What Makes Stored Biomass Self-Ignite?
Many power producers are using biomass as fuel for their boilers. Biomass comes in many forms, including corn stover, olive pulp, bagasse, and wood pellets.
Provided the raw materials are sustainably sourced, biomass fuels offer a low-carbon alternative to fossil fuels such as coal and natural gas. However, these fuels present some problems that do not apply to more traditional fuels, and self-heating and spontaneous combustion are among the most problematic.
The problems arise from two exothermic processes: oxidation and biological action. Oxidation occurs when volatile components in the biomass react with oxygen in the ambient air, generating carbon monoxide and heat. The rate of oxidation depends strongly on temperature: as a general rule, it doubles for every 10 °C increase in temperature. Because of this, a runaway condition can develop where spontaneous heating causes an increase in the oxidation rate, leading to more rapid heat production. If this continues for long enough, the fuel can eventually become hot enough that it catches fire.
A second mechanism involved biological action by fungi and bacteria. In these cases, the biomass is converted to carbon dioxide and methane. As the fungi and bacteria cause the temperature to increase, oxidation will also play a part and, again, the stored fuel can become hot enough that it can self-ignite.
Several factors contribute to the heat buildup. The size of the storage pile is significant as the heat generated can escape more easily from a small pile. Moisture content determines the likelihood of biological action, since dry biomass is not a hospitable environment for bacteria and fungi. The age of the material and its origin also play a part since freshly chopped wood contains a higher proportion of volatile, reactive material than wood that has been in storage for some time.
In addition to the fire risk, there are direct costs from these reactions, since they consume the raw material and reduce the amount that remains to burn in the boiler. Studies by the US Forrest Products Laboratory have shown stored wood chips lose between 0.5% and 4% of their mass per month.
Fortunately, AMETEK Land has a range of tools that can detect these reactions and give early warning that a dangerous condition could be developing. The LWIR-640 thermal imager can detect the temperature rise in open storage piles and in material stored under a dome. The Silowatch analyzer detects the carbon monoxide emitted when biomass oxidizes. And the HotSpotIR is specifically designed to detect hot material moving on a conveyor belt, so that it can be rejected before it moves into the next stage in its journey.
All biomass is susceptible to self-heating and spontaneous combustion, but careful management and accurate monitoring can minimize the dangers, allowing it to be stored and transported safely.
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