Page 221 - Advances In Productive, Safe, and Responsible Coal Mining

200 Advances in Productive, Safe, and Responsible Coal Mining

11.2 DPM characteristics

DPM is a general term used for submicrometer aerosols, which are emitted by diesel
engines as a product of incomplete combustion. The US Code of Federal Regulations
[15] (30 CFR Parts 7, 36, and 72) describes the composition of diesel engine exhaust as a
complex mixture of several compounds, which contains both particulate and gaseous
fractions. In any mine, the exact composition of diesel exhaust is variable. The physical,
toxicological, and chemical properties of DPM are controlled by the type/design of
engine, engine life, engine maintenance, engine tuning, equipment operator, type of fuel
used, load cycle, and exhaust after-treatment devices. In addition to these factors, dif-
ferent environmental settings in which diesel engines are used also affect the gaseous
and particulate matter composition of diesel engine exhaust. In a DPM mixture, the gas-
eous concentrations of diesel exhaust are oxides of nitrogen, carbon, and sulfur; alkenes
and alkanes; aldehydes; and monocyclic and polycyclic aromatic hydrocarbons;
whereas particulate constituents are diesel soot and other solid aerosols, including
metallic abrasion particles, ash particulates, silicates, and sulfates [16].
The major particulate fraction of diesel exhaust consists of very tiny individual par-
ticles with a solid elemental carbon (EC) core that absorbs many toxic substances. In
general, DPM is primarily composed of an EC core and other organic and inorganic
aerosols [17]. EC core particles are slowly covered by a thin layer of volatile material
[18–20]. The organic carbon (OC) fraction of DPM forms different compounds with OC
differing from EC because it is composed of volatile and semivolatile organic material.
>1800 different organic compounds have been known to adsorb on an EC core. A part
of these organic fractions result from incomplete fuel combustion in the diesel engine
and are formed when lubricating oil is not completely oxidized during the process of
combustion [21]. Diesel particles also contain a fraction of nonorganic absorbed com-
pounds [16]. The process of formation of EC during combustion and expulsion is mainly
governed by temperature, oxidant availability, and residence time [22].
In terms of size, aerosols contributed to the environment by diesel engines are typ-
ically polydispersed and log normally distributed in one, two, or even three distinctive
modes: [18,23] (a) nucleation mode (3–30nm), (b) accumulation mode (30–500nm),
and (c) coarse mode (>500nm). The approximate geometric mean diameters are
given in parentheses, but all of these modes are more appropriately defined by their
distinct nature than by their fixed-size boundaries [18]. The residence time of diesel
aerosols in the atmosphere depends on the size and concentrations of other particles in
the air. Aerosols between 100-nm and 10-μm have the longest residence time, which is
typically about 1week [18]. The typical residence time for 10-nm particles is about
15min. These particles primarily coagulate with larger particles from accumulation,
coarse modes, and dust [17].

11.3 DPM health effects

Health concerns related to DPM are a relatively new concept in mining, beginning in
the late 1970s to early 1980s [24–27]. As per MSHA, all diesel aerosols can be clas-
sified as respirable aerosols [17]. Particle size and distribution has a major impact on