Straw as an energy feedstock.



I/S Skive District Heating has since 2009 gasified wood pellets and made electricl power from the gas. The gasifier is a fluidbed type. Gas engines operate smoothly, but still 2011)the gasifier is triblesome.

Grain takes a lot of potassium and sodium from the soil - It is a characteristic grain shares with others fast-growing crops such as willow. These constituents make special requirements for the gasifier. Either operetaes the gasifier at temperatures below ash melting point or it must be so hot that the ashes becomes a thin liquid. Both types are available.
Asnæsværket at Kalundborg has recently (2011) put a 6MW low-temperature demonstration plant (Pyroneer - An LT-CFB gasifier) into operation for the gasification of straw.
Traditionally straw is returned to the soil after harvest, either by plowing or as part of the manure or litter. In recent times straw is an energy commodity and eventually there may be three typical applications: as fuel, feedstock for second generation ethanol or bio-methanol.

The lower heating value (LHV) of straw on a water and ash-free basis is 18.7 MJ/kg. As fuel straw is problematic because the ash gets soft already at 800 °C as opposed to wood, where the ash first softens at approx. 1100 °C . It can be solved by co-firing with other fuels. Gasification also requires constructive modifications. Straw has a high content of sodium and potassium, which makes straw ash soft and sticky at low temperature (Potassium chloride melts at 773 °C ). The ash content of straw is also quite high - 4% on average, but sometimes up to 8%. It can be compensated for by washing the straw in hot water - all in all difficult.

Mass balances for 1 t of straw dry matter (DM):

Straw as a fuel:
Straw with 14% moisture, 1000 kg DM
Heating 75% utilization 131 MJ
Ash, 4% on straw dry matter 40 kg DM
Smoke 960 kg DM

Straw as raw material for bio-ethanol:
Straw with 14% moisture, 4% ash 1000 kg DM
Additives 7 kg DM
Ethanol, 100% 248 kg DM
DDGS, Dried Distillers Grain and Solubles 423 kg DM
C5-molasses 134 kg DM
Yield according to  1000 kg straw (14,7 GJ) yields 146 kg ethanol (99,5% DM; 3,6 GJ) + 442 kg C5-molasses (65% DM; 3,3 GJ) + 174 kg Solid biofuel (90% DM; 10,3 GJ).

Straw as raw material for bio-methanol;
Straw with 14% moisture, 4% ash 1000 kg DM
Biomethanol 524 kg DM
Ash 40 kg
Heat 126 MJ