1.3 Steam explosion pretreatment

1.3.1 Operation of steam explosion reactor

The bulk density of the straw in the steam explosion reactor depends very much on the condition of the straw and the feeding method. When filling the pilot reactor with chopped straw manually, a bulk density of about 60 kg m-3 was achieved. Loading baled straw would lead to a bulk density of approximately 150 kg m-3 (bulk density of straw bales according Jenkins (1989): 100 — 200 kg m-3). The bulk density of straw pellets is 500 kg m-3 and higher (Theerarattananoon et al., 2011). For reliable discharge of the treated straw from the reactor in the explosion step, addition of water to the dry straw is usually required. The thermal energy requirement of the steam explosion treatment is met by steam directly fed into the reactor. In small steam explosion units, steam is also optionally used for jacket heating of the reactor. In adiabatic operation, the thermal energy is required for heating up the biomass and the added water. The steam in the vapour phase of the reactor is lost through a vent during the sudden pressure discharge of the reactor. The steam required for heating up the biomass and the added water mst,1 (in kg) can be calculated thus:

mst,1 = mS — cp, S ■ Щ + m-Cp, W ■ ATW +AhR |•

The mass of straw ms and the mass of the added water mW are in kg. The specific heat capacity of straw cp, S and the specific heat capacity of water cp, W are in kJ kg-1 K-1. The temperature difference between pretreatment temperature and feed temperature for straw ATS and water ATW are in K. The enthalpy of vaporization for water AhV at pretreatment temperature and the net reaction enthalpy AhR of the pretreatment process are in kJ kg-1.

The venting loss of steam mst,2 (in kg) can be calculated thus:

The bulk density of the straw in the reactor pS, b as well as the density of straw pS, the density of water pW and the density of steam pst, all at operation temperature and pressure, are in kg m-3. The factor for the volumetric use of reactor volume is r|V.

An increase in steam consumption of 10% can be estimated because of non-adiabatic operation of the steam explosion system and steam leakages (Sassner et al., 2008). The total steam consumption is therefore calculated thus:

mst = 1.1 . (mst,1 + mst,2) (4)

A reduction in the cost of pretreatment can be achieved by minimisation of the specific steam demand. Ahn et al. (2009) determined the specific heat capacity of wheat straw with a water content of 4.3 g water/g dry sample to be 1.63+0.07 kJ kg-1 K-1. The specific heat capacities of other types of straw were in the same range. The specific heat capacity of water is about 2.5 times higher than the specific heat capacity of straw. Therefore, the total water content of the input material is a main influencing factor on the thermal energy consumption of steam explosion pretreatment. Minimizing the rate of water addition to the straw is a way to reduce the steam consumption. Preheating of the added water using waste heat e. g. from the condenser of the distillation or increasing the bulk density of the straw in the reactor are also ways to reduce the steam consumption (Fig. 3).

A reduction in steam temperature would reduce the steam demand too, but at the same time reduce the effect of steam explosion treatment.

For the discharge of the treated straw from the reactor in the explosion step a certain fraction of the reactor volume has to remain filled with uncondensed steam. The remaining steam — filled fraction of the reactor volume under various operation conditions is shown in Fig. 4.

The steam explosion pretreatment of straw pellets is restricted by the pore volume available for the addition of water and condensing steam. From this point of view, a type of compacted straw with a density between 150 kg m-3 and 500 kg m-3 would be preferable.