Parratt & Associates Scoping Biorefineries: Temperate Biomass Value Chains

Chapter 4. Biorefineries and Technology

There have been numerous definitions of the term “biorefinery” and many different biorefinery types have been identified to accommodate the different classes of input feedstock and product outputs. At the core of biorefineries though is the conversion of plant biomass to power, chemicals (including fuels) and materials.

Plant biomass is chemically complex, and its specific composition varies greatly between different plant species. Some biomass components such as carbohydrates, lignin, proteins and fats are widespread, whereas secondary metabolites including dyes, flavours and essential oils are more specific. Biorefineries bring together the technologies needed to convert these raw materials into industrial products and energy.

Figure 4-1 provides a representation of the three key constituents of lignocellulosic plant material. The actual quantities of cellulose, hemicellulose and lignin vary between plants, varieties and species.

Figure 4-1: Schematic representation of the location and structure of different components in Lignocellulosic material. Adapted from Ritter¹⁵³

At this time, Australia does not cultivate dedicated bioenergy and industrial crops on any scale, with two exceptions: Delta Energy’s mallee co-generation production system, which uses coal-powered electricity plants in the Hunter Valley¹⁵⁴, and WA’s Verve Narrigen plant¹⁵⁵. Accordingly, in the near to medium term, the biomass for renewable fuel, energy and bioproducts is likely to depend on the waste streams arising from traditional food and fibre production (the latter including forest and crop residues). Australia currently exports a growing amount of wood chips to be burnt in either dedicated or co-generation plants in both Europe and Asia. However, this source material could be used effectively in Australia with the right investment setting. In temperate Australia, lignocellulosic material is the most likely type of biomass to be available in sufficient quantity and reliability of supply to warrant future investment in integrated biorefineries. The following sections provide a brief overview of biorefinery technologies most applicable to lignocellulosic feedstock. Although the main focus is on higher value products, the initial drivers for biorefinery development will most likely be energy and liquid fuels production.

Lignocellulosic biorefineries are the subject of intense global technology development, particularly in North America and Europe. However, the primary focus to date has been on fuel ethanol; this is also true of the very limited activity underway in Australia. The application and status of various technologies for lignocellulose conversion to biofuels in the Australian context were recently reviewed¹⁵⁶. Although there has since been significant technological development internationally, the underlying process technologies remain the same.

The basic steps for conversion of biomass to various product types are summarised in Figure 4-2

Figure 4-2: Basic biorefinery processes and product categories

4.1 Pre-treatment technologies

Regardless of the conversion technology used in the biorefinery, all require at least some pre-treatment step. ¹⁵⁷. The major technologies are;

• 
  mechanical size reduction – milling, chipping, grinding;
  
• 
  dilute acid hydrolysis;
  
• 
  concentrated acid hydrolysis;
  
• 
  alkaline hydrolysis;
  
• 
  steam explosion;
  
• 
  ammonia fibre explosion;
  
• 
  organosolve;
  
• 
  ionic liquids; and
  
• 
  supercritical fluids.
  

The choice of pre-treatment will be largely determined by the subsequent conversion technology to be used. Not all (e.g. ionic liquids) are currently considered economically viable at commercial scale. In the case of thermochemical conversions, pre-treatment may require little more than simple mechanical size reduction. In order for biochemical conversion of biomass into useful sugars and co-products it is necessary to break the lignocellulose into its major components, preferably with some additional fractionation of the lignin, cellulose and hemicellulose streams released.

The pre-treatment phase provides an opportunity for extraction of other components of the biomass, such as waxes (e.g. from cereal straw) or plant secondary metabolites (e.g. oil from eucalypts), which may form products or product intermediates in their own right. In many instances, these potential product streams may be lost or destroyed by the application of an inappropriate pre-treatment technology. In the context of integrated biorefineries seeking to develop multiple product streams that generate maximum value-add to the starting biomass, different pre-treatment technologies for each biomass feedstock must be evaluated.

4.2 Conversion technologies

Two main classes of conversion technology are most relevant to lignocellulosic biomass;

• 
  thermochemical treatment of unfractionated biomass to a synthesis gas or bio-oil; and
  
• 
  biochemical treatment of fractionated biomass to release sugar monomers that can then be fermented to give a range of fuel and other chemical products.
  

There is a fundamental difference between these two broad technology classes. Although it is possible to produce a wide variety of products through either approach, thermochemical methods largely destroy the great (bio)chemical diversity intrinsic to plant biomass, thereby losing access to some potential product classes. While biotechnological methods provide greater opportunity to exploit the chemical complexity of the biomass, the technology is at a much earlier stage of development. It is likely that future integrated biorefineries will likely use a combination of both biochemical and thermochemical conversion processes to extract maximum economic value from all components of the lignocellulose. Such plants are either in the planning stages or at pilot scale in the USA and Europe (e.g. Companies like Zeachem, Coskata, CMIV).

