Whilst concerns of energy security, climate change and the need to reduce greenhouse gas emissions, the demand and use of biomass for energy generation is on the rise. To meet this demand, biomass needs to be stored for extended periods of time in the form of pellets or wood chips. The question that poses itself here is what are the obstacles that need to be beat to safely and efficiently store such pellets or wood chips.
Gases Emitted During Biomass Storage
Recent research studies have revealed that during biomass storage, a substantial amount of gaseous compounds are emitted, which may pose environmental, health and safety risks to storage facilities and the workers handling the biomass. The typical gases produced from stored woody biomasses are carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and other volatile hydrocarbons, which include greenhouse gases with a high global warming potential. Additionally, when CO, CO2, and CH4 concentrations increase (together with oxygen depletion), serious accidents may occur.
In conjunction with complex physiochemical reactions, the temperature of stored biomasses can increase and result in a spontaneous ignition. While the actual mechanism for ignition is unknown, it is likely as a result of spontaneous heating due to bacterial activity. Although this process is difficult to model, it is safe to say that eliminating microbial action inside the storage pile would also eliminate the risk of fires due to spontaneous ignition.
Lastly, gases released from stored biomass may produce harmful odors and induce asthma-like respiratory tract symptoms.
Technologies Used To Prolong & Improve Storage Quality
In enclosed and confined spaces, where gases are emitted and oxygen is depleted, there is a high risk of biomass degradation, as well as, the auto-ignition of the storage pile. Research shows that biomass densification technologies, such as grinding, briquetting (cubing), and pelletization, might have lower risks associated with ignition.
During pelletization, a significant amount of water is removed from the biomass, which in turn reduces bacterial activity- the main pathway that initiates the emission of harmful gases. Therefore, densifying and dehydrating biomass might reduce some of these safety concerns and also maintain the quality of the biomass for a longer period of time.
Why Smaller Is Better
Large-scale wood chip storage should be avoided as this significantly promotes microbial growth and propagation. Heat accumulation in a storage pile is proportional to the heat capacity of the pile. Concurrently, the heat capacity is also proportional to the pile radius to the third power (volume), while heat removal is proportional to the pile radius to the second power (surface area). Therefore, the surface area to volume ratio of storage piles will need to be optimized to balance the effect of heat removal vs. heat accumulation.
Overcoming the Environmental Risks
The most significant greenhouse gases emitted from biomass storage are CO2 and CH4. The following suggestions could be implemented to mitigate some of these risks and create safer storage facilities and working environments.
A simple way to minimize environmental impact of greenhouse gas emissions from biomass storage production is to reduce the storage time and locate the storage sites near the production sites. This will also minimize greenhouse gas emissions associated with transportation and reduce the overall environmental impact.
There is a major need to build safer and more efficient biomass storage facilities, as well as, effective transportation strategies. Exhaustive and systematic scientific studies on the degradation processes and their effects are needed in order to fully understand and minimize risks of large-scale wood chip biomass storage on the environment, land and, ultimately, human health.
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