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Process | October 2011

Generation through waste heat recovery

The use of waste heat is imperative to optimise the output from any fuel burning source to enhance efficiency, says Anuj Malhotra.

The rising cost of fuel is a major factor affecting the cost of any manufacturing process. It is imperative to maximise the output from any fuel burning source in order to enhance thermal efficiency.

Any form of economically recoverable heat from gases from any process outlet and exhaust from equipment are termed as 'waste heat'. This can also be defined as the sensible heat content available in non-combustible gases. The source of gases could be from the chemical process or from the exhaust of furnaces or engines in both cases it is possible to recover this sensible heat and enhance the system efficiency. This can be done by installing heat recovery unit such as air pre-heater, boiler etc, to suit the plant's needs.

The usage of waste heat to enhance the efficiency has its origins in the early 1800 and these were primarily in the form of economisers used to cool gases from boiler exhaust. In the course of time the necessity of heat recovery found increasingly acceptance especially in the fall out of the increasing cost of fuel prices and the need to operate energy consuming devices at its highest efficiency.

Basically, there are three sources of waste heat:
1.Exhaust gases
2.Process gases
3.Combustible off gases

Exhaust gases are those that leave the furnace and engine exhaust or off gases which are exhausted to the stack without any additional equipment being installed in the path between the outlet of the equipment and the stack.

Process gases are those that are generated in a process and undergo a progressive change in its composition before the final product can be made.As the gases pass through various stages in the process, it is necessary to cool the gas at an intermediate point to achieve production of the final product.

Combustible off gases are essentially tail gases leaving the manufacturing plant but have some combustible constituents. The constituents govern the calorific value of the gas. These gases when burnt in a combustor or furnace lead to the formation of high temperature flue gases which are sources of waste heat.

Methods of heat recovery:
The various ways of heat recovery adopted in the industry are as under:
•Steam generation
•Pre-heating boiler feed water
•Pre-heating combustion air
•Super heating steam
•Pre-heating a process feed stream or heating a process stream to some intermediate point in the process.
•Providing utility steam and space heating in cold countries

Steam generation reason and usage
The heat recovery effected by one of the means identified earlier is adopted depending upon the requirements in the plant. It is a common practice to employ the steam generation route. Alternatively, the steam produced is used for process requirements in the plant.

Principal use of steam
1.Process heating: For this application the steam pressure would generally be in the range of 5 to 45 Kg/cm2 (g).
2.Power generation: For this application steam pressures would generally be in the range of 35 to 105 Kg/cm2 (g) and would generally require   as superheated.

System selection
The system selection of waste heat recovery is a very important aspect. The choice of the system primarily depends on the following parameters:
•Manufacturing process
•Plant's utility requirements
•Input details for equipment design
•Space availability

Classification of waste heat recovery boilers (WHRBs)
WHRBs may be broadly classified into three categories according to their applications. They are:
•Electric power station WHRBs
•Industrial WHRBs
•Marine WHRBs

WHRBs can also be classified into three categories according to the way the water or steam mixture is made to circulate through the evaporator.
•Natural circulation type
•Assisted or forced circulation type
•Once through type

Boilers can be classified as:
A.Fire tube type
B.Water tube type

Use of waste heat from flue gases in sponge iron manufacturing

Sponge iron making
Sponge iron is used as a substitute for scrap in the steel-making process.Iron ore (5-20 mm size) with dolomite and coal (3-15 mm size, non-coking grade) is charged to rotary horizontal kiln with a controlled air injection to kiln through a number of shell air blowers. Sponge iron is produced by the direct reduction process by removing oxygen from iron ore. The sponge iron produced is magnetically separated after cooling.

Need for waste heat recovery
Process and metallurgical industries are energy-intensive where the end prices are very sensitive to the input energy costs. Long term energy conservation can only enable competitive position in the market. The recovered heat helps to produce steam for power generation or for process requirement. Waste heat recovered also helps in meeting pollution norms and water conservation. As the heat is recovered, the flue gas outlet temperature being cooler, there is improvement and ease in operating condition of equipments at the down stream of the process.

Power is almost incidental and the generation cost of power is as low as Rs 0.50 per unit. A 100 tonne per day kiln can generate 1.6 MW of usable power that is 11 million units/year leading to saving per annum (grid@Rs 3.5 /kWh) of Rs 33 million/yr. The waste gases generated produces steam which in turn generates power. Moreover waste like coal fines and char from sponge iron process can be used in fluidised bed boilers as supplementary fuel. Hence one can say that all the waste from sponge Iron plant can be processed to obtain a by-product. Even the ash separated from bottom hoppers of boiler and electro-static precipitators (ESP) can be used for mixing with cement and brick making process.  

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