Page 647 - Dust Explosions in the Process Industries

6 14 Dust Explosions in the Process Industries

appropriate means to prevent accidental ignition of explosive dust clouds in industry.
Schacke, Viard, and Walther (1996) proposed some basic concepts to facilitate dust
explosionprevention and control when designing and running a chemicalprocess plant.
Identifying, knowing, and eliminating possible ignition sources is of prime concern.
Maximum benefit is obtained if this concern is taken into account from the early design
phase, during which it can also be decided whether further measures, such as inerting
and venting, are required. Klais and Niemitz (1996) addressed the specialproblems aris-
ing, and the precautions to be taken, when semi-stable chemical substances capable of
reacting or decomposing exothermally, even in the absence of oxygen from the air, are
exposed to thermal or mechanical loads in drying and grinding operations. Substances
that may show this behavior include azide, azo, and di-azo compounds;hydrazine; per-
oxides; nitro and nitroso compounds; epoxides; and some nitrates.

9.3.5.2
Self-Heating, Spontaneous Combustion, and Smoldering Combustion
in Dust Layers and Deposits

Some research work up to 1990 is discussed in Section 5.2 in Chapter 5. Test methods
are reviewed in Section 7.7 in Chapter 7.
Gibson (1993a) gave a summary of methods for preventing ignition of powders and
dusts in drying operations. Zockoll(1994a, 1996a, 1996b) described the development
of a new system for early detection of self-heating and self-ignition in deposits of
organic powders in spray dryers, based on detection of low concentrations of CO, in the
1-10 ppm range, with particular reference to drying plants for milk powder production.
It is important that the system supplier and the user work closely together to ensure an
optimal systemdesign for any given application. Loebel(l996) described the design and
installation of, and first experiencewith, a smolderingcoal fire detection system, based on
a semiconductor gas sensor. A multisensor system for detection of gases developed by
slow smoldering fires in lignite was described by Kohl and Kelleter (1996). H, and CO
were monitored continuously, using electrochemical sensors, and it was found that the
ratio of the measured concentrations of the two gases could provide information about
the state of development of the smoldering fire.
Zeeuwen (1999) outlined how critical conditions for self-heatingin stored bulk pow-
ders in industrial situations can be determined by employing suitable laboratory-scale
test methods.
Carson (1996) presented a systematic approach for characterizing the tendency of
combustible dusts to self-heat and start burning spontaneously, comprising various
laboratory-scale tests and mathematical models. The experimental methods included
differential thermal analysis (DTA),thermogravimetricanalysis (TGA), isothermaloven
tests, and adiabatic calorimetry. By means of this methodology, the critical conditions
at which the actual dust begins to pose a threat of spontaneouscombustion can be deter-
mined. Matyukhina (1996) studied the self-heating and onset of spontaneous combus-
tion in coal deposits, taking into account both air penetration due to the pressure exerted
by strong winds and the kinetics of slow coal consumption.
Krause and Schmidt (1997)reported experimental studies of critical thermal conditions
for the onset and sustained propagation of smoldering processes. Particularattention was
paid to the critical initial temperature of an embedded hot solid body and the critical size