Geoff Covey, Gerke Faber and Reg Harvey
At present, biorefineries are not economic unless very large, receiving government subsidies or in special circumstances. Building a large biorefinery presents technical and economic risks which are unacceptable to most companies. However, many organisations would like to gain an entry to the market for strategic reasons. If a high-value, low-volume by-product can be produced, the economics can be greatly improved and comparatively small operations can be worthwhile. This paper describes technology that is already in use to extract high value products from certain biomass types prior to their conversion to fuels.
Studies show that at present simple fuels from biomass cannot be produced to compete economically with fossil fuels except on a very large scale (feed rate of at least 500-1000 t/d (1) ). Such plants require a substantial investment and as such pose a large techno-economic risk at the present time. Therefore, they will generally not be built without substantial government guarantees and/ or funding contributions and only a limited number of such plants will be built.
The risk of entering the bio-fuels/bio-product market can be reduced by starting with small plan
ts, but as already noted, these are normally uneconomic. One solution to this problem is to build a plant where much of the revenue is generated by comparatively small volumes of high value chemicals and the remainder of the feedstock is converted to fuels or other lower value products (1) . These high value products can be extracted at various stages in the process1 but the present paper is concerned with the case where they are extracted before the biomass feed stock has been subjected to any chemical processing or significant thermal processing.
The approach to be considered here is one that is used in the atmospheric pressure extraction of many essential oils and vegetable oils and is outlined in Figure 1. The solvent-free extract produced in this way will usually be a mixture of multiple chemicals only some of which are valuable in themselves. Therefore in most cases, further processing will be required to purify the extract e.g. with a second solvent or distillation to refine the extract. This purification may be performed by the operator of the extraction plant, or the crude extract may be sold as is for further processing.
As already noted, the first extraction plant will usually be quite small (perhaps 50-300 t/d of feedstock) and the purification plant is typically even smaller as the crude extract is only (say) 0.5-10% of the biomass fed into the process.
Fig. 1: Typical schematic for extraction of soluble chemicals from biomass
As will be apparent from above, such a process will typically produce at least three products:
• One (or more) high value chemicals from the secondary processing of the extract. These will be critical to the profitability of the process. Therefore it is always worth considering whether more components can be economically recovered from the crude extract.
• The rejected portion of the extract. Depending on the solvent used this will usually be a liquid ‘oil’ and is likely to be suitable as a fuel either as is or after further upgrading. The scale of operation described here is such that only very simple upgrading of this liquid residue is likely to be economic on site.
• The solid residue from the extraction. This will be suitable for processing into a fuel – solid, liquid or gaseous. This is the material that feeds the main bio-fuel process, which will not be discussed further here. If the biorefinery is built in conjunction with a pulp mill and the feedstock is woody material, the residue may often be suitable for pulp production.
An example of this type of plant is that of Botanical Resource Australia Pty Ltd (BRA) in Ulverstone, Tasmania. They built an extraction plant and a refinery more than ten years ago. Covey Consulting has subsequently upgraded both of these plants and added a new extraction line using rather different equipment (Fig. 2) and two new refining lines. The extraction plant produces a crude solvent-soluble material which is further treated in the refinery to produce a pyrethrum concentrate, which is the main product, an oil (the ‘rejected’ part of the extract) which is used as a diesel substitute to fuel boilers at the site (some surplus is sold) and a solid residue which is to be briquetted and used as a renewable fuel at various nearby establishments. One of the problems faced in designing the extraction plants was the absence of process equipment of suitable size. Pilot plant type equipment is readily available, but much too small. Commercial equipment is intended mainly for extraction of edible oils from seeds and similar applications. Plants of this type tend to be quite large, and even the smallest equipment is economically unattractive for the scale of operation at Ulverstone. As a result, it was necessary to design equipment specifically for this process. However, the equipment designed is of a modular nature and is suitable for use in other plants extracting chemicals from biomass – particularly when the solid residue from the extraction process is to be used to prepare renewable fuels. Some of this equipment, and the way that it can be used is described below.