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Combustion management - Efficiency gas on furnace
Efficient
process heating requires recovery of the largest possible part of a fuel’s heat of combustion into the material being processed.[9][10] There are many avenues of loss in the operation of a heating process. Typically, the dominant loss is sensible heat leaving with the offgas (i.e., the flue gas). The temperature and quantity of offgas indicates its heat content (enthalpy), so keeping its quantity low minimizes heat loss.
In a perfect
furnace,
the combustion air flow would be matched to the fuel flow to give each
fuel molecule the exact amount of oxygen needed to cause complete
combustion. However, in the real world, combustion does not proceed in a
perfect manner. Unburned fuel (usually CO and H
2) discharged from the system represents a heating value
loss (as well as a safety hazard). Since combustibles are undesirable in
the offgas, while the presence of unreacted oxygen there presents
minimal safety and environmental concerns, the first principle of
combustion management is to provide more oxygen than is theoretically
needed to ensure that all the fuel burns. For methane (CH
4) combustion, for example, slightly more than two molecules of oxygen are required.
The second principle of combustion management, however, is to not use
too much oxygen. The correct amount of oxygen requires three types of
measurement: first, active control of air and fuel flow; second, offgas
oxygen measurement; and third, measurement of offgas combustibles. For
each heating process there exists an optimum condition of minimal offgas
heat loss with acceptable levels of combustibles concentration.
Minimizing excess oxygen pays an additional benefit: for a given offgas
temperature, the
NOx level is lowest when excess oxygen is kept lowest.[2]
Adherence to these two principles is furthered by making material and heat balances on the combustion process.
[11][12][13][14] The
material balance directly relates the air/fuel ratio to the percentage of O
2 in the combustion gas. The heat balance relates the heat
available for the charge to the overall net heat produced by fuel
combustion.[15][16] Additional material and heat balances can be made to quantify the thermal advantage from preheating the combustion air,[17][18] or enriching it in oxygen.[19][20]
Reaction mechanism
Combustion in oxygen is a chain reaction in which many distinct
radical
intermediates participate. The high energy required for initiation is
explained by the unusual structure of the dioxygen molecule. The
lowest-energy configuration of the dioxygen molecule is a stable,
relatively unreactive diradical in a triplet spin state. Bonding can be described with three bonding electron pairs and two antibonding electrons, whose spins
are aligned, such that the molecule has nonzero total angular momentum.
Most fuels, on the other hand, are in a singlet state, with paired
spins and zero total angular momentum. Interaction between the two is
quantum mechanically a "forbidden transition",
i.e. possible with a very low probability. To initiate combustion,
energy is required to force dioxygen into a spin-paired state, or singlet oxygen. This intermediate is extremely reactive. The energy is supplied as heat, and the reaction then produces additional heat, which allows it to continue.
Combustion of hydrocarbons is thought to be initiated by hydrogen
atom abstraction (not proton abstraction) from the fuel to oxygen, to
give a hydroperoxide radical (HOO). This reacts further to give
hydroperoxides, which break up to give
hydroxyl radicals.
There are a great variety of these processes that produce fuel radicals
and oxidizing radicals. Oxidizing species include singlet oxygen,
hydroxyl, monatomic oxygen, and hydroperoxyl.
Such intermediates are short-lived and cannot be isolated. However,
non-radical intermediates are stable and are produced in incomplete
combustion. An example is acetaldehyde produced in the combustion of ethanol. An intermediate in the combustion of carbon and hydrocarbons, carbon monoxide, is of special importance because it is a poisonous gas, but also economically useful for the production of syngas.
Solid and heavy liquid fuels also undergo a great number of
pyrolysis
reactions that give more easily oxidized, gaseous fuels. These
reactions are endothermic and require constant energy input from the
ongoing combustion reactions. A lack of oxygen or other poorly designed
conditions result in these noxious and carcinogenic pyrolysis products
being emitted as thick, black smoke.
The rate of combustion is the amount of a material that undergoes
combustion over a period of time. It can be expressed in grams per
second (g/s) or kilograms per second (kg/s).
