Heat recuperators

Typical heat-recuperation devices are finned gas exchangers, ceramic heat wheels, and Ljungstrom air preheaters. 

MIDREX Process. The primary components of a MID REX process plant include the shaft furnace, reformer, and heat recuperator. These components are supported by ancillary systems for handling iron ore, gas, water, and direct reduced iron. A flow sheet is shown in Figure 1. 

Fig. 9.2 shows how a simple open circuit gas turbine can be used as a cogeneration plant (a) with a waste heat recuperator (WHR) and (b) with a waste heat boiler (WHB). Since the products from combustion have excess air, supplementary fuel may be burnt downstream of the turbine in the second case. In these illustrations, the overall efficiency of the gas turbine is taken to be quite low ((tjo)cg = ccJf ca 0.25), where the subscript CG indicates that the gas turbine is used as a recuperative cogeneration plant. 

Fig. 9.2. Cogeneration plants (a) with waste heat recuperator (WHR) and (b) with waste heat boiler (WHB).

When ATad < 30°C, heat recuperation is important. The outlet switching temperature will be low so the reaction contribution of the inlet and outlet regions is small. In this situation, inert packing with high heat capacity and low porosity can be used in place of catalyst. The variant is shown in Fig. 7. 

A new evaluation standard for the dehydrogenation catalysts in the superheated liquid-film states is introduced here. This standard is called as the "ratio of heat recuperation" [39], being defined as the ratio of endothermic reaction heat to the denominator of heat supplied from the external thermo-reservoir to the catalyst layer shown as follows (Equations 13.10 and 13.11) ... 

Note Catalyst = platinum nanopartides supported on ACC (Pt/ACC, 5 wt-metal%) 3.0 g. Feed rate of methylcyclohexane = 3.5 mmol/min (superheated liquid-film state). a [Ratio of heat recuperation] = [reaction heat]/([reaction heat] + [evaporation heat]). 

A series of experimental results on the ratio of heat recuperation is shown in Table 13.4. Here temperatures of the catalyst layer, composed of a sheet of ACC dispersed with platinum nanoparticles (5 wt-metal%, 3.0 g), were varied at the constant feed ratio (3.5 mmol/min) of liquid methylcyclohexane (MCH) [39]. At the catalyst-layer temperature of 285°C, the heat recuperation ratio of 63.3% was attained, being much larger than the magnitudes of 15.6 and 47.5% for the catalyst-layer temperatures of 210°C and 245°C, respectively. 

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. 

Housley, G.K., J.E. O Brien, G.L. Hawkes (2008), Design of a Compact Heat Exchanger for Heat Recuperation from a High Temperature Electrolysis System , 2008 ASME International Congress and Exposition, Boston, MA, USA, November, paper IMECE2008-68917. 

A recuperative bayonet sulphuric acid decomposition reactor has been designed by researchers at Sandia National Laboratories (SNL) that features all-silicon carbide (SiC) construction for the heated parts, can be made from readily available SiC shapes, makes the most use of heat recuperation, and has all of its fluid connections at sufficiently low temperatures that conventional seal materials can be used. Bench-scale experiments using electric resistance heaters as the energy source have verified that the design functions as intended. 

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 special tank furnaces, e.g. for the manufacture of textile glass fibers, metal regenerators have been successful for heat recuperation. The regular reversal of the gas direction is then not necessary, simplifying temperature control over that with regenerative heat recuperation. 

Figure 1.18 shows how the available heat increases with the oxidizer preheat temperature. The thermal efficiency of the air/CH4 doubles by preheating the air to 2000°F (1400 K). For the 02/CH4 flames, the increase in efficiency is much less dramatic by preheating the 02. This is because the initial efficiency with no preheat is already 70% and because the mass of the 02 is not nearly as significant in the combustion reaction as compared with the mass of air in an air/fuel flame. There are also safety concerns when flowing hot 02 through piping, heat recuperation equipment, and a burner. 

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