The major thermochemical and biochemical conversion technologies are briefly described below and the general fuel and co-product types arising from the different technologies are summarised in Table 4-1.

Thermochemical

Several different types of thermochemical treatment that can be used to convert biomass to either liquid or gaseous forms. These can then either be used directly or with additional processing to yield various combinations of power, heat, gaseous or liquid fuels and chemicals. The three most developed technologies are gasification, pyrolysis and liquefaction. Biorefineries employing these thermochemical processes, together with additional processing steps, have the capacity to produce many of the fuels and chemicals, or their close equivalents, currently produced from fossil-fuel sources. However, the processes are currently not cost-competitive with petrochemical refining. A recent Princeton University study examined the potential to retrofit new processes onto existing paper and pulp mills. This study indicated that as pulp and paper pulp mill plant boilers age the replacement with a range of technologies for biomass transformation to liquid fuels and precursors could yield significant economic benefit¹⁵⁸.

4.2.1.1 Gasification

Biomass gasification involves heating the biomass in the presence of air or oxygen to yield a gaseous mixture of carbon monoxide, carbon dioxide, hydrogen, methane and nitrogen, known as synthesis gas (Syngas) or producer gas depending on the relative quantities of the different gases. The technology is comparatively mature and gasifiers capable of processing a variety of biomass types are commercially available. The gasification products can be burnt for energy or further processed to produce hydrogen, hydrocarbon fuels (via Fischer-Tropsch synthesis), or methanol. These in turn can be processed into a wide range of chemicals using well-established chemical synthesis routes.

4.2.1.2 Pyrolysis

Pyrolysis involves heating biomass in the absence of air. Depending on the temperature and heating rate, different products are formed including charcoal, bio-oil or gases. Bio-oil is a complex mixture of many different chemicals that may be burned directly as a fuel or further treated to provide a variety of chemicals. The spectrum of chemicals produced is determined by the specific pyrolysis conditions used and to some extent by the chemical composition of the starting biomass.

3. Liquefaction

Solid biomass can be converted to a liquid form by heating under high pressure in the presence of water. The complex oil produced is known as bio-crude, which is generally a mixture of many different chemicals that is subsequently upgraded by catalytic processing to give either liquid fuels or synthesis gas.

Biochemical

The biochemical conversion of lignocellulose takes place at much lower temperature and slow reaction rates than when compared to thermochemical processing. In general biochemical conversion requires more elaborate pre-treatment of the biomass to dissociate the cellulose, hemi-cellulose and lignin components. The polysaccharides are subsequently hydrolysed to their sugar monomers in a process known as saccharification. The sugars released can then be fermented to produce alcohol fuels, or potentially a very wide range of other ‘platform’ chemicals that can be used as replacements for many of the products currently derived from petrochemical refineries as described in Chapter 5. Herein lies the significant advantage of an industrial biotechnological approach over thermochemical transformation – conversion to a multitude of platform chemicals is possible once C5 and C6 sugars are available.

The main fuel product from biochemical processing is currently bioethanol, though technologies for production of higher alcohols such as butanol are in active development¹⁵⁹. Bioethanol is the major product of the first-generation biorefineries that use waste starch and sugar, or grains as feedstock. These first-generation biorefineries have little scope for value-added co-product development and provide limited scope for greatly increased production volumes.

Table 4-1: Lignocellulosic biorefinery technologies for fuel and co-product production

	Conversion Technology	Intermediate Product	Fuel Products	Co-products
Thermochemical	Gasification	Biosyngas	Hydrogen F-T diesel Methane Ethanol Methanol	Chemicals (wide variety possible using existing technologies)
	Pyrolysis	Bio-oil	Bio-oil (generator use), but can be upgraded to other products, including syngas	Biochar Heat Electricity Chemicals (from syngas)
	Liquefaction	Bio-oil
Biochemical	Wet mill	Sugar	Ethanol	Distillers dried grains plus solubles, Carbon dioxide
	Dry mill	Starch/sugar	Ethanol	Animal feed, carbon dioxide
	Lignocellulosic fermentation	Cellulose Hemicellulose Lignin	Ethanol	Chemicals (very broad range possible based on biotechnological and chemical processing)

Although a wide variety of chemicals can be produced as co-products from either thermochemical or biochemical processing routes, there are significant differences between the two. The pyrolysis and liquefaction technologies give rise to complex bio-oils with considerable chemical diversity but generally low yields of any individual chemical. Direct chemical production therefore requires complex and expensive downstream fractionation. In practise it is more likely that the bio-oil will be converted to syngas, and as with gasification conversion technology, the path to chemical production then relies on Fischer-Tropsch synthesis and conventional, energy-intensive petrochemical refining. In contrast, biochemical conversion allows for the production of a very wide range of potential platform chemicals (as outlined in Chapter 5), providing flexibility in the range of biorefinery products. Biochemical processing also has an added advantage as it does not destroy the complexity and chiral specificity of the natural plant chemistry.