Detailed descriptions of combustion processes, from the chemical
kinetics perspective, requires the formulation of large and intricate
webs of elementary reactions.
[21]
For instance, combustion of hydrocarbon fuels typically involve
hundreds of chemical species reacting according to thousands of
reactions (see, e.g., the GRI-mech mechanism,
http://combustion.berkeley.edu/gri-mech/).
Inclusion of such mechanisms within computational flow solvers still
represents a pretty challenging task mainly in two aspects. First, the
number of degrees of freedom (proportional to the number of chemical
species) can be dramatically large; second the source term due to
reactions introduces a disparate number of time scales which makes the
whole dynamical system stiff. As a result, the direct numerical
simulation of turbulent reactive flows with heavy fuels soon becomes
intractable even for modern supercomputers.
[22]
Therefore, a plethora of methodologies has been devised for reducing
the complexity of combustion mechanisms without renouncing to high
detail level. Examples are provided by: the Relaxation Redistribution
Method (RRM)
[23][24][25][26] The Intrinsic Low-Dimensional Manifold (ILDM) approach and further developments
[27][28][29] The invariant constrained equilibrium edge preimage curve method.
[30] A few variational approaches
[31][32] The Computational Singular perturbation (CSP) method and further developments.
[33][34] The Rate Controlled Constrained Equilibrium (RCCE) and Quasi Equilibrium Manifold (QEM) approach.
[35][36] The G-Scheme.
[37] The Method of Invariant Grids (MIG).
[38][39][40]
Temperature
Antoine Lavoisier conducting an experiment related combustion generated by amplified sun light.
Assuming perfect combustion conditions, such as complete combustion under
adiabatic
conditions (i.e., no heat loss or gain), the adiabatic combustion
temperature can be determined. The formula that yields this temperature
is based on the first law of thermodynamics and takes note of the fact that the heat of combustion
is used entirely for heating the fuel, the combustion air or oxygen,
and the combustion product gases (commonly referred to as the flue gas).
In the case of fossil fuels burnt in air, the combustion temperature depends on all of the following:
The adiabatic combustion temperature (also known as the
adiabatic flame temperature) increases for higher heating values and inlet air and fuel temperatures and for stoichiometric air ratios approaching one.
Most commonly, the adiabatic combustion temperatures for coals are
around 2,200 °C (3,992 °F) (for inlet air and fuel at ambient
temperatures and for
), around 2,150 °C (3,902 °F) for oil and 2,000 °C (3,632 °F) for
natural gas.[41][42]
In industrial
fired heaters, power station steam generators, and large gas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air.
For example, excess combustion air of 15 percent means that 15 percent
more than the required stoichiometric air is being used.
Instabilities
Combustion instabilities are typically violent pressure oscillations
in a combustion chamber. These pressure oscillations can be as high as
180 dB, and long term exposure to these cyclic pressure and thermal
loads reduces the life of engine components. In rockets, such as the F1
used in the Saturn V program, instabilities led to massive damage of the
combustion chamber and surrounding components. This problem was solved
by re-designing the fuel injector. In liquid je
t engines the droplet
size and distribution can be used to attenuate the instabilities.
Combustion instabilities are a major concern in ground-based gas turbine
engines because of NOx emissions. The tendency is to run lean, an
equivalence ratio less than 1, to reduce the combustion temperature and
thus reduce the NOx emissions; however, running the combustion lean
makes it very susceptible to combustion instability.
The
Rayleigh Criterion
is the basis for analysis of thermoacoustic combustion instability and
is evaluated using the Rayleigh Index over one cycle of instability[43]
where q' is the heat release rate perturbation and p' is the pressure fluctuation.
[44][45]
When the heat release oscillations are in phase with the pressure
oscillations, the Rayleigh Index is positive and the magnitude of the
thermo acoustic instability is maximised. On the other hand, if the
Rayleigh Index is negative, then thermoacoustic damping occurs. The
Rayleigh Criterion implies that a thermoacoustic instability can be
optimally controlled by having heat release oscillations 180 degrees out
of phase with pressure oscillations at the same frequency.
[46][47] This minimizes the Rayleigh Index.
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