Lignins in their native form are the most abundant renewable aromatic polymers on earth (Kirk and Farrell, 1987). Consequently, lignins present great potential as a source of energy due to their high fuel content (26—28 MJ/ton dry lignin) rivaling the fuel content of some coals (Lora, 2006; Tomani, et al., 2011). Lignins can be combusted to produce "green" electricity, power, fuel, steam, or syngas; all these are forms of energy which are being or will be used in the future to operate industrial plants where lignins are generated as by­products. The lignin by-products are called "technical lignins" or "industrial lignins" and they differ dramati­cally in properties from the native lignins found in plants. Examples of the use of technical lignins as a source of energy to run industrial plants are the pulp
mills deployed worldwide and the emerging lignocellu — lose biorefineries. Energy production is, compared to all other technical lignins applications, the one with the lowest market value, estimated at approximately 10 US$ cents/kg as coal replacement (Holladay et al., 2007). However, energy generation is the lignin applica­tion with the highest demand by volume and currently the one with the lowest technical risk. Almost every ma­jor pulp chemical mill today utilizes lignin as a source of energy. The latter is today’s common industrial practice which will likely be mirrored by future cellulosic biomass biorefineries which will use lignin as the main energy source in combination with other fuels such as raw biomass.

Technical lignins are available in large volumes, pri­marily in kraft mill spent liquors ("black liquors"), and, to a less extent, in the spent liquors of the few remaining sulfite mills ("brown liquors"). According to our conservative estimate, ca. 6—7% of the spent liquor

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00018-8

produced at a kraft pulp mill could be used for lignin extraction without significantly affecting the plant energy balance. This represents a potential average lignin production capacity per plant in the order of 30—75 tons of lignin per kraft pulp plant per day (Domtar, 2013) assuming an average annual pulp pro­duction capacity of ca. 0.5 million tons odw pulp (Table 18.1). On the contrary, in sulfite pulp mills, the majority of the produced spent liquor can be used for

lignosulfonate production given the fact that not many sulfite pulping players burn lignosulfonates for energy generation. In 2004, it was reported that 2% of the lignin available in the pulp and paper industry was commer­cially used comprising about 1,000,000 tons/year ligno — sulfonates from sulfite pulping brown liquors and <100,000 tons/year from kraft spent liquors (Gosselink et al., 2004). Assuming the forecasted annual growth rate of 2.5% (IHS-Chemical, 2012), the current lignosul- fonates production should be ca. 1,200,000 tons/year.

The global annual production of chemical pulps is estimated at 150 million tons/year (Vappula, 2011) with an average odw chemical pulp production per plant of ca. 0.5 million tons (Table 18.1) and a total volume of dis­solved lignin in pulp making of ca. 70 million tons glob­ally (Lora, 2010). Assuming a lignin annual production capacity per chemical pulp mill of ca. 27,000 metric tons kraft lignin per year (Domtar, 2013), it can then be concluded that the estimated annual global potential for kraft lignin production capacity from pulp mills is ca. 6—9 million tons depending on the feedstock species and the plant design which is in good agreement with the estimates reported by other authors (Glasser, 2010; Lora,

2010) . When considering the potential replacement of the main petrochemicals, namely, ethylene, propylene, butadiene and benzene, toluene, and xylene (BTX) iso­mers, which are produced at a rate of about 300 million tons/year with a total value of over $400 billion (Lucin- tel, 2013), by lignin it is relevant to consider the current global potential production capacity of technical lignins. It becomes clear from the analysis performed above that in the best case scenario purified technical lignins pro­duced globally at chemical pulp mills could potentially replace a maximum of ca. 2% of the global volume of main petrochemicals. However, the emerging lignocellu — lose biorefinery industry for production of biofuels and chemicals might completely change this picture. For instance, the US Department of Energy estimates that 1.3 billion tons of biomass is available in the United States alone for biorefining into transportation fuels and chemicals (Perlack et al., 2005). This amount of biomass could make available additionally 225 million tons of lignin which could be utilized for power, trans­portation fuels, products and various combinations of the above (Holladay et al., 2007). Assuming that 20% of this biorefinery lignin (45 million tons) will be converted into BTX and linear hydrocarbons, the result could be ca. 10% replacement of these petrochemicals by lignins produced at American biomass biorefineries alone. In other words, if we consider, in addition to the American biorefineries, a scenario where these biomass biorefining technologies will be deployed globally, we could witness, in the future, a hypothetical situation where a large fraction of petroleum-derived BTX, and perhaps of other petrochemicals too, could be replaced by lignin.

