Progress with Bio-refinery

by the CADDET New Zealand National Team

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Introduction

Technology is now being developed for the large-scale conversion of various woody biomass sources into power generation together with the production of organic chemicals.

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Energy Storage

Biomass, in the form of plants and trees, captures solar energy through photosynthesis and stores it as chemical energy in the bonds between the carbon, hydrogen and oxygen atoms that form cellulosic plant material. In effect, biomass is solar energy stored in a chemical form.

Fossil fuels are derived from biomass material deposited millions of years ago. While the chemical composition of biomass and fossil fuels are different, it is possible to produce a similar range of end products (such as fuels and commodity chemicals) from either source.

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Bio-refinery Technology

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The process demonstration plant

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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PictureSchematic of the Convertech process

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Current Progress

The Convertech process has been under development since 1989. All components of the technology have been proven at the batch pilot plant stage. A commercial-scale plant has now been built, in order to demonstrate the dynamics and chemical processing features of a full-scale system. This plant consists of one module which can be used to reproduce the conditions in any of the five modules of a complete system. During 1997, this plant has successfully yielded data to predict the performance of a complete plant and successfully tested a range of biomass types.

Until now, the development effort has been 95% private sector funded with total expenditure exceeding
$7 million (where $ is the New Zealand dollar). The demonstration stage project included support from the Australian Energy Research and Development Corporation.

For more information contact
Ian Bywater at Convertech Ltd,
PO Box 13-776, Christchurch,
New Zealand;
Tel: +64 3379 3301;
Fax: +64 3379 3303;
E-mail: bywateri@convertech.co.nz

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Part of the single-module plant

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.