Efficiency of Heat Recovery

A further enhancement to the HRS process whereby the exhaust from a gas fired turbine is used to superheat steam from the HRS process is also possible (129). The superheated steam is then fed through a turbogenerator to produce additional electricity. This increases the efficiency of heat recovery of the turbine exhaust gas. With this arrangement, electric power generation of over 13.6 kW for 1 t/d (15 kW/STPD) is possible. Good general discussions on the sources of heat and the energy balance within a sulfuric acid plant are available (130,131). 

The cost of recovery will be reduced if the streams are located conveniently close. The amount of energy that can be recovered will depend on the temperature, flow, heat capacity, and temperature change possible, in each stream. A reasonable temperature driving force must be maintained to keep the exchanger area to a practical size. The most efficient exchanger will be the one in which the shell and tube flows are truly countercurrent. Multiple tube pass exchangers are usually used for practical reasons. With multiple tube passes the flow will be part counter-current and part co-current and temperature crosses can occur, which will reduce the efficiency of heat recovery (see Chapter 12). 

Efficiency of heat recovery to avoid supplementary fuel 492 - 780 = 63%... 

The exhaust gas and lime discharge temperatures are generally monitored. The former has a safety role for the exhaust fan (if used) and also is an indication of any problems with heat transfer in the calcining and pre-heating zones. The lime discharge temperature should be monitored to protect rubber belt conveyors handling the lime it is also an indication of the efficiency of heat recovery in the cooling zone. 

Cold air can be sucked into the furnace convective section through holes in the furnace skin. This reduces the efficiency of heat recovery from the hot flue gas. At lower crude rates, flue gas flow drops, but cold air in-leakage remains constant. Thus, at lower crude rates, holes in the furnace exterior will hurt efficiency more than at higher throughputs. A roll of aluminum tape can go a long way toward correcting this problem. 

Figure 2-5 shows the improvement in eyele effieieney beeause of heat recovery with respect to a simple open-cycle gas turbine of 4.33.T ratio pressure and 1,200°F inlet temperature. Cycle efficiency drops with an increasing pressure drop in the regenerator. 

Simple-cycle efficiency does not usually mean as much to process users as total-cycle efficiency, because the gas turbine is not usually economic in process applications without some type of heat recovery. Total-cycle efficiency is most important in any economic evaluation. In a cycle with heat recovery, the only major loss that is charged to the cycle is the heat exhausting from the boiler stack. With the good comes the bad. Gas turbine maintenance is generally somewhat higher in cost and should be included in the total evaluation. 

The cleanliness of the products of combustion is such that the use of heat-recovery equipment is possible without the risk of corrosion. This has led to the development of combined heat and power packages where the overall efficiency is high. 

Practical conversion processes can only approach the theoretical efficiencies shown in Table 3. The coal conversion reactions do not proceed to completion at ambient temperatures within practical time limitations. As a result, a portion of the coal feedstock must be burned to supply heat so that the reactions can be carried out at elevated temperatures and pressure where the rates of conversion are rapid. In practical systems, this additional heat can only be partially recovered. Consequently, practical conversion processes have actual heat recovery efficiencies of about 60-70% for production of high H/C ratio products. Production of secondary fuels having somewhat lower H/C ratio, i.e. about 2.0, permits attainment of heat recovery efficiencies of 70 to 80j. 

Moving bed reactors for oil recovery from shale is one example of this kind of operation. Another somewhat analogous operation is the multistage counterflow reactor, and the four- or five-stage fluidized calciner is a good example of this. In all these operations the efficiency of heat utilization is the main concern. 

The overall process is a net producer of heat for efficient operation, heat recovery (using waste heat boilers) is important. 

It is worth mentioning that both processes employ total recycle of C02 and NH3 and very efficient methods of heat recovery. As a consequence, discharge of chemicals into the environment is kept at a very low level, characterized by 1 ppm urea and 1 ppm... 

The TOSCO-II process is capital intensive because it requires a large volume of heating gases and mechanically complex equipment the PARAHO and Union processes are also capital intensive because they have long residence time requirements that entail massive hardware. The PARAHO and Union processes are, however, heat efficient as a result of countercurrent shale and gas flow. But the TOSCO process, although having some degree of heat recovery, uses heat relatively inefficiently. 

The desirability of this process results from a high heat flux and efficient countercurrent heat recovery from the spent shale. This, in turn, results in a thermally efficient plant of relatively small size and low capital expense. 

There are six components that may be important in industrial combustion processes (see Figure 1.16). One component is the burner that combusts the fuel with an oxidizer to release heat. Another component is the load itself that can greatly affect how the heat is transferred from the flame. In most cases, the flame and the load are located inside of a combustor, which may be a furnace, heater, dryer, or kiln that is the third component in the system. In some cases, there may be some type of heat recovery device to increase the thermal efficiency of the overall combustion system, which is the fourth component of the system. The fifth component is the flow control system used to meter the fuel and the oxidant to the burners. The sixth and last component is the air pollution control system used to minimize the pollutants emitted from the exhaust stack into the atmosphere. The first four system components are considered next. 

---