Waste-gas heat losses are unavoidable in the operation of all fuel-fired boilers, ovens, dryers, furnaces and kilns. Air and fuel are mixed and burned to generate heat, and a portion of this heat is transferred to the heating device and its load. When the energy transfer reaches its practical limit, the spent combustion gases are removed (exhausted) from the heat processing equipment via a flue or stack to make room for a fresh charge of combustion gases. At this point, the exhaust flue gases still hold considerable thermal energy -- at times, more than what was left behind in the process. In many fuel-fired heating systems, this waste heat is the greatest source of heat loss in the process and can be greater than all the other losses combined.
Fortunately, it is possible to reduce waste heat losses. The first step is to take measures to reduce all heat losses associated with the heat processing equipment because any reduction will be multiplied by the overall available heat factor. This could result in much higher energy savings. These losses include:
- Heat storage in the oven, boiler, dryer, furnace or kiln
- Losses from the heat processing equipment’s outside walls or
- Heat transported out of the equipment by the load conveyors, fixtures
- Radiation losses from openings, hot exposed parts, etc.
- Heat carried from the heat processing equipment by the cold air
- Heat carried out by the excess air used in the burners.
- A lower energy component of product costs.
- Improved heat processing equipment productivity.
- Lower emissions of carbon monoxide, nitrogen oxides and unburned hydrocarbons.
Ways to Increase Energy EfficiencyTo increase energy efficiency by reducing exhaust gas heat losses, you first must calculate the losses. The exhaust gas heat losses can be calculated using the equation:
Furnace Exhaust Heat Losses = W X CP X (TEXHAUST – TAMBIENT)
where W is the mass of the exhaust gases; CPis the specific heat of the exhaust gases; TEXHAUSTis the flue gas temperature entering the furnace exhaust system (stack); and TAMBIENTis the ambient temperature.
The highest priority is to minimize exhaust gas temperature and mass or volume of exhaust gases. The heat processing equipment exhaust gas temperature depends on many factors associated with the equipment operation and heat losses. It can be measured directly, or it can be assumed to be 100 to 200°F (55 to 111°C) above the control temperature for the heated zone where the flue gases are exhausted.
The exhaust mass flow depends on the combustion airflow, fuel flow and the air leakage into the heat processing equipment. Measurement of fuel flow, together with the percentage of oxygen in the flue gases, can be used to estimate mass or volume of exhaust gases.
The flue gas specific heat (CP) for most gaseous fuel-fired furnaces can be assumed to be 0.25 BTU/lb/°F or 0.02 BTU/scf/°F for a reasonably accurate estimate of flue gas heat losses.
Minimize Exhaust Gas Temperatures.Excessive gas temperatures can be the result of poor heat transfer in the heat processing equipment. If the combustion gases are unable to transfer the maximum possible heat to the oven, furnace or other heat processing equipment -- and its contents -- they will leave the equipment at higher temperatures than necessary.
Overloading heat processing equipment also can lead to excessive stack temperatures. To get the proper rate of heat transfer, combustion gases must be held in the heating chamber for the right amount of time. The natural tendency of an overloaded oven or furnace is to run colder than optimal unless the temperature is set artificially high. This causes the burners to operate at higher than normal firing rates, which increases combustion gas volumes. The higher the gas flow rates and shorter time in the heat processing equipment cause poor heat transfer, resulting in higher temperature for the flue gases. Increased volumes of higher temperature flue gases lead to sharply increased heat losses.
Some reduction in exhaust volumes will be the indirect result of efficiencies applied elsewhere. For instance, flue gas losses are a fixed percentage of the total heat input to the oven or furnace. Any reduction in heat storage, wall, conveyor or radiation losses will be multiplied by the available heat factor.
Use of Oxygen Enriched Combustion Air.Ambient air contains approximately 21 percent oxygen, with nitrogen and other inert gases as the balance. The total volume of exhaust gases could be reduced by increasing the oxygen content of the combustion air, either by mixing extra oxygen into the ambient air or by using 100 percent oxygen. The reduced exhaust gases would result in substantial fuel savings. The exact amount of energy savings depends on the percentage of oxygen in combustion air and flue gas temperature. Higher values of oxygen and flue gas temperature offer higher fuel savings. Obviously, the fuel savings would need to be compared to the cost of oxygen to estimate actual economic benefits.
Waste Heat Recovery.Reducing exhaust losses should always be the first step in a well-planned energy conservation program. Once that goal has been met, consider the next level: waste heat recovery. Waste heat recovery elevates oven or furnace efficiency to higher levels because it extracts energy from the exhaust gases and recycles it to the process. Significant efficiency improvements can be made even on furnaces that operate with properly tuned ratio and temperature controls. There are four widely used methods.
Direct Heat Recovery to the Product.If exhaust gases leaving the high-temperature portion of the process can be brought into contact with a relatively cool incoming load, energy will be transferred to and preheat the load. This reduces the energy that finally escapes with the exhaust. This is the most efficient use of waste heat in the exhaust.
Use of waste heat recovery to preheat combustion air is commonly used in medium- to high-temperature ovens and furnaces. Using preheated air for the burners reduces the amount of purchased fuel required to meet the process heat requirements. Preheating combustion air requires the use of a recuperator or regenerator.
Regenerators.A regenerator is an insulated container filled with metal or ceramic shapes that can absorb and store relatively large amounts of thermal energy. During the operating cycle, process exhaust gases flow through the regenerator, heating the storage medium. After a while, the medium becomes fully heated (charged). The exhaust flow is shut off and cold combustion air enters the unit. As it passes through, the air extracts heat from the storage medium, increasing the air’s temperature before it enters the burners. Eventually, the heat stored in the medium is drawn down to the point where the regenerator requires recharging. At that point, the combustion airflow is shut off, and the exhaust gases return to the unit. This cycle repeats as long as the process continues to operate.
For a continuous operation, at least two regenerators and their associated burners are required. One regenerator provides energy to the combustion air while the other recharges. In this sense, it is much like using a cordless power tool; to use it continuously, you must have at least two batteries to swap out between the tool and the recharger. An alternate design of the regenerator uses a continuously rotating wheel containing metal or ceramic matrix. The flue gases and combustion air pass through different parts of the wheel during its rotation to receive heat from flue gases and release heat to the combustion air.
Waste Heat Boiler.Use of a waste heat boiler to recover part of the exhaust gas heat is an option for plants that need a source of steam or hot water. The waste heat boiler is similar to conventional boilers with one exception: it is heated by the exhaust gas stream from a process oven or furnace instead of its own burner. Waste heat boilers may be the answer for plants seeking added steam capacity. Users should remember, however, that the boiler generates steam only when the process is running.
Not all processes are candidates for waste heat recovery. Exhaust volumes and temperatures may be too low to provide financial justification. A comprehensive program for reducing oven or furnace energy consumption involves achieving the best performance from the existing equipment and adding modifications and upgrades that can make substantial reductions in energy consumption.