Safety Implications of Coal and Biomass Fuel Mixes

Practically everyone would agree that the energy policy of the U.S. is in a great state of flux. Not since the introduction of commercial nuclear power some five decades ago has our country come to such an energy crossroads. No matter what your political ideology, no one can refute that conventional coal-fired power plants are being paralyzed by recent and potential U.S. Environmental Protection Agency (EPA) regulations designed to cut the nation’s reliance on coal.

The agency appears to be making a systematic assault on coal as a domestic energy source. Brandon Bell, PE commented in the April 2012 issue of POWER (“Has Boiler MACT Improved the Future for Biomass Power?”) that recent regulatory actions by the EPA may “cause 40 GW to 50 GW worth of coal plant closures.”

Concurrent with the assault on coal, increased numbers of pundits are advocating the use of alternative fuels. A lot of well-meaning people believe that burning wood pellets is as simple as leaving the coal in the ground and cranking up the chipper. In and of itself, this is a noble thought worthy of research and investment. But the cold hard reality is that, for the foreseeable future, the focus most likely will be on renewable biomass sources that can be effectively blended (cofired) with coal in existing or reasonably modified coal-fired units with minimal capital outlay.

Biomass is effectively carbon neutral: The amount of carbon released during combustion is equal to the carbon absorbed during plant growth. There are numerous examples of operationally successful test cofiring of biomass and coal on an experimental scale. But although the future of biomass as a viable full-scale alternative to coal is still uncertain, bringing biomass material on site and running it through traditional infrastructure designed for coal can create unanticipated hazardous conditions. Whether that blending takes place in the fuel yard or at the burners, commingling of fuels can trigger severe safety implications that must be taken into account.

Typical Biomass Candidates

Biomass is simply plant material, or even animal waste, that can be used as a source of energy. Biomass is nothing new; in fact, coal itself is a form of ancient biomass. Table 1 lists the five categories of biomass that can ultimately take the form of solid or liquid fuels. The discussion here centers on solid biomass.

Table 1. Five sources of biomass. Some of the common forms of solid biomass that have been tested are wood pellets, saw dust, pelletized grain, grain screenings, wheat shorts, and pelletized agri-waste. Source: The Utility FPE Group Inc.

Some of the more common biomass candidates that have been tested are wood pellets, saw dust, pelletized grain, grain screenings, wheat shorts, and pelletized agri-waste.

The Hazards of Biomass

With the introduction of biomass to the fuel mix, a common assumption has been that the explositivity of coal dust would be minimized. This could not be further from the truth. Biomass allows greater fuel flexibility, but it does not reduce the risk of fire and explosion. Extensive research in 1999 by the Technical Research Center of Finland confirmed that biomass and coal-biomass mixtures are extremely hazardous because of the combustibility of the dust, its propensity for spontaneous combustion, and the reactivity of fuel mixtures.

Combustible Dust. It’s a fact that coal movement generates fugitive dust. Whether coal is moving along on a conveyor, passing through a transfer point, or being discharged into or exiting a container, dust is produced. The main hazard introduced with biomass is that most biomass materials also produce combustible dust as they flow though the handling system. Adding combustible biomass dust to combustible coal dust can greatly increase an already hazardous condition. (For more on fugitive dust issues, see “Dusty Trail: The Movie,” p. 68.)

A value called the deflagration index, denoted as Kst, is assigned to all combustible dusts based on standard ASTM testing protocol. The higher the Kst value, in essence, the bigger the explosion. Table 2 provides some indication of relative Kst values for coal and biomass.

Table 2. Explosion propensities of coal and biomass material. A value called the deflagration index, denoted as Kst, is assigned to all combustible dusts based on standard ASTM testing protocol. The higher the Kst value, the bigger the explosion to be expected. Source: The Utility FPE Group Inc.

Spontaneous Combustion. A second hazard is that of spontaneous combustion. Well known for its contributions to fires and explosions in coal plants, spontaneous combustion can be just as prevalent with most biomass materials. Heat production in biomass typically begins as an aerobic process. Then, as the temperature rises, the chemical chain reaction takes over. Self-ignition has been found to correlate with silo/bunker volume: The larger the volume of the silo, the lower the self-ignition temperature of the biomass.

Duality of Fuel Mixes. A third hazard arises from blending coal and biomass products. Significant information is available regarding the reactive and explosive hazards of various ranks of coals. However, there has been very little research into the same characteristics of biomass and coal-biomass blends. The Technical Research Center of Finland found that coal-biomass mixtures are less reactive than just coal alone but more reactive than biomass alone. This means that blending coal and biomass in the same silo will contribute to higher levels of reactivity than if the silo contained just biomass.

Incidents of Fires and Explosions

On February 12, 2012, a fire broke out at the biomass-fired 750-MW Tilbury power station, which is owned by RWE npower, a UK energy company (Figure 1). The fired raged for two days. More than 120 firefighters were deployed.

1. In better times. The coal-fired four-unit 1,131-MW Tilbury Power Station began operating in the late 1960s. Last February, significant damage to portions of the fuel-handling portion of the plant occurred when a fire broke out in two silos containing wood pellets. The units had recently been converted to burn biomass. Courtesy: RWE npower

This late-1960s plant had recently been converted from coal to biomass and was billed as a pioneer in its use of biomass technology. The fire involved 4,000 tons of wood pellets in two silos. The plant, which ended its commissioning period on February 6, was set to begin commercial operations. Before the fire, the plant was intended to be the largest coal-to-biomass conversion in Europe, using 2 million tons of wood pellets per year.

The blaze at the Tilbury plant followed a raging fire in October 2011 at a 100,000–cubic meter wood pellet storage facility at the Port of Tyne in the north of England. Other biomass-related fires and explosions include the following incidents:

  • In June 2009 an explosion at the University of South Carolina’s wood-burning boiler caused a “potentially lethal accident.” That explosion followed two previous smaller explosions and a series of mechanical breakdowns.
  • In June 2011 a dust explosion crippled a biomass wood pellet production facility near Waycross, Ga. Production was down for approximately one month.
  • On July 15, 2009, a large stack of discarded dock piers burned at the chipper for a power plant located in Clarion County, Pa. Crews were on scene all day and into the night. Every fire department in the area had representatives and apparatus there to help fight the large fire.
  • On February 5, 2010, a biomass plant in Brilon, Germany, exploded, killing three workers.

What Does All This Mean?

The numerous instances of fires and explosions at biomass-related facilities prove that biomass is a fuel to be reckoned with. The potential for devastating impacts on lives, property, and electrical reliability does not diminish the potential advantages of using biomass. However, due to the lack of research on biomass and coal-biomass mixtures, many facilities are treating biomass as if it were some inert fuel being added to the mix. Using 30-year-old silos and material-handling systems to move biomass materials whose properties we do not yet fully understand is like putting the horse before the cart. The resulting fires and explosions are proof that this is a serious safety problem that needs to be dealt with.

—Contributed by Edward B. Douberly, Jr. (edouberly@ufpeg.com), founder and president of The Utility FPE Group Inc., a U.S.-based consultancy offering risk engineering services to coal and biomass plants around the world.