High-grade heat

There are only four conventional heat exchangers and this is the minimum number allowed by thermodynamics. In addition to an evaporator and condenser, one is needed to get high grade heat in and one to reject the heat of adsorption to the environment. 

The reformed gas leaves the furnace at a high temperature where high grade heat is recovered successively to a reformed gas boiler, steam superheater process feedstock heater and boiler, feedwater heater. The reformed gas then passes to the distillation area where low grade heat is efficiently recovered via column reboilers and a demineralized water heater. 

Description Gas feedstock is compressed (if required), desulfurized (1) and sent to the optional saturator (2) where some process steam is generated. The saturator is used where maximum water recovery is important. Further process steam is added, and the mixture is preheated and sent to the pre-reformer (3), using the Catalytic-Rich-Gas process. Steam raised in the methanol converter is added, along with available C02, and the partially reformed mixture is preheated and sent to the reformer (4). High-grade heat in the reformed gas is recovered as high-pressure steam (5), boiler feedwater preheat, and for reboil heat in the distillation system (6). The high-pressure steam is used to drive the main compressors in the plant. 

Heat from deep underground is extracted through pipes connected to a heat pump, which converts it into high-grade heat for radiators and hot water. You will need to power the heat pump, but you can expect a 4 1 return on the amount of electricity generated. 

There are, however, two major disadvantages of using producer gas. The first is the high total heat usage per tonne of lime, as a result of heat losses in the gas producer and of the low proportion of high-grade heat produced by the gas. The second is the additional capital and operating costs associated with the gas producer. 

Part of the skill of the lime kiln designer is to minimise heat requirements by integrating efficient heat transfer with controlled combustion. This maximises both heat recovery and the amount of high grade heat available for calcination. 

Coke ignites at about 800 °C, corresponding with the onset of surface calcination, so that there is little wastage of high grade heat. Exhaust gas temperatures of about 100 °C and lime discharge temperatures as low as 50 °C reflect the high thermal efficiency of the kiln. 

High-grade heat may be defined simply as that heat which is available for calcination. The residue is low-grade heat. The concept may appear obvious, but it is not universally accepted. There are, however, significant ramifications arising from the concept, which merit further consideration. A detailed mathematical model is given in [16.56]. 

The amount of high-grade heat increases directly with increases in the amount of heat recovered by the air and stone entering the calcining zone. It also increases when oxygen-enriched air is used for combustion as this reduces the amount... 

Thus, an increase in heat recovery by the combustion air of 25 kcal/kg lime, increases the amount of high grade heat by 25 kcal/kg. If the theoretical flame temperature is 1800 °C, 50 % of the heat is high grade, and the fuel input can be reduced by 50 kcal/kg to re-establish the original input of high-grade heat. Thus, the reduction in heat input is double the additional amount recovered. 

Rednced energy demand low-grade heat replaces high-grade heat, and a vacuum pump is required in the initial stages. 

A more rational approach would be to utilise the temperature potential more effectively by incorporating the furnace into a gas turbine cycle, perhaps using a catalytic plate reactor or the equivalent This could be achieved with pressurised combustion, in which the furnace acts as the combustor of a conventional gas turbine cycle. The design of the catalytic plate reactor (see Chapter 5) lends itself well to operation at elevated pressure in view of its millimetre-sized gas channels. Thus power may be extracted from the high-grade heat before it is used at a lower temperature to drive the process. 

If the high-grade heat that emerges from the fuel cell stack is used inefficiently within the system, then the heat that emerges from the system for practical use will be degraded. 

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