to read the full report - Ecolateral by Peter Jones

54
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
Currently the majority of these residues are recycled to agricultural land. The high water content makes
large scale transportation unfeasible, and so utilisation for energy or fuel generation would likely be at a
local scale.
Dry Agricultural Residues
There has been much interest in the use of dry agricultural residues for energy for some time. The UK
Government (then Department of Energy) supported a significant programme in the 1980s-early 1990s to
examine the potential and support demonstration projects. Two main dry agricultural residues were
identified as: poultry litter and straw.
Straw
Straw is available from cereal and other 'combinable' crops such as oil seeds. The recent report by CSL
for NNFCC gives a broad description of the straw market. 78 It is produced seasonally and is localised to
arable farming areas. Straw is produced during crop harvest and often remains on the ground after the
passage of stage one collection – the seed crop. It must then be collected and baled in a second
handling stage. For an estimation of regional crop area potentially generating straw, by crop type and
region of the UK please see Appendix 1.
The three main types of straw that are available in the UK are barley, wheat and oil seed rape (OSR). It
is common for barley and oat straw to be consumed almost entirely by the livestock industry as part of the
winter diet with additional straw used for bedding, so this must be borne in mind when considering the
following figures. Cereal straw is also used to protect some crops, such as carrots, against frost and to
keep others, such as strawberries, off the ground. OSR straw is not suitable for either of these uses.
Other potential industrial uses for straw are for paper-making, or in the production of industrial fibre,
including constructional board manufacture. 79 Straw can also be used as a feedstock for power
generation, as is the case at the EPRL Power station in Ely (see later chapter). OSR straw is popular for
combustion, as it burns fiercely due to the presence of residual oilseeds. To moderate this, such straw is
mixed with cereal straws to contain this volatility at lower levels.
To prevent deterioration during storage, crops are generally harvested dry, i.e. moisture content below
15% and, as straw is usually harvested immediately after harvesting the associated cereal or oilseed
crop, its moisture content will be around the same. During poor weather, however, the crops may be
harvested at higher moisture contents, meaning the straw will be wetter, or the straw may spoil in the
field, reducing that year’s yield.
Straw is harvested over a short period (typically around 70 days), and for the remainder of the year
storage is required. This storage may be done in Dutch barns or in stacks on the field. There will usually
be some losses in dry matter during storage, mainly due to degradation of biomass. This loss is
estimated to be around 10-20% of dry matter. Straw stored in the open (piles of bales in the field) can
also be damaged by rainwater and the outer bales in the stack may rot and be lost. Problems can be
avoided by choosing the storage sites carefully. If stored damp and under cover, some self-heating can
occur – a potential fire hazard.
A number of estimates for the quantity of straw available across the UK have been made, however in
reality it varies from year to year, depending on the harvest and on alternative market demand for straw.
Existing traditional agricultural uses such as livestock bedding and feed, animal feed compounding, crop
protection and composting consume significant and varying amounts of straw each year. 80
78
National and Regional Supply/demand balance for agricultural straw in Great Britain, Copeland, J., Turley, D., Central Science Laboratory for the
NNFCC, 2008.
79
ETSU New and Renewable Energy: Prospects in the UK for the 21st Century: Supporting Analysis.
80
ETSU BM/04/00056/REP/3 (1999) Energy from Biomass: summaries of the biomass projects carried out as part of the Department of Trade and
Industry’s New and Renewable Energy Programme Volume 5: Straw, poultry litter and energy crops.