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Waste Heat Recovery

In industrial process, heat is generated during fuel combustion or chemical reaction. The heat that is not in use is waste heat. Sources of waste heat could be hot combustion gases discharged to the atmosphere, heated products exiting industrial processes, and heat transfer from hot equipment surfaces. It is not an easy task to quantify exact value of industrial waste heat but studies have estimated that 20 to 40% of industrial energy consumption is discharged as waste heat.

Though some waste heat losses from industrial processes are inevitable, facilities can reduce these losses by improving equipment efficiency or installing waste heat recovery technologies. Waste heat recovery entails capturing and reusing the waste heat in industrial processes for heating or for generating mechanical or electrical work. Example uses for waste heat include generating electricity, preheating combustion air, preheating furnace loads, absorption cooling, and space heating.

Factors Affecting Waste Heat Recovery

Evaluating the feasibility of waste heat recovery requires characterizing the waste heat source and the stream to which the heat will be transferred. Two important waste stream parameters that must be determined include:

  • heat quantity,
  • heat temperature/quality,

These parameters allow for analysis of the quality and quantity of the stream and also provide insight into possible materials/design limitations. Nevertheless corrosion of heat transfer media is of considerable concern in waste heat recovery, even when the quality and quantity of the stream is acceptable.

Heat Losses –Quality

Depending upon the type of process, waste heat can be rejected at virtually any temperature from that of chilled cooling water to high temperature waste gases from an industrial furnace or kiln. Usually higher the temperature, higher the quality and more cost effective is the heat recovery. In any study of waste heat recovery, it is absolutely necessary that there should be some use for the recovered heat. Typical examples of use would be preheating of combustion air, space heating, or pre-heating boiler feed water or process water. With high temperature heat recovery, a cascade system of waste heat recovery may be practiced to ensure that the maximum amount of heat is recovered at the highest potential. An example of this technique of waste heat recovery would be where the high temperature stage was used for air pre-heating and the low temperature stage used for process feed water heating or steam raising.

Heat Losses – Quantity

In a heat recovery situation it is essential to know the amount of heat recoverable and also how it can be used. An example of the availability of waste heat from a re-heating furnace is given below: In a re-heating furnace, the exhaust gases are leaving the furnace at 600oC at the rate of 40000m3/hour. The total heat recoverable at 180oC final exhaust can be calculated as

Q = V x ρ x Cp x ΔT
Q is the heat content in kCal
V is the flow rate of flue gas in m3/hr
ρ is density of the flue gas in kg/m3
ΔT is the temperature difference in oC
Cp (Specific heat of flue gas) = 0.24 kCal/kg/oC
Heat available (Q) = 40000 x 0.405 x 0.24 x (600-180) = 1,632,960 kCal/hr

By installing a recuperator this heat can be recovered to pre-heat the combustion air. The fuel savings would be 19% (@ 1% fuel reduction for every 22oC reduction in temperature of flue gas.

 

Furnace Waste Heat recovery

Since furnaces operate in high temperature domains; the stack temperatures are very high, and waste heat recovery is a major energy efficiency opportunity in furnaces. The recovered waste heat is conventionally used for either pre-heating of combustion air or to heat the material itself.

Figure 1: waste heat diagram
Source: APO, 2010 Training Manual on Energy Efficiency for Small and Medium Enterprises.

Some common waste heat systems are listed below.

Simple double pipe type heat exchanger

Figure 2: Heat exchanger
Source: APO, 2010 Training Manual on Energy Efficiency for Small and Medium Enterprises.

These generally take the form of concentric cylinders, in which the combustion air passes through the annulus and the exhaust gases from the furnace pass through the center. Such recuperators are very cheap to make, are suitable for use with dirty gases, have a negligible resistance to flow, and can replace the flue or chimney if space is limited.

Convection recuperators

Figure 3: Convention recuperators
Source: APO, 2010 Training Manual on Energy Efficiency for Small and Medium Enterprises.

Convection recuperators consist essentially of bundles of drawn or cast tubes. Internal and/or external fins can be added to assist with heat transfer. The combustion air normally passes through the tubes and the exhaust gases outside the tubes, but there are some applications where this is reversed. For example, with dirty gases, it is easier to keep the tubes clean if the air flows on the outside. Design variations include ‘U’ tube and double pass systems. Convection recuperators are more suitable for exhaust gas temperatures of less than about 900°C. Beyond 900ºC, ceramic recuperators can be used which can withstand higher temperatures.

Regenerative burners

Figure 4: Regenerative burners
Source: APO, 2010 Training Manual on Energy Efficiency for Small and Medium Enterprises.

Convection recuperators consist essentially of bundles of drawn or cast tubes. Internal and/or external fins can be added to assist with heat transfer. The combustion air normally passes through the tubes and the exhaust gases outside the tubes, but there are some applications where this is reversed. For example, with dirty gases, it is easier to keep the tubes clean if the air flows on the outside. Design variations include ‘U’ tube and double pass systems. Convection recuperators are more suitable for exhaust gas temperatures of less than about 900°C. Beyond 900ºC, ceramic recuperators can be used which can withstand higher temperatures.

Payback of energy saving options

Experience from the past has shown that implementing energy saving options in waste heat recovery is highly profitable with payback of investment of less than 3 years.

Table 1: Payback of investment of energy saving options for waste heat recovery

Options Estimated payback period
Optimize excess air levels < 1 year
Control leakages of air/flue gas < 1 year
Install properly sized waste heat recovery units in furnaces/boilers/heaters 1-2 years
Clean the heat transfer areas frequently < 1 year
Adopt on-line cleaning system 1-2 years

Source: NEEP 2012-2016, IGEA

 

Energy Efficiency in Furnace and WHR

  • Establish a management information system on loading, efficiency, and specific fuel consumption.
  • Prevent infiltration of air, using doors or air curtains.
  • Monitor O2/CO2/CO ratios and control excess air level.
  • Improve burner design, combustion control, and instrumentation
  • Ensure that the furnace combustion chamber is under slight positive pressure.
  • Use ceramic fiber linings in the case of batch operations
  • Match the load to the furnace capacity.
  • Retrofit with heat recovery devices
  • Investigate cycle times and avoid extended hours of runtime and excess heating.
  • Provide temperature controllers.
  • Ensure that the flame does not touch the stock.
  • Repair damaged insulation.
  • Recover maximum heat from flue gases.
  • Ensure upkeep of heating surfaces by regular cleaning.
  • Use an infrared gun to check for hot wall areas during hot weather.
  • Ensure that all insulated surfaces are clad with aluminum lining.
  • Insulate all flanges, valves, and couplings

References

Bureau of Energy Efficiency, 2010 Guidebook for National Certification Examination for Energy Managers and Energy Auditors: Book 2

Asian Productivity Organization, 2010 Training Manual on Energy Efficiency for Small and Medium Enterprises

Nepal Energy Efficiency Programme, 2012-2016, Investment Grade Energy Audits