Lignin Valorisation Opportunities from Cellulosic Sugar Production
At present, the majority of lignin generated around the globe originates from either an acid sulfite or a kraft pulping process. However, with regards to cellulosic ethanol production, neither the kraft process nor the acid sulfite pulping processes are compatible pretreatment techniques, due to substantial costs associated with the generation of high quality cellulose for pulp and paper manufacturing. Alternative pretreatment technologies include: alkali, dilute acid, concentrated acid, liquid hot water, steam explosion, AFEX, and organosolv processes. All of these technologies, with the exception of concentrated acid, AFEX and organosolv, are available for the pretreatment of lignocellulosic biomass at commercial scales.
In terms of lignin recovery, steam explosion, organosolv, and alkaline pretreatments demonstrate the highest attainable yields. Followed by concentrated acid, dilute acid, and liquid hot water pretreatments. These pretreatment technologies generate a wide range of molecular weight lignins. Those that produce higher molecular weight lignins include dilute acid, concentrated acid and AFEX pretreatments, whereas alkaline, liquid hot water, steam explosion and organosolv pretreatments produce lower molecular weight lignins. In general, all lignin fractions display a high number of phenol and hydroxyl groups. The presence of β-O-4 linkages and other labile bonds tends to correspond with the degree of lignin depolymerization, i.e. smaller lignin fragments generally contain less of these reactive sites as they have been more extensively degraded. Higher molecular weight lignins are therefore, typically more reactive than lower molecular weight compounds.
With regards to biomass compatibility, in general all pretreatment technologies are suitable for processing agricultural residues. Therefore, when considering the types of feedstocks that are readily available in Indonesia (e.g. rice/maize straw, oil palm empty fruit bunches, sugarcane residues and bagasse), all technologies could potentially be used as a means of pretreating biomass prior to cellulosic ethanol production. In terms of wood processing, hardwoods are largely compatible with all pretreatment processes apart from AFEX, and softwoods are believed to be processed most effectively using liquid hot water pretreatment.
There are two main options for the technical development of pretreatment derived lignins: 1) to take advantage of established markets for lignosulfonates through the sulfonation of suitable pretreatment lignins; and 2) to establish new applications based on those currently under development for kraft lignins.
In terms of identifying interesting market opportunities for pretreatment derived lignins, there are a number of factors that must be considered and these form the basis of this review. These include:
- The TRLs of the applications in question (i.e. is there an existing market to sell into?);
- The suitability of the lignin for the given application (i.e. do the properties of lignin match up to those required for the application?);
- The value of the application (i.e. is the chosen application/market likely to yield good economic returns?).
Based on these key factors, a near-term market opportunity has been identified in the production of biomass pellets for heat applications. This is an existing application that has been developed commercially for lignosulfonate/kraft lignin use. The properties of lignins derived from pretreatment appear to be compatible with this function. However, this is a low-value market application and thus, it may be more profitable to target an end-use that could yield better returns.
Another interesting near-term application is in the production of phenol formaldehyde resins. The phenolic properties of lignin make it an ideal candidate to replace fossil-derived phenol. This application has been extensively developed using kraft lignin and at present, is considered technically ready for commercialization.
Although less developed than phenol formaldehyde resins, the production of carbon fiber for use in composites is also considered an attractive target for lignin product development. Although technical restrictions may prohibit their use in high performance applications such as aerospace, there could be an opportunity to incorporate these materials into automotive applications for example, as part of industry efforts to enhance the light weighting of vehicles.
Finally, the conversion of pretreatment derived lignins into water soluble lignosulfonates may also be possible depending on the target application, and on technical requirements such as molecular size or distribution. This offers up a wider range of possible applications to these types of lignins.
In all of the possible end-uses outlined above, it is important to be aware however, that any novel lignins entering the market will not only compete with fossil-based alternative materials, but also with the conventional lignins that currently dominate the lignin market (i.e. kraft lignins and lignosulfonates). Therefore, any commercial project targeting specific lignin applications will require a detailed technoeconomic and market analysis to determine the true extent of the opportunity proposed.