Fuels for Biofuels part 4: lignocelluose as a biofuel: pretreatment.

One of the problems with lignocellulosic feedstocks is the complexity of composition and the presence of lignin - the polymer that allows plants to fill their cells with a rigid mass and stand up straight - see my post on plant structure. Wood and the fibrous stems of crops and other plants have an abundance of lignin and this needs to be broken down before the cellulose fibers that provide the edible stuff for micro-organisms becomes accessible. Simultaneously the hemicellulose and cellulose fibers need to be disrupted to make them more readily digestible.

Figure 1 shows the a schematic of conversion of lignocelluosic feedstock to biofuel. In this post I will discuss pretreatment processes  (the green bit in Figure 1). 

Figure 1. Converting lignocellulose to biofuel (4).

The basic process for biological conversion of lignocellulose to ethanol is a pretreatment to break down the wood polymers, followed by enzyme mediated hydrolysis of those polymers to produce monosaccharides that can then be fermented by micro-organism to produce ethanol. 

Pretreatment
Even before pretreatment, the biomass must be converted to smaller pieces. For example, wood is shredded to chips and then further converted to fibers. Great energy savings can result if the chips are chemically treated before further milling to fibers (Zhu et al, 2010) i.e. the order of shredding and then pretreatment can be manipulated to give optimal results. There are different types of pretreatment and the type that is used depends a great deal on the biomass resource. For example one treatment that works well for wheat straw may be much less efficient for use with wood chips. Here are a few examples of pretreatments:
1) Dilute acid pretreatment uses low levels of acid (e.g. 2% sulfuric acid) and heat (190dC). Compounds called furferals are produced which inhibit fermentation but are a value added co-product.  The process works best with hardwoods rather than softwoods and also produces a condensed form of lignin that is of little value apart from as boiler fuel. 
2) Steam explosion (Figure 2) also uses dilute acid and combines chemical pretreatment with size reduction in one step increasing efficiency. However it is energy intensive, has relatively low sugar recovery, though this can be improved, and works better with hardwoods than with softwoods. It has not yet been commercially scaled (2010).

Figure 2  Oil palm trunk chip (a) and steam exploed oil palm trunk pulp (b)

3) Organosolv uses organic solvents such as ethanol, acetone, butanol or polyethylene glycol. To date ethanol has proven to be the most economical as it is cheap and easy to recover even if it is not the most efficient. Organosolv processes produce high quality lignin and celluose which has good digestibility. Large quantities of ethanol are required and the process is energy intense. As a result commercial viability lies in the recovery of the high quality value added products that are a by-product of this process. 
4) The most promising pretreatment however is the Sulfate Process (also known as the Kraft Process or Sulfite pretreatment to Overcome Recalcitrance of Lignocellulose (SORL). As above, dilute acid is used together with the addition of sulfite (for the chemical reaction, click here). Conditions are relatively mild at 130-190dC and produce low amounts of fermentation inhibitors but also lignosulfonate which is a good value added coproduct. Additionally the process softens lignin by increasing its hydrophilicity which reduces energy consumption and aids downstream polymer digestion. Economy is also increased because the process can be carried out directly on wood chips without further size decrease. 

Figure 3. Sodium chloride compared with ionic liquid [bmim]NTf₂

5) Ionic liquid  treatment is a relatively recent approach using salts that are liquid at room temperature (figure 3) and that are able to dissolve cellulose and lignin. However, more research is needed into ionic salt recovery from the dissolved biomass before they can be commercially viable.   
6) Biological pretreatment. If you have had the good fortune of being able to walk through

Figure 4 - link

a quiet wood and step over or on soft rotting branches (Figure 4), you'll understand even better that there are biological processes capable of breaking down lignin. These involve enzymes and as we will discuss in the next post, enzymes are a major economic burden in biofuel production. Their advantage is that they require mild conditions and consolidation of the breakdown of lignin (pretreatment), cellulose hydrolysis and fermentation could potentially all be done in one step, substantially reducing cost.  There'll be more on this in a subsequent post I think.

In each pretreatment scenario, the lignin and other non-digestible material must be separated from the degraded cellulose fibers. How this occurs depends on the pretreatment type and the details of the pretreatment. For example in organosolv pretreatment, temperature and solvent (e.g. ethanol) to water ratio effect the morphology of lignin. The many aspects of such fractionation are beyond the scope of this blog and at this moment, my interest!

Engineering plants.
As discussed in my April blog post, wood consists of three polymers - cellulose, hemicellulose and lignin. Have a look back to remind yourself of their structures. Wood cellulose has a twist to the arrangement of the polymers and to be relatively amorphous, possible making it more amendable to breakdown. Soft and hardwood hemicellulose differs in the composition of the monosaccarhides that predominate and in chemical linkages between the monosaccharides. While cellulose and hemicellulose are polysaccharides, lignin is an unstructured polymer of monolignols and again the composition and structure of lignins varies between hardwoods and softwoods.  As the chemical links joining the monomers are all different, so are the enzymes  necessary to break them. In order to understand the best procedure for doing this, much research is performed into the structure and composition of the three polymers and to how changes in the genetic components of their synthesis pathways can ease the breakdown pathway during biofuel production without affecting plant vigor (3). Other avenues of genetically engineering plants have also been explored (5) and these include increasing the amount of cellulose, engineering plants to produce their own cellulase and reducing the amount of lignin in the plant.  Of course, planting great swaths of genetically engineered trees or grasses is controversial.  This topic will have its own post in the future.

Thermal and biological conversion of wood feedstock
As interesting aside, there are two ways of converting wood to biofuel: biological and thermal. Above we discussed  pretreatment for biological conversion. Thermal conversion (1) has two pathways:  gasification and pyrolysis. Gasification uses high heat and pressure and yields carbon monoxide and hydrogen from which a variety of fuels can be catalytically obtained. Pyrolysis (breakdown by fire!) uses heat in the absence of oxygen to generate bio-oils and char. Bio-oil can lead to a number of different products but requires further treatment before it can be used. Char is the solid carbon enriched material that remains. It can be used as a soil fertilizer and perhaps as a method of carbon sequestration to remove CO2 from the atmosphere in attempts to mitigate climate change.

References

1) Pu, Y, Kosa M., Kalluri U, Tusakn GA, and Ragauskas AJ. 2011 Challenges of the utilization of wood polymers: how can they be overcome? Applied Microbiology and Biotechnology September 2011, Volume 91, Issue 6, pp 1525-1536
2)Zhu, JY, Pan, X. and Zalesny Jr., RS.  2010 Pretreatment of woody biomass for biofuel production:energy efficiency, technologies, and recalcitrance Appl Microbiol Biotechnol 87:847–857
3) Feltus FA and Vandenbrink JP. 2012 Bioenergy grass feedstock: current options and prospects for trait improvement using emerging genetic, genomic, and systems biology toolkits. Biotechnol Biofuels Nov 2;5(1):80
4)Verardi, A, De Bari, I, Ricca, E, and Calabrò, V. Hydrolysis of Lignocellulosic Biomass:
Current Status of Processes and Technologies and Future Perspectives. 2013. www.intechopen.com 
5) Sticklen, M. 2006. Plant genetic engineering to improve biomass characteristics

for biofuels. Current Opinion Biotechnology. 17:315-319