With a bigger focus than ever on meeting strict emissions targets, Steve O’Neill, Technical Specifications Engineer at EOGB Energy Products Ltd, looks specifically at NOx and how combustion modifications can significantly reduce the amount that is produced…
Due to their effect upon the environment and human health, it is clear that there will be a continued interest into the reduction of pollutant gases from the combustion process within the process, hydrocarbon and other combustion industries well into the future.
When a fuel is burnt, pollutants are generated through the combustion process. Different fuels will contain different levels of impurities which will contribute to the varying types and levels of emissions being released. For example, some heavy oils and diesels may contain sulphur and traces of metals, coal contains sulphur and nitrogen and refuse derived fuels may contain chlorine or heavy metals.
Oxides of Nitrogen (NOX)
In combustion terms, NOX will commonly include nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O) and less commonly pollutants such as nitrogen tetroxide (N2O4).
Nitrogen oxides can form acid rain, by a similar mechanism to oxides of Sulphur (SOx), and will contribute to the formation of particulates during smog episodes. In the air, NOx reacts readily with common organic chemicals and ozone, to form a wide variety of toxic products. Therefore, nitrogen oxides are strictly regulated, resulting in the rise of low-NOx technology for industrial and commercial processes.
The production of thermal NOX is understood to increase exponentially with an increase in temperature, typically in flame regions with a temperature higher than 11000C. Above this temperature, thermal NOX is known to be the predominant mechanism for the formation of NOX.
Most of the proposed reaction schemes for thermal NOX utilise the extended Zeldovich mechanism. The main parameters of thermal NOx formation are defined by this equation as it can be observed that temperature, oxygen and nitrogen concentrations and residence time in the flame zone are critical factors.
Another factor to be considered at elevated flame temperatures is the molecular dissociation that would take place in these high temperature zones, which will give rise to the formation of unburnt hydrocarbons (UHC’s).
The formation of thermal NOX becomes a significant factor in burner design when considering high temperature application that would normally incorporate air pre-heat technology or any other procedure, which would elevate the flame temperature.
Various coals and oils may contain up to 2% N2 (by mass). During combustion, between 10% – 50% of this nitrogen will react with hydrocarbon radicals like CH or CH3 and will form HCN, which will then lead to NO. Little can be done to stop the generation of fuel NOX.
In the combustion of most gaseous fuels the quantity of nitrogen held within them is considered to be negligible and thus fuel NOX is not considered to be a critical factor in their combustion.
By approximately the same mechanism as thermal NOX formation, the nitrogen contained in the air will be attacked by the hydrocarbon radicals CH to form HCN, which then leads almost invariably to NO.
The formation of prompt NOX involves hundreds of different reactions of which the hydrocarbon radicals are simply an intermediary component. This mechanism is called ‘prompt’ NOX because it forms quickly as the combustion reactants separate and re-form.
The prompt NO mechanism occurs at lower temperatures than thermal NOx and in fuel rich conditions with short flame residence times.
NOX reduction by combustion modification
Many different strategies have been developed over the years to assist in the reduction of NOX during the combustion process. Some of the abatement strategies available include reducing pollutants by passing the exhaust gas over a metal surface, pretreatment of fuels, using an alternative fuel to that which was traditionally used for a specific process and using oxygen enriched combustion air.
Although the above can have a potentially significant economical impact on the process, the financial impact could invariably outweigh the benefit of the reductions obtained. By modifying the combustion process, it is possible to obtain the same outcomes in NOX reduction so this is much more economically sound option than most pre or post combustion reduction methods.
Air staged combustion
Combustion staging controls the formation of NOx (particularly thermal NOx) by controlling the amount and location of air introduced into the combustion chamber. The air is either reduced or excess air is used to cool the flame temperature making the conditions less conducive to thermal NOX formation.
The air must be carefully controlled to ensure that carbon monoxide is not produced by insufficient air. There is also the danger that if too much excess air is injected into the high temperature flame zone, the nitrogen and oxygen will combine to form excessive levels of Thermal NOx. The excess air entering the flame must be carefully controlled through the use of a combination of oxygen trim controls and burner design.
Fuel staged combustion
This is a similar technique to air staged combustion but introduces fuel in stages instead of combustion air.
The burner design creates two specific combustion regions of the fuel/ air injection point and the air registry and this creates the swirl to promote turbulent mixing of the fuel/air. A lean primary region is created in which the total quantity of combustion air is supplied with a fraction of the fuel. The secondary region is then injected with the remainder of the fuel and combusted by the excess air from the primary region, thus limiting the formation of NOx by promoting lower flame temperatures.
Low excess air
Low excess air firing can be used on most boilers and can yield overall NOx reductions of 5-10% when firing on natural gas.
Increased thermal efficiency is achieved at low excess air levels due to less heat energy being lost through heating increased volumes of air. More heat is transferred to the process per unit of energy input, thus requiring less fuel to provide the required heat flux.
Consideration must be given to how much excess and tertiary air can be trimmed away as the reduction could cause the overall flame area to expand and promote flame chilling (depending upon burning velocity), or alternatively the outer flame surface could impinge on the wall of the furnace thus promoting the production of CO.
Reduction in air pre-heat temperature
Combustion air preheat is often used to increase the thermal efficiency of the boiler.
Air preheaters will typically have an exhaust gas temperature of around 250 degrees centigrade. The addition of air preheat is conducive to the formation of thermal NOx. This trade off between thermal efficiency and NOx levels can be partially resolved by introducing a convection section heat recovery unit.
Off condition combustion
To reduce the peak temperature of a flame, which would occur at stoichiometric conditions, the fuel/air can be introduced in such a way to produce off-stoichiometric combustion. Over fired air keeps the mixture fuel rich and completes the combustion process using staged air injectors.
Fuel reburning operates along similar techniques as fuel staging whereupon the main burners are operated with very low excess air to produce fuel rich conditions.
Approximately 10-20% of the fuel is diverted to a point above the primary combustion zone, whereupon it is injected into the ductwork downstream of the main furnace known as a ‘fuel rich reducing zone’. Due to the reduced flame temperatures, prompt NOX will form. As the flue gases pass through the fuel rich reducing zone, the prompt NOX is reduced back to its base constituents. Any UHC’s or CO is subsequently burnt off downstream of the primary combustion zone at significantly reduced temperatures, resulting in reduced NOX emissions. The remaining combustion air is then injected above the reburn zone to provide the necessary burnout air.
In many cases, reburning can be more economical than post combustion NOx controls that would otherwise be used in these instances
Water or steam injection is a NOx reduction method whereby water is injected into the flame, thus lowering the flame temperature and reducing thermal NOx production.
Water injection can reduce NOx emissions by up to 80%. However, there is a limit to the amount of water that can be injected due to condensation and there can be a reduction in boiler efficiency of between 3-10%. There is also the danger of the production of UHC and CO.
Flue gas recirculation
Flue gas recirculation (FGR) is a popular method of reducing NOx emissions from industrial boilers.
FGR entails using a portion (15% – 30%) of the relatively low temperature flue gases and recirculating then back into the combustion region. The recirculation of flue gases dilutes the combustion reactants, and reduces the oxygen concentration, thereby inhibiting thermal NOx formation. The technology can be split into two methods:
Overall, with the increased focus on reducing harmful pollutant gases from the atmosphere, it’s important that companies consider combustion modifications as an economically viable way to meet stricter emissions targets.