Gas Turbine Exhausts

The hot gases from the combustor, temperature controlled to 980°C by excess air, are expanded through the gas turbine, driving the air compressor and generating electricity. Sensible heat in the gas turbine exhaust is recovered in a waste heat boiler by generating steam for additional electrical power production. 

The product gas after cleanup consists of primarily CO and H2. Combustion of coal gas in high firing-temperature gas turbines converts virtually all of the CO to CO2, and gas turbine exhaust is expected to contain no more than 10 ppm CO when operating at design conditions. Carbon monoxide emissions from a CGCC plant are thus expected to be around one-tenth those of a modem coal-fired plant equipped with low NO burners. 

Fig. 4. Use of gas turbine air preheat for ethylene cracking furnace. The gas turbine exhaust duct contains 17% oxygen at 400°C.

Most gas turbine appHcations in the chemical industry are tied to the steam cycle, but the turbines can be integrated anywhere in the process where there is a large requirement for fired fuel. An example is the use of the heat in the gas turbine exhaust as preheated air for ethylene cracking furnaces as shown in Figure 4 (8). 

Cracking reactions are endothermic, 1.6—2.8 MJ/kg (700—1200 BTU/lb) of hydrocarbon converted, with heat supplied by firing fuel gas and/or fuel oil in side-wall or floor burners. Side-wall burners usually give uniform heat distribution, but the capacity of each burner is limited (0.1—1 MW) and hence 40 to 200 burners are required in a single furnace. With modem floor burners, also called hearth burners, uniform heat flux distribution can be obtained for coils as high as 10 m, and these are extensively used in newer designs. The capacity of these burners vary considerably (1—10 MW), and hence only a few burners are required. The selection of burners depends on the type of fuel (gas and/or liquid), source of combustion air (ambient, preheated, or gas turbine exhaust), and required NO levels. 

Instead of gas turbine exhaust, air preheat has been used in some plants to reduce fuel consumption. Flue gas leaving the furnace stack passes through an air preheater, and the preheated air is suppHed to the burners. By using mostly hearth burners, the duct work and the investment cost can be minimised with air preheat and gas turbine exhaust. It is also possible with 100% waH-fired furnaces, and has been proven in commercial operation (34). 

Another applieation for turboexpanders is in power reeovery from various heat sourees utilizing the Rankine eyele. The heat sourees presently being eonsidered for large seale power plants inelude geothermal and oeean-thermal energy, while small systems are direeted at solar heat, waste heat from reaetor proeesses, gas turbine exhaust and many other industrial waste heat sourees. Some of these systems are diseussed below in greater detail. 

The utilization of gas turbine exhaust gases, for steam generation or the heating of other heat transfer mediums, or in the use of eooling or heating... 

Raising the inlet temperature at the waste heat boiler allows a signifieant reduetion in the heat transfer area and, eonsequently, the eost. Typieally, as the gas turbine exhaust has ample oxygen, duet burners ean be eonveniently used. 

The efficiency of the steam section in many of these plants varies from 30-40%. To ensure that the steam turbine is operating in an efficient mode, the gas turbine exhaust temperature is maintained over a wide range of operating conditions. This enables the HRSG to maintain a high degree of effectiveness over this wide range of operation. 

Figure 3-19 shows the thermal efficiency of the gas turbine and the Brayton-Rankin cycle (gas turbine exhaust being used in the boiler) based on the LHV of the gas. This figure shows that below 50% of the rated load, the combination cycle is not effective. At full load, it is obvious the benefits one can reap from a combination cycle. Figure 3-20 shows the fuel consumption as a function of the load, and Figure 3-21 shows the amount of steam generated by the recovery boiler. 

Because fuel costs are high, the search is on for processes with higher thermal efficiency and for ways to improve efficiencies of existing processes. One process being emphasized for its high efficiency is the gas turbine combined cycle. The gas turbine exhaust heat makes steam in a waste heat boiler. The steam drives turbines, often used as lielper turbines. References 1, 2, and 3 treat this subject and mention alternate equipment hookups, some in conjunction with coal gasification plants. 

Consider next a recuperative STIG plant (Fig. 6.5, again after Lloyd [2]). Heat is again recovered from the gas turbine exhaust but firstly in a recuperator to heat the compressed air, to state 2A before combustion and secondly in an HRSG, to raise steam S for injection into the combustion chamber. 

In the first type, heating of the steam turbine cycle is by the gas turbine exhaust with or without additional firing (there is normally sufficient excess air in the turbine exhaust for additional fuel to be burnt, without an additional air supply). In the second, the main combustion chamber is pressurised and joint heating of gas turbine and steam turbine plants is involved. 

Unfired cycle This cycle is very similar to the mentary-fired case except there is no added fuel heat input. The approach temperature and pinch point are even more critical, and tend to reduce steam pressures somewhat. Similarly, the gas turbine exhaust temperature imposes further limits on final steam temperature. 

Figure 16.33 A gas turbine exhaust matched against the process (same as a flue gas).

Aero-derivative gas turbines are typically used for offshore applications where weight and efficiency are a premium, to drive compressors for natural gas pipelines, and stand-alone power generation applications for peak periods of high power demand. For stand-alone applications, gas turbine efficiency becomes a critical issue. However, if heat is to be recovered from the gas turbine exhaust, the efficiency becomes less important as the waste heat is utilized. 

Figure 23.15 shows the potential to generate steam from a gas turbine exhaust. The potential to generate steam can be increased by introducing bring of fuel after the gas turbine. There are three bring modes for gas turbines as follows. 

Unfired HRSG. An unbred HRSG uses the sensible heat in the gas turbine exhaust to raise steam. 

Example 25.5 A gas turbine exhaust is currently operating with a flowrate of 41.6 kg s-1 and a temperature of 180°C after a heat recovery steam generator. The exhaust contains 200 ppmv NOx to be reduced to 60 rng rn 3 (expressed as N02) at 0°C and 1 atm. The NOx is to be treated in the exhaust using low temperature selective catalytic reduction. Ammonia slippage must be restricted to be less than 10 mgm 3, but a design basis of 5 mg-rn 3 will be taken. Aqueous ammonia is to be used at a cost of 300 -1 1 (dry NH3 basis). Estimate the cost of ammonia if the plant operates... 

Biomass gasifiers have the potential to be up to twice as efficient as using conventional boilers to generate electricity. For even greater efficiency, heat from the gas turbine exhaust can be used to generate additional electricity with a steam cycle. These improvements in efficiency can make environmentally clean biomass energy available at costs more competitive with fossil fuels. 

The combination of the fuel cell and turbine operates by using the rejected thermal energy and residual fuel from a fuel cell to drive the gas turbine. The fuel cell exhaust gases are mixed and burned, raising the turbine inlet temperature while replacing the conventional combustor of the gas turbine. Use of a recuperator, a metallic gas-to-gas heat exchanger, transfers heat from the gas turbine exhaust to the fuel and air used in the fuel cell. 

There can be many different cycle configurations for the hybrid fuel cell/turbine plant. In the topping mode described above, the fuel cell serves as the combustor for the gas turbine, while the gas turbine is the balance of plant for the fuel cell, with some generation. In the bottoming mode, the fuel cell uses the gas turbine exhaust as air supply, while the gas turbine is the balance of plant. In indirect systems, high-temperature heat exchangers are used. 

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