The field of industrial combustion is very broad and touches, directly or indirectly, nearly all aspectsof our lives. Many aspects of our life are strictly connected to production processes where combustion is involved. The cars we drive use internal combustion engines. The planes we fly in use jet-fuel-powered turbine engines. Most of the materials we use have been made through some type of heating process. Combustion in many industrial processes is treated in a marginal way, without attention to efficiency, energy saving and pollutants.
We should define an emission as any unwanted egress of a substance or quantity. If we refer to fossil fuels combustion process, CO and NOx are its main byproducts. With proper design, they can be reduced to acceptable levels but never eliminated. CO emissions are generally an indicator of poor mixing, which could cause an high quantity of unburnt products.
A proper burner design, and a proper flame management CO, typically should not be a big problem, but, if we look to tissue paper production process, a reality check shows that many of basic flame management principles are not considered. The result is that most paper mills are running with unbalanced and unsafe burners, causing low process global efficiency and consequently high level of pollutants. NOx is formed in very low quantities (parts per million on a volume basis) as a byproduct of combustion in several ways. NOx contributes to ground-level ozone via a complex reaction with hydrocarbons and other reactive compounds.
We can find three main NOx formation mechanisms.
The Thermal NOx mechanism comprises several steps, but can be reduced for discussion purposes, to two main ones:
N + O2 = NO + O (1)
O + N2 = NO + N (2)
N2 + O2 = 2NO (3)
NOx in combustion generally refers to NO or NO2 (nitrogen dioxide), although nitrogen can form other molecular species (e.g., N2O, N2O5). However, only NO and NO2 are regulated or formed in any quantities of concern.
High temperatures are required to generate the atomic species in equations indicated above. Industrial flames can achieve such temperatures. Oxygen molecules comprise a double bond and molecular nitrogen comprises a triple bond. Therefore, the oxygen double bond is the easiest to rupture (Equation 1). Owing to the difficulty of rupturing the N triple bond, equation 2 is the rate-limiting step and paces the reaction. If we presume that atomic oxygen is in equilibrium with its molecular counterpart, we can write a rate law governing nitric oxygen production.
where the square brackets [ ] denote volume concentrations of the enclosed species, A and b are constants, T is the absolute temperature, and q is the time under those conditions.
Unfortunately, it is not possible to integrate equation 4 because of a lack of informationregarding the oxygen-temperature history within the flame. However, Equation 4.11 is useful as a heuristic for pointing out the main features of thermal NOx formation.
First, temperature is an exponent indicating that NOx formation is very sensitive to temperature. Hot spots in the flame contribute significantly to NOx formation. Second, high oxygen concentration inflates NOx. The relation is square root proportional within the integral. However, upon integration, NOx would be related to oxygen concentration raised to a higher power. Studies show that NOx is approximately proportional to oxygen concentration. Nitrogen varies little over the course of combustion and there is no convenient way to reduce nitrogen concentration. Therefore, it can be regarded as approximately constant.
Equation 3 suggests some NOx strategies and parameters to reduce:
- peak flame temperatures.
- oxygen concentration.
- time of high-temperature combustion in the vicinity of oxygen.
We can find two additional NOx forming mechanisms, generally less involved in NOx total amount :
- Fuel bound NOx. This mechanism is due to Nitrogen presence into fuel molecule. It is not influenced by Nitrogen addition into natural gas (generally this method is used in order to reduce combustion chamber temperatures and increase mixture heat capacity).
- Prompt NOx. This mechanism is similar to fuel bound NOx formation, but it is related to Nitrogen into combustion air. It is not practical to limit nitrogen concentration in combustion air for most applications. The main strategy for reducing prompt NOx is to dilute the fuel species before combustion.
Combustion stability and flame formation are two relevant parameters for managing efficiency and pollutants emissions. During last years, swirling burners are being used by main manufacturers with the aim of increase combustion stability and temperature distribution, achieving important reductions in terms of pollutants emissions.
Swirl Number could be defined as following formulation :
Where u is the axial velocity, w is the tangential velocity and r is the radial position. Looking at the big picture, we can classify swirling flames as follows :
S≈0,3 – Toroidal recirculation zone – moderate mixing rate – use in industrial furnaces.
S>0,6 – Recirculation flux into flame center – primary jet softer than Type I. Typical for furnaces and boilers.
High S – High recirculation flux into flame center and high mixing rate.
Very high S – Flat flame with high mixing rate.
In Mevas we are strongly committed to bring these concepts in Pulp&Paper industry, giving also our contribution to improve burners quality, so we developed a computational fluid-dynamic model, in order to understand how a swirling non-premixed burner should be made, paying particular attention to process requirements in Pulp&Paper industry. What we obtained is shown in the photo below.