The central region of the heap is then allowed to dry from 50 % moisture content in the spring to approximately 25 % by August. After 3 months of storage, a crust develops on the external surface which is believed to protect the remainder of the heap by allowing rainwater to run off. Open air heaps are often built to reach considerable dimensions in order to optimise the core to surface ratio. The process of piling wood chips in heaps is believed to lead to a redistribution of the moisture within the biomass, resulting in a wet outer surface and much drier inner part. Heaped storage also assists the logistics of the supply chain by buffering supply and enabling the movement of wood on demand so that, for example, the discrepancies between the supply window and heat demand during winter can be addressed. These can be placed under cover in open barns, or under tarpaulin or fleece sheets, though uncovered outside storage is cheaper and also benefits from direct sunlight and aeration in favourable drying conditions during April and May so that the chips can dry by natural ventilation. A low-cost solution is to pile the wood chips into heaps. ![]() ![]() Forced-drying is one option but can be a costly process and requires access to facilities. The conventional harvesting window of willow is in the early spring therefore, during this time large quantities of biomass are cut, which must be dried and stored until required. In wood chip biomass supply chains, trees are harvested at a moisture content of around 50 %, and although it is possible to utilise wood chips with up to 65 % moisture content in modified furnaces or gasifiers, it is beneficial to dry the material to increase its net calorific value and to reduce the quantity of water transported in the biomass. Additionally, 40 Mtoe of forest biomass are envisaged to be available to biomass energy systems by 2020 without compromising environmental criteria. By 2020, 19.3 million ha of agricultural land could be diverted to dedicated bioenergy production, providing 100 Mtoe, while complying with good agricultural practice and without affecting domestic food production. Solid biomass, particularly wood and wood waste, is currently the biggest source of renewable energy in the EU and is expected to make a key contribution to meeting the EU’s 20 % renewable energy target by 2020. Further research is required to detect whether there are fugitive emissions of CH 4 from wood chip heaps, as this will compromise the net GHG savings from utilising the biomass stored in this way.īiomass is considered to play an important long-term role in reducing future global greenhouse gas (GHG) emissions, and since the implementation of the Renewable Energy Directive (RED) in 2009, there has been growth in the uptake of renewable energy in Europe. It is necessary to explore other methods of harvesting SRC to minimise losses and optimise land use efficiency. The results from this study suggest that outside wood chip storage is not an efficient method of storing biomass, though this may be location-specific as there are some studies showing lower dry matter losses. In both instances, the peak CH 4 concentration occurred as CO 2 concentrations dropped, suggesting that after an active period of aerobic decomposition in the first 2 months of storage, the conditions in the heap became anaerobic. A peak in methane (CH 4) concentration was also detected in both heaps, though more noticeably in the East Midlands heap after around 55 days. After establishment of the heaps, the temperature rose rapidly and this correlated with a peak in carbon dioxide (CO 2) concentration within the heap. The process of mixing the heap, however, led to incorporation of wet outer layers and hence the average moisture content was higher in an average sample of chip. ![]() The results from analysing the whole Rothamsted heap indicated an overall loss of 1.5 GJ per tonne stored, although measurements from bagged samples in the core suggested that the chips dried sufficiently to have a minimal energy loss from storage. During this time, the wood chips dried from 54 to 39 % moisture content in the East Midlands heap and 50 to 43 % at Rothamsted. The Rothamsted heap showed a whole-heap dry matter loss of 21 %. The bagged samples showed dry matter losses of 18 and 19 % in the East Midlands and Rothamsted heaps after 210 and 97 days storage, respectively. A series of 1- and 3-m probes were embedded in the heaps in order to retrieve gas samples for analysis, and pre-weighed net bags were positioned in the core of the heap to detect dry matter losses. One heap was built on a grassland area (East Midlands) and the other (Rothamsted) on a concrete hard standing. This study examined the dry matter losses and the greenhouse gas (GHG) concentrations within two short rotation coppice (SRC) willow wood chip storage heaps.
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