Agriculture Reference
In-Depth Information
7.1
Introduction
Biomass storage occurs after harvest and before delivery to the conversion plant.
Biomass is harvested during a very short window and the harvested biomass must
support biorefinery operations year round. Storage can provide the necessary buffer
to ensure continuous provision of biomass; therefore, it is a necessary component of
the biomass feedstock production and provision system. Storage of biomass, how-
ever, presents a number of challenges that must be addressed as part of an efficient
storage solution. These challenges are:
• Volume: Lignocellulosic biomass is a low-value, high-volume product. The typi-
cal density of a bale from commercially available balers is about 150-200 kg m −3 .
The low density leads to a substantial storage volume requirement for a biorefin-
ery of reasonable size. For example, model-based analysis by Shastri et al. [ 1 ]
showed that a Miscanthus-based biorefinery processing about 2,800 Mg d −1 pro-
ducing about 350 million liters of ethanol per year would require a covered stor-
age area of more than 800,000 m 2 with a height of 4.88 m and an open storage
area of almost 20,000 m 2 with a height of 2.44 m.
• Dry matter loss: Storage of biomass for longer durations can result in significant
dry matter loss depending on the storage method. Losses as high as 25-30 %
have been reported for open storage without any protection. Such high losses
negatively impact the cost-competitiveness. As elaborated later in the chapter,
the loss of carbohydrates as part of the dry matter loss may be even more impor-
tant from the ethanol production standpoint. Moreover, Emery and Mosier [ 2 ]
showed that dry matter losses during storage reduced the net greenhouse gas
benefit of ethanol over gasoline by 10.9 %.
• Safety: Long-term storage of high-moisture biomass may lead to safety hazards.
Microbial activity in stored biomass increases the biomass temperature and
may lead to self-ignition. The emissions from storage piles can also create
health hazards.
Minimizing the total dry matter loss is the primary objective of efficient storage.
The dry matter loss is often caused by wind and rain erosion, leaching, high tem-
peratures, and handling activities such as loading and unloading. Biochemical activ-
ity also causes losses in cellulose-containing dry matter [ 3 ]. It is important to
understand the various factors affecting the dry matter loss and also the potential
solutions to minimize those losses.
The purpose of storage is to also deliver biomass feedstock that has lost the least
amount of cellulose and hemicellulose. The environmental conditions to which the
feedstock is exposed after its harvest determine its quality and suitability for a par-
ticular end use [ 4 ]. Biomass quality relevant to its conversion to ethanol is defined
primarily by its total dry matter content relative to when it was harvested, pH,
enzyme and yeast-relevant nutrient content, and its resistance to cutting and
crushing due to lignin and moisture content. Various biochemical and chemi-
cal reactions occur during storage, which influence these biomass properties.
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