Page 219 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

Chapter 7
Combustion Process and Air Emission
Formation

The combustion thermochemistry introduced in Chap. 3 applies to only simple
cases. In engineering practice, fuels and oxidizers are seldom premixed to avoid
explosion. Instead, they are often delivered separately into the combustion chamber
and then mixed immediately prior to combustion. The combustion takes place in a
flame rather than the entire combustion system. As a result, the physical processes
as well as the fuel properties govern actual fuel combustion. Fluid mechanics,
thermodynamics, and heat and mass transfer govern the fuel-oxidizer mixing, the
combustion process, and air emissions.
In the process of combustion, the chemical energy of fuel is converted into
thermal energy. There are a variety of fuels used in combustion for energy pro-
duction in different engineering applications. The fuels can be solid, liquid, gaseous,
or their mixtures. Each of them also has its own family. Most commonly used solid
fuels include coal, charcoal, coke, and biomass; liquid ones include gasoline, diesel,
and recently, bioethanol and biodiesel. Natural gas is the most widely used gaseous
fuel which contains primarily methane (CH 4 ) and other trace gases.
Depending on the fuel properties the flue gases may contain high concentrations
of the oxides of sulphur and nitrogen (SO x , and NO x ), fine particulates, and trace
elements like mercury. The actual combustion processes and their air emission
formation mechanisms are introduced in this chapter.

7.1 Gaseous Fuel Flame

Gaseous fuels and oxidizers enter a stationary combustion device separately and
combustion takes place in a diffusion flame (Fig. 7.1). The combustion sustains by the
heat released from combustion. Mixing ratio is described using the overall equiva-
lence ratio introduced in Sect. 3.2. However, this value is not uniform everywhere
and the local equivalence ration varies from 0 to 1, representing zones of pure
oxidizer or pure fuel. When the gaseous fuel is injected into the combustion chamber,
there is a central core area containing pure fuel. The surrounding oxidizer and outer

© Springer Science+Business Media Singapore 2014 195
Z. Tan, Air Pollution and Greenhouse Gases, Green Energy and Technology,
DOI 10.1007/978-981-287-212-8_7