One option to reduce possible heat losses in a combustion process consists of reducing the temperature of the flue-gases leaving the stack. This can be achieved by:
●dimensioning for the maximum performance plus a calculated safety factor for surcharges
●increasing heat transfer to the process by increasing either the heat transfer rate, (installing turbulators or some other devices which promote the turbulence of fluids exchanging heat), or increasing or improving the heat transfer surfaces
●heat recovery by combining an additional process (for example, steam generation by using economisers, see Section 3.2.5) to recover the waste heat in the flue-gases
●installing an air (or water) preheater or preheating the fuel by exchanging heat with flue- gases ( see S ection 184.108.40.206). Note that the manufacturing process can require air preheating when a high flame temperature is needed (glass, cement, etc.). Preheated water can be used as boiler feed or in hot water systems (such as district schemes)
●cleaning of heat transfer surfaces that are progressively covered by ashes or carbonaceous particulates, in order to maintain high heat transfer efficiency. Soot blowers operating periodically may keep the convection zones clean. Cleaning of the heat transfer surfaces in the combustion zone is generally made during inspection and maintenance shutdown, but online cleaning can be applied in some cases (e.g. refinery heaters)
●ensuring combustion output matches (and does not exceed) the heat requirements. This can be controlled by lowering the thermal power of the burner by decreasing the flowrate of fuel, e.g. by installing a less powerful nozzle for liquid fuels, or reducing the feed pressure for gaseous fuels.
Reducing flue-gas temperatures may be in conflict with air quality in some cases, e.g:
●preheating combustion air leads to a higher flame temperature, with a consequence of an increase of NOx formation that may lead to levels that are higher than the emissions limit value. Retrofitting an existing combustion installation to preheat the air may be difficult to justify due to space requirements, the installation of extra fans, and the addition of a NOx removal process if NOx emissions exceed emission limit values. It should be noted that a NOx removal process based on ammonia or urea injection induces a potential of ammonia slippage in the flue-gases, which can only be controlled by a costly ammonia sensor and a control loop, and, in case of large load variations, adding a complicated injection system (for example, with two injection ramps at different levels) to inject the NOx reducing agent in the right temperature zone
●gas cleaning systems, like NOx or SOx removal systems, only work in a given temperature range. When they have to be installed to meet the emission limit values, the arrangement of gas cleaning and heat recovery systems becomes more complicated and can be difficult to justify from an economic point of view
●in some cases, the local authorities require a minimum temperature at the stack to ensure proper dispersion of the flue-gases and to prevent plume formation. This practice is often carried out to maintain a good public image. A plume from a plant's stack may suggest to the general public that the plant is causing pollution. The absence of a plume suggests clean operation and under certain weather conditions some plants ( e.g. in the case of waste incinerators) reheat the flue-gases with natural gas before they are released from the stack. This is a waste of energy.
The strategies above, apart the periodic cleaning, require additional investment and are best applied at the design and construction of the installation. However, retrofitting an existing installation is possible (if space is available).
Some applications may be limited by the difference between the process inlet temperature and the flue-gas exhaust temperature. The quantitative value of the difference is the result of a compromise between the energy recovery and cost of equipment.
Recovery of heat is always dependent on there being a suitable use (see Section 3.3).
See the potential for pollutant formation, in Cross-media effects, above.
Payback time can be from under five years to as long as to fifty years depending on many parameters, such as the size of the installation, and the temperatures of the flue gases.
Energy Efficiency (2009) 3.1.1