An explosive issue

Geof Brazier and Mitch Rooker, members of the US-based NFPA committee on explosion venting and protection systems discuss coal dust explosion risks in power plants.

In February 2009, a silo at a coal-fired power plant exploded, severely injuring six workers and resulting in US$300,000 in fines. This recent power plant explosion, located in the Midwest United States, serves as another dangerous reminder of the risks faced by power plants that handle combustible coal dust.

Coal handling, processing and storage systems can produce hazardous conditions with the potential to produce a dust explosion. Explosions occur as a result of ignition of a combustible material (dust, gas, or vapour) when mixed with oxygen, typically that which is present in the air. When this takes place inside a confined enclosure, such as a dust collector or a conveyor gallery at a power plant, a rapid pressure rise is developed in addition to the expected flame of combustion. This pressure could develop to over 100psig in a fraction of a second if the dust explosion is not mitigated in some way, exerting destructive forces within a few milliseconds that will place both personnel and equipment at risk. Examples of potential ignition sources within the coal systems of a power plant are:
• tramp metal producing sparks within milling, grinding and conveying systems
• pyrites producing impact sparks
• static electricity
• hot surfaces
• smouldering fire nests which can self-ignite at relatively low temperatures, as low as 160°C, compared to a dust cloud self ignition temperature as low as 440°C
• easily ignited pockets of “coal gas” that can generate a secondary dust explosion.

In this recent example of a combustible dust explosion within a power plant, the event occurred at a silo used to collect fugitive before the dust is then used as fuel. Contract workers were onsite setting up scaffolding on the outside of the silo when the explosion occurred. Flames, embers and dust rained down on the workers causing severe burns which required hospitalisation.

By mitigating the impact of an explosion, the pressure generated remains below a safe level for the process equipment (see blue line). The higher the Kst value, the faster the rate of pressure rise due to combustion. Coal dust has a fairly wide explosive band depending on the type and moisture content, which is why it is important to test for explosibility. There are more than 100 published Kst values for coal ranging from 37 to 176 recorded. A Kst value of 138 will generate a pressure in excess of 100psi within an enclosed volume in less than 100 milliseconds.

Measuring explosion hazards

Three parameters are used to assess the reactivity of coal dust and form the heart of the risk assessment process that leads to the design and specification of appropriate safety measures:

1. Pmax – Were a coal dust explosion allowed to fully develop within an enclosure of great strength, a peak pressure of up to 133psig for bituminous coal would be reached. Different types of coal and variation in particle size result in different values for Pmax. However, the figure is consistently above 100psig which is far above the design pressure of most coal handling and storage equipment as well as most processing equipment.

2. Kst – A factor determined by measurement of rate of pressure rise for a combustible dust sample, the Kst value for a given material indicates the power potential of the combustion event. Coal dust Kst values range between 80 and 130bar-m/sec for bituminous coal and can exceed 200bar-m/sec for Powder River basic coal.

3. MIE – The Minimum Ignition Energy required to ignite a cloud of coal dust is as low as 30mJ. This is sufficiently low for an inadvertent electrostatic discharge to start a coal dust explosion. While an elevated moisture content will increase the amount of energy required to trigger combustion by around 10-fold, this is still only 300mJ.

Other factors which are important include process information such as dust concentration, airflow velocity, operating pressure, temperature and humidity.

In the US, under National Fire Protection Association (NFPA) 68-2007 Standard, dust sample testing is required to correctly define the combustion risk. The concern is that assumed reference data may result in decisions that leave a facility under protected. Under-protection can lead to consequences as severe as no protection. Often actual dust sample testing indicates lower Kst and Pmax values than reference data. This supports a lower level of protection resulting in cost savings to the plant owner/operator. Particle size can profoundly affect explosibility of combustible dusts; the finer the dust, the higher the Kst value.