Steam recuperation

Fresh reducing gas is generated by reforming natural gas with steam. The natural gas is heated in a recuperator, desulfurized to less than 1 ppm sulfur, mixed with superheated steam, further preheated to 620°C in another recuperator, then reformed in alloy tubes filled with nickel-based catalyst at a temperature of 830°C. The reformed gas is quenched to remove water vapor, mixed with clean recycled top gas from the shaft furnace, reheated to 925°C in an indirect fired heater, and injected into the shaft furnace. For high (above 92%) metallization a CO2 removal unit is added in the top gas recycle line in order to upgrade the quaUty of the recycled top gas and reducing gas. 

Smelting. The fuel suppHed to the reverberatory furnace is in the range of 5—6 GJ/t (4.7-5.7 x 10 Btu/t) concentrate. Steam produced in the waste heat boiler is equal to ca 60% of the energy suppHed by the fuel. The additional heat recovered from the exit gases in the recuperator to preheat the combustion air is equal to ca 10% of the energy from the fuel. Hence, the heat recovered from the furnace is equal to ca 70% of the heat from the fuels. 

Chodkiewicz, R. A Recuperated Gas Turbine Incorporating External Heat Sources in the Combined Gas-Steam Cycle, ASME Paper No. 2000-GT-0593. 

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. 

However in practice, for the same states 1-5 the steam raised S will be less hence there is no advantage in operating a STIG plant in this variation of the basic CBTX recuperative gas turbine plant. Nonetheless, this form of analysis as developed by Lloyd will prove to be useful in the discussion of the chemical recuperation plant in Chapter 8. 

Thermo-chemical recuperation using steam (steam-TCR)... 

The basic idea of using TCR in a gas turbine is usually to extract more heat from the turbine exhaust gases rather than to reduce substantially the irreversibility of combustion through chemical recuperation of the fuel. One method of TCR involves an overall reaction between the fuel, say methane (CH4), and water vapour, usually produced in a heat recovery steam generator. The heat absorbed in the total process effectively increases... 

Fig. 8.9. Cycle Bl. Chemically recuperated cycle with steam reforming.

Fig. 8.9 shows a chemically recuperated cycle [Bl] of the first type, i.e. chemical recuperation with steam reforming (steam/TCR). 

However, many reactions of commercial interest have chemistry, mechanical, or system requirements that preclude the use of cross-flow reactors. Processes cannot use a cross-flow orientation primarily because of high temperatures and the need to internally recuperate heat such as steam methane reforming (SMR) [12, 13] and oxidation reactions [14]. Counter- and coflow devices require a micromanifold to dehver sufficiently uniform flow to each of the many parallel channels. 

The subsequent steam reforming section is operated at very high temperatures 850-900 °C. The SMR catalysts themselves are already active below 400 °C, but high temperatures are necessary to drive the strongly endothermic reaction forward [8]. In industry, nickel catalysts are used in high-alloy reaction tubes, which are heated by external burners. This design is expensive and leads to heat losses, although much of the heat is recuperated. Noble metal catalysts such as sup-... 

Figure 1-14 shows a simplified layout for an SOFC-based APU. The air for reformer operation and cathode requirements is compressed in a single compressor and then split between the unit operations. The external water supply shown in figure 1-14 will most likely not be needed the anode recycle stream provides water. Unreacted anode tail gas is recuperated in a tail gas burner. Additional energy is available in a SOFC system from enthalpy recovery from tail gas effluent streams that are typically 400-600°C. Current thinking is that reformers for transportation fuel based SOFC APUs will be of the exothermic type (i.e. partial oxidation or autothermal reforming), as no viable steam reformers are available for such fuels. 

In conclusion, it would seem that either recuperated case would always be preferred. However, the cost of a recuperator can be significant. Also, the analyses performed fixed the turbine inlet temperatures at fairly low values. Typically with larger heavy-duty turbines the temperature is pushed to material limits and heat recuperation from the turbine exhaust is done in a combined cycle (steam bottoming cycle). Such a system can achieve close to 60% LHV efficiency on natural gas. Section 8.4 will consider whether a steam bottoming cycle is potentially appropriate for a hybrid system. A steam cycle is generally not economical for smaller systems. 

Figure 8.9 shows the turbine exhaust gas temperatures for the cathode recycle and recuperated cases. The cathode recycle has a comparatively higher quality of heat. Systems with temperatures above 800 K are a good candidate for heat recovery steam generation. Thus, up to a PR of about 20 there is some potential for the addition of a steam cycle to the cathode recycle analysis, which would increase both system power and efficiency. 

More detailed studies, including economic, are warranted. A combination of cathode recycling and recuperation, or steam generation, could be the best way to optimize efficiency, power and cost for a particular power plant size. 

Recuperation BASF "isothermal" process conventional primary steam reforming Degussa BMA process... 

VT Voltage transformer (from 15 kV to 400 V), SG Steam generator (or the intermediate heat exchanger), SH Superheater (steam heated from 770°C up to 800°C), HR Heat recuperators (at low and high temperature), WT Water treatment. 

In the subsequent product stripping section, the product is stripped free of benzene and water using a live steam stripper or a reboiler column. The effluent discharged from the washing area is steam-stripped to recuperate MNB. 

Case 4 Thermal recuperation as in Case 3 with the addition of catalytic reforming of methanol/steam mixture to give C02 + 3H2 at 800F as the fuel. 

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