any modifications to existing combustion methods, which means current power plants can be retrofitted with this process. In fact, this method has been demonstrated at some small-scale power plants.14... 

With modifications, IGCC power plant designs could recover CO2. The CO in the coal gas can convert to hydrogen (H2) and CO2 by reaction with H2O. This so-called shift reaction is common in the manufacture of ammonia and hydrogen gas. Modified IGCC could remove COj at pressure by conventional acid gas removal technology and then fire the remaining H2-rich fuel gas in a combustion turbine. 

Modification of direct coal combustion for CO2 removal would be more difficult. CO2 could be removed from the flue gas after conventional combustion by acid gas removal technology. Although this approach has found some commercial application, the low pressure and low concentration of the CO2 in the flue gas makes it a relatively expensive method. Removing 90% of the CO2 from flue gas of a conventional coal-fired boiler would increase the capital cost by a factor of 3.0 and thermal efficiency drops by 12% compared to a conventional direct coal combustion power plant. The larger capital increase and efficiency loss with CO2 recovery is principally due to recovery at low pressure which requires a larger flue gas compression, CO2 absorbers, and increased steam requirements. Depending on the cost of coal and capital, the increased electric cost for CO2 removal with a direct coal combustion power plant is 2.0-3.0 times that of a conventional direct coal combustion power plant. 

With Flue Gas Recirculation (FGR), flue gas is introduced with the combustion air and acts as a thermal diluent to reduce the combustion temperature. Usually, the amount of flue gas recirculated corresponds to 10-20% of the combustion air. FGR reduces only thermal NO (. It is suitable only for oil- and gas-fired boilers. Results with coal have been generally disappointing. In coal-fired stoker units, FGR provides better grate cooling. FGR has been successfully applied on industrial solid fuel-flred units and is considered appropriate for waste-to-energy plants. Retrofit modifications include new ductwork, gas recirculation fan(s), flue gas/air mixing devices and controls (Makansi, 1988 Wood, 1994). Gas recirculation fans can be troublesome. 

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