Heat recovery and cogeneration

Heat is evolved during many chemical reactions, and the products/process streams come out from the process reactors/units at considerably high temperatures. These streams are to be cooled in many plants for maintaining proper temperatures in downstream units for running the plant efficiently. 

The possibility of recovery of thermal energy from such streams must be examined, and suitable heat recovery equipments, such as boilers, economisers, air preheaters, and recuperators, should be installed. 

Certain exothermic reactions are carried out in reactors where cooling systems are to be arranged. Heat recovery can be considered from cooling jackets used for maintaining the reaction temperatures. 

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The minimisation of gas emissions as CO2, SO2, NO, and other acid gases is a key topic in sustainable process design. This problem can be handled only by a systematic approach of all the pollution sources generated by energy integration, namely the utility system, heat recovery and cogeneration, as well as by process modifications. 

Boilers may be used for domestic hot water heating, space heating, waste heat, or chemical recovery. They also may be used for mechanical work, electrical power generation, cogeneration, and innumerable industrial process applications using direct (live) steam or indirect steam (e.g., coil heated) processes. Both FT and WT designs are commonly employed for heat-recovery applications. 

While this basic definition of cogeneration efficiency seems straightforward, complications are created by the process steam generated from waste heat recovery that can be used for power generation or process heating and that does not require any fuel to be fired in the utility system. The heat supply can be defined as the sum of the heat from fuel (both in the utility boilers and fired heaters) and steam generation from the waste heat recovery (see Figure 23.44)17 ... 

The results of the performance calculations for the fuel cell, Rankine cycle heat recovery system, summarized in Table 9-24, indicate that the efficiency of the overall system is increased from 57% for the fuel cell alone to 72% for the overall system. This Rankine cycle heat-fuel recovery arrangement is less complex but less efficient than the combined Brayton-Rankine cycle approach, and more complex and less efficient than the regenerative Brayton approach. It does, however, eliminate the requirement for a large, high temperature gas to gas heat exchanger. And in applications where cogeneration and the supply of heat is desired, it provides a source of steam. 

Monolith catalysts are used for the control of carbon monoxide and hydrocarbon (known as volatile organic compounds or VOCs) emissions from chemical plants and cogeneration facilities. In this case, square bricks are stacked on top of one another in a wall perpendicular to the flow of exhaust gases at the appropriate temperature location within the heat recovery boiler. The size of the brick can vary from 6 in (ceramic) to 21 ft (metal). Pt and Pd catalysts are used at operating temperatures between 600 and 1200°F. Cell sizes typically range between 100 and 400 cells per square inch. Typical pressure drop requirements for monoliths are less than 2 in of water. 

Table VI shows a sample of a qualitative decision matrix for the base-case design which summarizes the suggestions from the thermoeconomic evaluation of each component. Decreasing the values of the pressure ratio P2/P1 and the isentropic compressor efficiency tqsc as well as increasing the isentropic turbine efficiency r]st are expected to improve the cost effectiveness of the cogeneration system. Note, that the decrease in the p2 /p 1 value contradicts the corresponding suggestions from the heat-recovery steam generator and the combustion chamber. However, changes snggested by the evaluation of a component should only be considered if they do not contradict...

Optimize compressor operation, reduce friction and air leakages in air distribution system, low pressure drop air coolers and instruments, reduction of air leaks, use of heat of compression dryers, cool compressor suction air, waste heat recovery at compressor inter-stage coolers, better control Install additional compressors in parallel. Use of heat of compression dryer Use cogeneration, ORC, KC or (hydraulic) power recovery turbines... 

Ankur, K., Bulatova, I., Smith, R., and Kim, J.K. (2012) Site-wide low-grade heat recovery with a new cogeneration targeting method, Chemical Engineering Research and Design, 90, 677-689. 

Excess power, steam, hot water, hot air generated available from waste heat recovery units, and cogeneration facilities,... 

Cogeneration of Power Through Waste Heat Recovery Boilers and Economisers... 

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