COMBUSTION SYSTEMS
Combustion systems provide heat for processes or for comfort. Typical equipment using combustion as their process are ovens, heat treatment tunnels, boilers, furnaces, etc. Combustion is at the heart of energy-intensive industries requiring high temperatures (metals, cement, glass).
This Section briefly describes the combustion phenomenon and gives some recommendations on the main parameters to monitor and the potential energy conservation actions.
A combustion is a chemical reaction between an oxidizer (oxygen) and a fuel (gas, liquid, solid), and generates energy under the form of heat. Most fuels used in industry are carbon-based molecules (“fossil fuels”):
• Coal
• Solid, liquid or gaseous hydrocarbons
• Biomass
The combustion of fossil fuels is the oxidation of their carbon and other constituents (mainly hydrogen for hydrocarbons). These reactions yield water and carbon dioxide.
Natural gas is a mixture of different gases, and is mainly made of nitrogen (5-10%) and alkanes (saturated hydrocarbons) : methane (85-95%), ethane (4-8%), propane, butane.
During the combustion of methane, carbon combines with oxygen to yield carbon dioxide, and hydrogen combines with oxygen to yield water. Both reactions generate useful combustion heat.
When water exits the stack as a vapor, the released heat is called the Lower Heat Value. When water exits as a liquid, the released heat is called the Higher Heating Value (because water condensation yields additional heat).
Because the flue gases exit at relatively high temperatures (typically 180°C to 600°C depending on the application) and their heat content is lost, not all the heat released by the combustion will be recovered. This heat loss with the flue gases must be deducted from the available heat of combustion. Combustion efficiency is thus defined as the net heat recovered by the boiler over the fuel lower heating value. Note that the overall equipment efficiency (e.g., a boiler) will be even lower because of additional heat losses. Typically, a water boiler will have an overall efficiency of circa 85% based on LHV, and circa 75% based on HHV.
This means that when a site is billed 100 MWh of natural gas (purchase is based on HHV), only about 75 MWh will be recovered as “useful” heat, unless the installation is designed to recover the heat from the condensation of water in the flue gases.
The combustion reaction requires oxygen. In a few industrial applications like some glass plants, pure oxygen is used. However, in the huge majority of cases, air is used as the source of oxygen, which means that nitrogen will be part of the combustion gases. The accompanying nitrogen does not take part in the combustion reaction. It is therefore just heated and exhausted in the fumes.
If the air flow is insufficient, the oxygen flow will also be insufficient to complete the combustion reaction. Incomplete reaction yields toxic carbon monoxide (CO) and decreases the fuel heating value. To ensure complete reaction and avoid the release of CO, oxygen is provided in excess of the theoretical requirements. Since oxygen is fed with air, combustion will require “excess air”, which will be expressed as the ratio of practical air flow to theoretical air requirement.
Common excess air ratios to achieve highest possible efficiencies are:
• Natural gas: 3-10%
• Fuel oil: 5-20%
• Coal: 15-60%
By measuring the residual oxygen in the combustion exhaust gases, the excess air ratio can be determined. Therefore, burner tuning is enabled through the measurement of the oxygen content of the stack exhaust gases.
These measurements are mandatory in most countries and must be carried out on a regular basis. They ensure that combustion systems are tuned and that air pollution is minimized.
From the sample output of a boiler combustion analyzer, the flue gas analysis in the above figure shows:
- Exit gas temperature = 136°C
- CO2 content = 8.7%
- Oxygen content = 5.6%
- Measured combustion efficiency = 93.9%
- CO content = 0.
The figure below shows a measurement report for an annual maintenance certificate in France. This report shows all information relative to the equipment, as well as the detailed data from the gases analyses.
Energy savings recommendations
Optimize excess air
A high excess air will lead to a high volume of exhaust gases (mainly consisting of inert nitrogen). This high volume of gases will increase the waste of heat, and reduce system efficiency. Therefore, energy savings (and carbon emissions reductions) can be realized by proper burner tuning. The table below (for natural gas combustion) shows that when decreasing excess air to reduce outlet oxygen content from 6% to 3.5%, the combustion efficiency raises from 92.7% to 93.8% for an outlet flue gases temperature of 150°C.
Heat recovery from flue gases and preheating combustion air
The flue gases waste an appreciable amount of heat. The heuristics is circa 1% point of efficiency for each 22°C of stack gases temperature. For example: if the flue gas temperature is reduced from 250°C to 206°C in a heat recovery system, then overall system efficiency has been raised by 2%.
If combustion air can be preheated (with waste heat), then overall system efficiency can be increased because the preheat energy will be added and decrease the required fuel energy. Just like heat recovery from the exhaust gases, combustion air preheating will raise overall system efficiency by 1% for each 22°C air temperature rise.
Energy savings tips with combustion systems
- Reduce the demand for heat: for example thru better building insulation, lower temperature set point, better controls, etc.
- Install high efficiency burners (depending on burner age, gains of up to 5% are possible)
- Tune the burners on a regular basis (optimize excess air – typically 3.5% oxygen content in the exhaust gases when using natural gas.
- Install efficient controls (in comfort heat applications, vary hot water temperature setpoint with external temperature) : 10-20% savings possible
- Insulate piping and valves
- Install economizer on flue gases (e.g.water preheating with flue gases) : 1% gain for each 22°C drop in hot gas temperature drop
- Install variable speed drives for water pumps: 15-30% gains potential on power consumption
- Reduce blowdown rate and recover heat from blowdown
- Maintenance: check filters, clean water side on a regular basis, etc.
- For steams systems: reduce steam pressure, return condensate to the boiler
