The combustion of fossil fuels produces not only the complete oxidation products CO₂ and H₂O (as well as SOₓ in the case of sulphur-containing fuels) but also soot, unburned hydrocarbons CₓHᵧ and nitrogen oxides NOₓ. With the exception of H₂O, all of the substances mentioned are classified as pollutants. CO₂ must also be regarded as such, since it contributes to the warming of the Earth's atmosphere.

Carbon Dioxide (CO₂)

A reduction in CO₂ emissions is only possible by increasing plant efficiency. In gas- or oil-fired domestic heating systems, the use of condensing boiler technology (cooling the flue gas below the dew point) can achieve an efficiency improvement of 8–10 % [Dötsch 97]. In industrial combustion systems, preheating the combustion air increases efficiency, but often causes a significant increase in NOₓ emissions.

Sulphur Oxides (SOₓ)

Sulphur in the fuel is completely oxidised to SOₓ during combustion (approx. 95 % SO₂, approx. 5 % SO₃). Reduction is only possible through flue gas aftertreatment.

Soot

Soot is formed mainly in diffusion burners, recognisable by the yellow luminosity of the flame. Premix burners are virtually soot-free under steady-state operation; soot emissions only occur during start-up and shut-down.

Unburned Hydrocarbons CₓHᵧ and CO

These substances arise from incomplete combustion. They can be avoided by homogeneous fuel-air mixing and sufficient residence times at temperatures above 800 °C. Under steady-state operation of modern burners, emissions are in the range of a few ppm. The main sources are transient operating phases (start/stop).

Nitrogen Oxides (NOₓ)

The predominant proportion of emitted nitrogen oxides is NO; NO₂ is formed secondarily from primarily formed NO. NO formation occurs via four mechanisms.

Thermal NO Formation (Zeldovich Mechanism)

NO is formed from atmospheric nitrogen through reaction with atomic oxygen [Zeldo. 46]. The rate-determining reaction has a high activation energy due to the strong N₂ triple bond and proceeds rapidly only at high temperatures. What matters are the temperature peaks in the flame – caused by poor mixing quality – not just the mean flame temperature. Thermal NO can be largely avoided by low flame temperatures and homogeneous premixing. Flue gas recirculation effectively reduces the adiabatic flame temperature [Baehr 84, Flamme 89].

Fuel NO Formation

In fuels with organically bound nitrogen, this is converted to HCN and further to NH₃, which reacts atmospherically to form NO.

Prompt NO Formation (Fenimore Mechanism)

NO is formed from atmospheric nitrogen through reaction with hydrocarbon radicals (CH + N₂ → N + HCN) [Fenim. 71]. Due to the low activation energy, prompt NO occurs from approx. 1000 K – significantly earlier than thermal NO. It forms preferentially under fuel-rich conditions in the flame front [Glarb. 86, Bartok 72].

Nitrous NO Formation (N₂O Mechanism)

N₂O is formed from molecular atmospheric nitrogen and atomic oxygen and reacts further to NO. High pressure and low activation energy favour this mechanism, which is particularly relevant for lean premix combustion in stationary gas turbines [Correa 90]. Under near-stoichiometric conditions and ambient pressure, it is neglected.

References

[Baehr 84]	Baehr, H.D.: Thermodynamik, Springer-Verlag Berlin, 5. Auflage, 1984, ISBN: 3-540-10777-0
[Bartok 72]	Bartok, W.; Engleman, V.S.; Goldstein, R.; Del Valle, E.G.: Basic Kinetic Studies and Modelling of Nitrogen Oxide Formation in Combustion Processes, AIChE Symposium Series 68 (126):30, 1972
[Correa 90]	Correa, S.M.; Smooke, M.D.: NOx in Parametrically varied Methane Flames. In: 23rd Symposium (International) on Combustion, The Combustion Institute, 1990, S. 289–295
[Dötsch 97]	Dötsch, C.; Haase, F.; Köhne, H.: Emissionsarmes Öl-Brennwerttechnik-Konzept. In: Wärmetechnik 42 (1997), Heft 12, S. 652–658
[Fenim. 71]	Fenimore, C.P.: Formation of nitric oxide in premixed hydrocarbon flames. In: 13th Symposium (International) of Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1971
[Flamme 89]	Flamme, M.: Minimierung der Stickoxidbildung, Dissertation Ruhr-Universität Bochum, 1989
[Glarb. 86]	Glarbog, P.; Miller, J.A.; Kee, R.J.: Kinetic modelling and Sensitivity Analysis of Nitrogen Oxide Formation in Well-Stirred Reactors. In: Combustion and Flame 65 (1986), S. 177–202
[Zeldo. 46]	Zeldovich, Ya.B.: The Oxidation of Nitrogen in Combustion and Explosions. In: Acta Physicochimica URSS 21 (1946), S. 577–628

