Biomass has long been recognised as a potential source of both renewable energy and organic chemicals. The large-scale use of biomass has been impeded by three main problems:
- the high moisture content and low energy density of most green biomass;
- the presence of alkali salts which make it very difficult to burn some forms of biomass economically in high efficiency, high temperature boilers or to power gas turbines;the
- difficulty in accessing the valuable chemicals contained in biomass, which have the potential to substitute for petro-chemicals.
A novel process has been developed which overcomes the major problems by:
- washing out the mineral salts in the biomass, thus reducing the alkali content;
- using steam hydrolysis to achieve a breakdown of the biomass into its chemical constituents (in particular, various sugar fractions, cellulose, lignin and volatile components);
- drying the biomass efficiently using superheated steam
The technical breakthrough has been achieved by Convertech Ltd, based in Christchurch, New Zealand. The steam hydrolysis, chemical extraction and drying operations are carried out in a series of five continuously operated
modules, as illustrated in the figure.
The process involves progressive heating and pressurisation of the moist particles of biomass, followed by rapid decompression-washing, further heating and pressurisation and final
superheated drying of the resulting solids. This multi-stage operation subjects the biomass to a regime of temperature, pressure and moisture content, leading to its sequential fractionation.
Depending on the nature of the
biomass feedstock, it is shredded or milled and fed into the plant as a stream of small particles. The biomass is given a front-end wash before it is directed through a rotary interlock into module 1.
A fan propels the steam
and biomass through the module. This first stage extracts any volatile products contained in the biomass. Some of these, such as essential oils, may have a substantial market value. The remaining volatile extracts may be used as a
fuel to drive the refining process itself.
Hydrolysis of the hemicellulose fraction is carried out in the second module. It releases residual alkali salts for their subsequent removal, and transforms the hemicellulose into a
range of chemicals, such as pentosans (sugars) and furfural (a valuable chemical in the nylon and foundary industries).
Both hydrolysis stages of the process (at the second and fourth modules) produce low molecular weight
lignin and cellulose. The lignin can be extracted and used to manufacture epoxy resins and phenolic resins; as binders in building material; or as an initiator for urethane products, etc. The cellulose fibre produced downstream of
the fifth module has a wide range of applications in the panel board, paper and power generation industries. The nutrients removed from the raw biomass in the wash water also have a fertilising value.
The economic viability
of the process depends strongly on the yields of high-value materials obtained from what is essentially a wood-refining process. The solid residue from the end of the process can be used either as a clean, demineralised, solid
fuel, or as a raw material for a new construction material which might attract a higher value. Removal of the alkali component from the biomass can solve the slag build-up problems which often result during biomass combustion when
the fuel has a high silica content.
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 Schematic
of the Convertech process
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