The subject of waste heat recovery, though now new, is talked about all over the world, today, and almost in every sphere of activity. Fast depletion of coal reserves and ever-increasing oil prices are resulting in the increased unit cost of electrical power which necessitates the system designer to select a process or a system with a maximum heat recovery for plant economics. Waste heat utilization generates energy, practically at no fuel cost. Thus energy production from waste heat reduces the cost of energy and conserves the other scarce sources of energy, which are fast depleting.

A Waste Heat Recovery System can be broadly defined as any system, which recovers the heat, which is otherwise wasted. The WHR system can be designed to meet the process steam requirement in which case, the system consists of a Heat Recovery Steam Generator (HRSG) also called as Waste Heat Recovery Boiler with associated auxiliaries. In case there is no requirement of process steam, the WHR system can be designed as a power plant consisting of a HRSG, a steam turbine with an alternator, a condenser and associated auxiliaries. In either case, the HRSG, which recovers the waste heat to generate steam at required parameters, is the heart of the Waste Heat Recovery system and needs careful design.

It is clear that a HRSG means a boiler generating steam from waste heat.  A conventional boiler is fired with a fuel like coal, oil or gas to generate steam.  However, a heat recovery steam generator (waste heat boiler) generally does not burn any fuel.  It utilizes heat from the waste gasses or exhaust gasses

Waste Heat Potential: 

There are various sources of waste heat. Many industries/installations exhausts flue gases at high temperatures. For example, a typical 20 MW gas turbine working on open cycle exhausts 108 kg/sec of flue gas at 512oC. By installing an unfired Waste Heat Recovery system 

at the downstream of the GT, 48 t/hr of super heated steam at 23 Ata, 405oC can be generated to ultimately produce 10 MW of electrical power by the steam turbine. 

In typical 3000 TPD cement plant 110 kg/sec of flue gases are available at 370oC from the rotary kiln. A waste heat boiler can be suitably designed to recover the heat from the above waste gases to generate 28-tons/ hr of steam at 12.5 Ata and 305oC and to ultimately produce an electrical power of 4.2 MW.

Normally, Diesel engine exhaust heat recovery gives rise to only low-pressure steam and as such can be used for only process requirement. Similarly, the quantity of steam generated and the electrical power output can be calculated, once the flue gas quantity and temperature are known from any furnace. However, the waste heat from gas turbines and diesel engines are mainly considered here.  The exhaust gases from gas turbine or diesel engines contain oxygen also, which can support combustion of fuel. Therefore, additional fuel is burnt in waste heat recovery boilers behind gas turbines or diesel engines to generate extra steam or high temperature steam.

The details of the waste heat boilers of gas turbine based on combined cycles and co-generation plants are discussed further.  The same is generally applicable to diesel engine plants also.  However, the special aspects that are different between them are also presented.

Classification Of  Heat Recovery Steam Generators:

Heat Recovery Steam Generators can be broadly classified into the following two categories.

  • From the process point of view as category I.

  • From the heat recovery point of view as category II.

HRSG Of Category I:

In processes like manufacture of sulfuric acid, hydrochloric acid and fertilizers like urea, the products like reformed gas, synthesis gas, converted gas etc. have to be cooled down to a particular temperature for favorable reaction. Cooling any gas by generation of steam is the most economical and simple method and hence, these process gasses are cooled in a HRSG. The steam generated by the HRSG is normally utilized either for process use or power generation in a cogeneration system.   

HRSG Of Category II:

HRSG of this category recovers the heat, which is otherwise wasted to the atmosphere, and generates steam at required parameters either for process use or for power generation. These boilers are always optional from the point of the cost-economics of the heat recovery. HRSG of this category are generally used in Combined Cycle Plants and Co-Generation Plants.