Classification in the Electromagnetic Spectrum

Infrared radiation (IR) is electromagnetic radiation in the wavelength range of approximately 0.7 µm to 1000 µm. The range of 0.7–100 µm is particularly relevant for technical heat transfer problems. A distinction is made between:

Near IR (NIR): 0.7–2.5 µm
Mid IR (MIR): 2.5–25 µm – technically most significant range
Far IR (FIR): 25–1000 µm

Generation of Infrared Radiation

Infrared radiation is generated by thermal motion at the molecular level. Three mechanisms are relevant:

Lattice vibrations in solids: The atoms of a solid vibrate about their equilibrium positions (phonons). The accelerated charges thereby emit electromagnetic waves. The higher the temperature, the greater the vibration amplitudes and the shorter the wavelength of the emission peak.

Rotational and vibrational transitions in gas molecules: Gas molecules emit radiation only at discrete wavelengths where the photon energy hν corresponds to an allowed transition between two energy levels. Only molecules with a variable dipole moment are IR-active – N₂ and O₂ are therefore IR-inactive, while CO₂ and H₂O are strongly IR-active. CO₂ emits mainly at 2.7 µm, 4.3 µm and 15 µm; H₂O at 1.4 µm, 1.9 µm, 2.7 µm and 6 µm.

Electronic transitions: At very high temperatures (> 700 °C), electronic transitions between energy levels contribute to radiation. The photon energy follows from E = hν = hc/λ. This explains the red glow and further the white glow at increasing temperature.

Fundamental Laws

Stefan-Boltzmann Law: The radiant power per unit area emitted by a real body is

q̇ = ε · σ · T⁴

with the Stefan-Boltzmann constant σ = 5.670 × 10⁻⁸ W/m²K⁴ and emissivity ε. The T⁴ dependence is the crucial difference from convection (∼ T) – at high temperatures, radiation always dominates.

Planck's Law of Radiation: The spectral radiance of a black body:

E_bλ = C₁ / { λ⁵ · [exp(C₂/λT) − 1] }

with C₁ = 3.742 × 10⁸ W·µm⁴/m² and C₂ = 1.439 × 10⁴ µm·K.

Wien's Displacement Law: The emission peak shifts with temperature:

λ_max · T = 2898 µm·K

Examples: Sun (5778 K) → 0.5 µm (visible light); industrial furnace (1000 K) → 2.9 µm; human body (310 K) → 9.3 µm.

T₁ 500 K

T₂ 1500 K

T₃ 3000 K

Radiation Properties of Real Surfaces

For an opaque surface, α + ρ = 1 (absorptivity + reflectivity). At thermal equilibrium, Kirchhoff's Law applies: ελ = αλ – the spectral emissivity equals the spectral absorptivity. Typical total emissivities:

Material	ε
Black body (ideal)	1.00
Fireclay / Refractory	0.85–0.95
Oxidised steel (500 °C)	0.79
Polished aluminium	0.04–0.06
Polished copper	0.03

View Factor

The view factor F₁₂ describes the fraction of radiation emitted by surface 1 that strikes surface 2 directly. It depends solely on geometry:

F₁₂ = (1/A₁) ∬ cos θ₁ cos θ₂ / (π r²) dA₂ dA₁

Key calculation rules: Reciprocity A₁F₁₂ = A₂F₂₁; summation rule ΣFᵢⱼ = 1; for convex surfaces Fᵢᵢ = 0.

Net Radiation Exchange

For two infinitely large parallel grey plates:

q̇₁₂ = σ(T₁⁴ − T₂⁴) / (1/ε₁ + 1/ε₂ − 1)

In the general case, each surface is described in the radiation network by a surface resistance Rₒ = (1−ε)/(εA) and a space resistance Rᵣ = 1/(A·Fᵢⱼ).

Radiation Exchange with Gases

CO₂ and H₂O emit and absorb at discrete bands. The effective gas emissivity according to Hottel:

ε_g = ε_CO₂ + ε_H₂O − Δε

where Δε is the correction for overlapping absorption bands. The mean beam length for a cavity is Lₘ = 3.6 V/A.

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