Oxy-fuel combustion process
Encyclopedia
Oxy-fuel combustion is the process of burning a fuel using pure oxygen instead of air as the primary oxidant. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures are possible. Historically, the primary use of oxy-fuel combustion has been in welding and cutting of metals, especially steel, since oxy-fuel allows for higher flame temperatures than can be achieved with an air-fuel flame.
There is currently research being done in firing fossil-fueled power plants with an oxygen-enriched gas mix instead of air. Almost all of the nitrogen is removed from input air, yielding a stream that is approximately 95% oxygen. Firing with pure oxygen would result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas
, or staged combustion
. The recycled flue gas can also be used to carry fuel into the boiler and ensure adequate convective heat transfer to all boiler areas. Oxy-fuel combustion produces approximately 75% less flue gas than air fueled combustion and produces exhaust consisting primarily of CO2 and H2O (see figure).
The justification for using oxy-fuel is to produce a CO2 rich flue gas ready for sequestration. Oxy-fuel combustion has significant advantages over traditional air-fired plants. Among these are:
Economically speaking this method costs more than than a traditional air-fired plant. The main problem has been separating oxygen from the air. This process needs lots of energy, nearly 15% of production by a coal-fired power station can be consumed for this process. However, a new technology which is not yet practical called chemical looping combustion
can be used to reduce this cost. At present in the absence of any need to reduce CO2 emissions, oxy-fuel is not competitive. However, oxy-fuel is a viable alternative to removing CO2 from the flue gas from a conventional air-fired fossil fuel
plant. However, an oxygen concentrator
might be able to help, as it simply removes nitrogen.
In industries other than power generation, oxy-fuel combustion can be competitive due to higher sensible heat availability.
Oxy-fuel combustion is common in various aspects of metal production.
The glass industry has been converting to oxy-fuel since the early 1990s because glass furnaces require a temperature of approximately 2800 degrees F, which is not attainable at adiabatic flame temperatures for air-fuel combustion unless heat is regenerated between the flue stream and the incoming air stream. Historically, glass furnace regenerators were large and expensive high temperature brick ducts filled with brick arranged in a checkerboard pattern to capture heat as flue gas exits the furnace. When the flue duct is thoroughly heated, air flow is reversed and the flue duct becomes the air inlet, releasing its heat into the incoming air, and allowing for higher furnace temperatures than can be attained with air-fuel only. Two sets of regenerative flue ducts allowed for the air flow to be reversed at regular intervals, and thus maintain a high temperature in the incoming air. By allowing new furnaces to be built without the expense of regenerators, and especially with the added benefit of nitrogen oxide
reduction, which allows glass plants to meet emission restrictions, oxy-fuel is cost effective without the need to reduce CO2 emissions. Oxy-fuel combustion also reduces CO2 release at the glass plant location, although this may be offset by CO2 production due to electric power generation which is necessary to produce oxygen for the combustion process.
Oxy-fuel combustion may also be cost effective in the incineration of low BTU value hazardous waste fuels.
Oxy-fuel combustion is often combined with staged combustion
for nitrogen oxide
reduction, since pure oxygen can stabilize combustion characteristics of a flame.
There is currently research being done in firing fossil-fueled power plants with an oxygen-enriched gas mix instead of air. Almost all of the nitrogen is removed from input air, yielding a stream that is approximately 95% oxygen. Firing with pure oxygen would result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas
Flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...
, or staged combustion
Staged combustion
Staged combustion is a method for reducing nitrogen oxides emissions from combustion. There are two methods:* Air staged supply* Fuel staged supply-Air staged supply:Characteristics:* NOX reduction rate from 50-75% is possible....
. The recycled flue gas can also be used to carry fuel into the boiler and ensure adequate convective heat transfer to all boiler areas. Oxy-fuel combustion produces approximately 75% less flue gas than air fueled combustion and produces exhaust consisting primarily of CO2 and H2O (see figure).
The justification for using oxy-fuel is to produce a CO2 rich flue gas ready for sequestration. Oxy-fuel combustion has significant advantages over traditional air-fired plants. Among these are:
- The mass and volume of the flue gas are reduced by approximately 75%.
- Because the flue gas volume is reduced, less heat is lost in the flue gas.
- The size of the flue gas treatment equipment can be reduced by 75%.
- The flue gas is primarily CO2, suitable for sequestration.
- The concentration of pollutants in the flue gas is higher, making separation easier.
- Most of the flue gases are condensable; this makes compression separation possible.
- Heat of condensation can be captured and reused rather than lost in the flue gas.
- Because nitrogen from air is not allowed in, nitrogen oxideNitrogen oxideNitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
production is greatly reduced.
Economically speaking this method costs more than than a traditional air-fired plant. The main problem has been separating oxygen from the air. This process needs lots of energy, nearly 15% of production by a coal-fired power station can be consumed for this process. However, a new technology which is not yet practical called chemical looping combustion
Chemical looping combustion
Chemical looping combustion typically employs a dual fluidized bed system where a metal oxide is employed as a bed material providing the oxygen for combustion in the fuel reactor...
can be used to reduce this cost. At present in the absence of any need to reduce CO2 emissions, oxy-fuel is not competitive. However, oxy-fuel is a viable alternative to removing CO2 from the flue gas from a conventional air-fired fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
plant. However, an oxygen concentrator
Oxygen concentrator
An oxygen concentrator is a device providing oxygen therapy to a patient at minimally to substantially higher concentrations than available in ambient air. They are used as a safer, less expensive, and more convenient alternative to tanks of compressed oxygen. Common models retail at around US$800...
might be able to help, as it simply removes nitrogen.
In industries other than power generation, oxy-fuel combustion can be competitive due to higher sensible heat availability.
Oxy-fuel combustion is common in various aspects of metal production.
The glass industry has been converting to oxy-fuel since the early 1990s because glass furnaces require a temperature of approximately 2800 degrees F, which is not attainable at adiabatic flame temperatures for air-fuel combustion unless heat is regenerated between the flue stream and the incoming air stream. Historically, glass furnace regenerators were large and expensive high temperature brick ducts filled with brick arranged in a checkerboard pattern to capture heat as flue gas exits the furnace. When the flue duct is thoroughly heated, air flow is reversed and the flue duct becomes the air inlet, releasing its heat into the incoming air, and allowing for higher furnace temperatures than can be attained with air-fuel only. Two sets of regenerative flue ducts allowed for the air flow to be reversed at regular intervals, and thus maintain a high temperature in the incoming air. By allowing new furnaces to be built without the expense of regenerators, and especially with the added benefit of nitrogen oxide
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
reduction, which allows glass plants to meet emission restrictions, oxy-fuel is cost effective without the need to reduce CO2 emissions. Oxy-fuel combustion also reduces CO2 release at the glass plant location, although this may be offset by CO2 production due to electric power generation which is necessary to produce oxygen for the combustion process.
Oxy-fuel combustion may also be cost effective in the incineration of low BTU value hazardous waste fuels.
Oxy-fuel combustion is often combined with staged combustion
Staged combustion
Staged combustion is a method for reducing nitrogen oxides emissions from combustion. There are two methods:* Air staged supply* Fuel staged supply-Air staged supply:Characteristics:* NOX reduction rate from 50-75% is possible....
for nitrogen oxide
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
reduction, since pure oxygen can stabilize combustion characteristics of a flame.