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Addressing Fire Risk in Biomass Storage.

Hay producers have long been aware of the risks of storing this combustible material. Images of barn fires caused by tight stacks of high-moisture bales are always in the back of their minds. So, as cellulosic biofuel demonstration projects were conducted over the past decade, great care was taken to develop biomass storage procedures to avoid spontaneous combustion. This effort was highly successful because, to our knowledge, no fires in herbaceous feedstocks (such as corn stover or switchgrass) have been attributed to spontaneous combustion. However, fire remains a significant risk for facilities that handle and store biomass. Nearly every company with large volumes of corn stover or switchgrass in storage has experienced a major fire. These fires are especially troubling because the most common ignition sources--lightning and arson--are essentially unavoidable.

Fears of a lightning strike or an arsonist's torch weigh heavily on feedstock procurement managers. Within minutes, a small fire can grow to engulf dozens of bales. Because storage sites are typically not staffed, fires can become unmanageable before they are detected and firefighters are notified. Such fires can become so intense that firefighters are unable to extinguish them and can only work to protect the surrounding property. Smoldering fires take months to fully die and must be monitored closely during that time. In addition to the safety, cost, and production risks, large fires can also severely erode community support for a cellulosic biorefinery.

An obvious solution to reducing fire risk, and the basis for existing codes, is to decrease the stack size and space the stacks farther apart. However, this approach is often cost-prohibitive because it significantly increases the footprint of the storage site. Furthermore, fires can spread by ember transmission, even in mildly windy conditions, across distances of 200 ft or more. This means that distance alone is not enough to protect nearby stacks from a fire.

Recent research at Oak Ridge National Laboratory in collaboration with DuPont, Iowa State University, and Underwriters Laboratories (UL), sponsored by the Department of Energy's Bioenergy Technologies Office, has focused on understanding the dynamics of fires in biomass stacks and the development of safer storage designs. We conducted experiments in a large calorimeter at UL to observe how a fire grows and spreads through a stack of corn stover bales.

For the first test, we ignited a stack of 12 rectangular bales of corn stover (bale size: 4 ft x 3 ft x 8 ft; stack size: 2 bales wide x 3 bales long x 2 bales high) along one side. As shown in the accompanying photos, the fire spread very quickly through the vertical channels between the bales and along the loose bale ends. Hypothesizing that the "chimney effect" of the vertical channels between the bales was contributing significantly to the fire growth, we designed a second experiment to test the effect of physical barriers across the vertical channels. We used cement boards for the barriers. The results were striking. Adding cement boards to block the vertical channels within the bale stack reduced the time needed to extinguish the fire by 50% and reduced the water required by 75%.

The next step was to develop a strategy to add physical barriers in large, outdoor stacks of corn stover bales. While the cement boards were highly effective in the indoor test fire, they are not feasible in the field at industrial scale due to the cost of the boards and the time and labor required to install and remove them. Instead, we conducted an outdoor test fire at an Iowa State University research center to determine if offsetting the stacked bales could effectively block the vertical channels. In this test, a vertical channel three bales high was blocked with another bale, which significantly slowed the fire growth. Follow-up testing in the calorimeter at UL confirmed that the bales themselves are effective at reducing the chimney effect, although not indefinitely. The heat release rate from this test was very similar to the test with cement boards for the first hour. After one hour, the fire had burned away enough of the bales to allow the fire to grow more rapidly. Still, an hour of fire suppression gives firefighters more time to mount a strong attack before the fire becomes unmanageable, and it dramatically reduces the amount of water required.

Based on the experiments conducted at UL and Iowa State, DuPont developed a new stack design for the 2017 stover harvest. In the new design, the fourth and sixth rows are offset to break the vertical channels. While we hope for no more fires, we are optimistic that stacking in this configuration would make such a fire more manageable.

Another positive outcome of this project is improved awareness of firefighter safety in responding to biomass bale fires. The indoor fire tests highlighted the instability of industrial-scale bale stacks during a fire. During past fires, firefighters have been known to climb on top of a stack, which can be up to 21 ft tall, to attack the fire from above. In our tests, we observed that outer bale sections, often weighing several hundred pounds, began to fall as the binder twine burned away. This poses a danger to firefighters on top of the stack as well as to firefighters standing near the stack. Work is underway to develop best practices for a collapse zone around a bale stack to protect firefighters.

ASABE member Erin Webb, Senior Research and Development Staff Member, Environmental Sciences Division, Oak Ridge National Laboratory, and Joint Associate Professor, Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, USA,

Jeff Chambers, Technical Staff Member, Fuels and Engines Research Group, Oak Ridge National Laboratory, Oak Ridge, Tenn., USA,

Keith Webster, Program Coordinator, Department of Agricultural and Biological Engineering, Iowa State University, Ames, USA,

Caption: Stacks of corn stover bales engulfed in flames.

Caption: Adding cement boards between rows of stover bales eliminates the chimney effect caused by vertical channels between bales and significantly slows fire growth. These photos show test fires with no cement boards (above) and with cement boards (below) approximately one hour after ignition.

Caption: In August 2017, a fire started by an arsonist destroyed 40,000 bales of corn stover in rural Iowa. Photo by Keith Webster, Iowa State University.

Caption: Following the tests showing that blocking the vertical channels slows fire growth, a new stack design was developed in the fall of 2017. The image on the left shows conventional stacking with vertical channels extending from top to bottom. The image on the right shows the new offset stacking with vertical channels blocked at rows 4 and 6.

Caption: The new stack design for corn stover bales implemented by DuPont in the fall of 2017. The red arrows show that offset stacking eliminates top-to-bottom channels to minimize the chimney effect and slow the growth of fires, should they occur.
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Author:Webb, Erin; Chambers, Jeff; Webster, Keith
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Jul 1, 2018
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