A review of mechanisms responsible for changes to stored woody biomass fuels

Abstract Large scale bioenergy facilities require vast amounts of biomass materials and take advantage of a variety of woody materials in various forms including logs, hog fuel, bark, forest harvest residue, short-rotation hardwoods and whole tree chips. Development of the supply chain logistics necessary to deliver and utilize these material in a cost effective manner is well underway but is strongly dependent on forest type, regional and local harvesting practices as well as location, size and design of storage facilities available. Storage of woody biomass is necessary at various points along the supply chain but the effect of storage on woody biomass is complex and not fully understood. The key mechanisms responsible for major changes to woody biomass on storage are (i) living cell respiration, (ii) biological degradation, and (iii) thermo-chemical oxidative reaction. All three mechanisms involve mass to energy conversion and contribute to self-heating of piles and dry matter losses. Living cell respiration is a short term effect that lasts only several weeks while starch and sugar are readily available and adequate temperature and oxygen levels are present. Biological degradation is caused by a large variety of organisms from bacteria to wood degrading fungi and function best under specific moisture, temperature and oxygen conditions. Finally, thermo-chemical oxidative reactions can contribute to excessive dry matter loss once elevated temperatures have been attained in the pile as a consequence of the first two mechanisms. This review paper discusses the science behind the mechanisms of change to biomass on storage, and draws examples from experimental research to support the explanations.