Another abundant source of technical lignins, which is often ignored, is the acid-hydrolysis (AH) lignin ("hy­drolysis lignin") which has been produced at Eastern European wood and agricultural wastes AH plants since the mid-1930s with yields in the range of 350—400 kg lignin/ton odw softwood. The annual production of such hydrolysis lignin in the former Soviet Union reached 1.5 million tons by the end of the 1980s. Howev­er, only 30—40% of the hydrolysis lignin was really uti­lized, whereas the rest was disposed in giant landfills nearby the wood hydrolysis plants, creating as a result serious environmental problems caused primarily by autoignition of these deposits. For example, the current lignin waste stocks in the Irkutsk region (Siberia), where only four plants are located, exceed 20 million tons (Rabinovich, 2010) equivalent to ca. 20 times today’s global commercial technical lignin market. The current annual production of AH lignin in Belarus alone is in the order of 100,000 tons (Podterob et al., 2004). The main application of the hydrolysis lignin is the produc­tion of pellets for energy generation. However, highly specialized applications, such as pharmaceutical entero — sorbents, have been successfully developed and commercialized on the basis of purified hydrolysis lignin. An example of these commercial sorbents is the enterosorbent "Polyphepan" (Podterob et al., 2004).

In addition to the low-value energy lignin applica­tion, a wide diversity of high-value industrial applica­tions have been envisioned or industrially realized or demonstrated including uses as novel materials, poly­meric, oligomeric, and monomeric feedstock. Some of these opportunities, such as the use of lignin or its derivatives in animal feed additives, agriculture, con­struction, textile, oil drilling, binders, dispersants, and composites, are today commercial realities but many others such as the production of carbon fiber precur­sors, the broad incorporation of lignin in synthetic polymeric blends, or the production of BTX remain longer term opportunities with great value and market potential.

Both low — and high-value lignin applications are often seen as efficient vehicles to increase the productiv­ity, reduce fossil fuel consumption, and increase the profitability of the industrial plants where lignin is pro­duced as a by-product. For instance, the Lignoboost™ process (Tomani, 2010), a recently commercialized pro­cess by Metso Corporation (Helsinki, Finland) for lignin production from alkaline black liquors, significantly im­proves the profitability of the pulp and paper mill by debottlenecking the wood pulp production as a result of increasing the recovery capacity of pulping chemicals and valorizing the lignin stream. Commercial-scale lignin production based on the Lignoboost™ process has begun in February 2013 by Domtar Corp. at the Ply­mouth Mill (NC, USA) with a targeted rate of 75 tons / day (~ 27,000 tons/year), destined for a wide range of industrial applications as a bio-based alternative to the use of petroleum and other fossil fuels (Domtar,

2013) .

As it was mentioned earlier, in the case of emerging industries, such as the cellulosic ethanol industry, the smart utilization of residual lignin could dramatically boost the profitability of the cellulosic biofuel plants if converted into value-added chemicals such as BTX, other monomeric and oligomeric phenolic compounds, and suitable for material applications macromolecules such as carbon fiber precursors, polymeric blends, ad­hesives, dispersants, and others. While energy and monomeric applications for technical lignins and their derivatives often target direct replacements of fossil fuels and petrochemicals, the development of novel lignin-derived oligomeric and macromolecular entities has the potential of generating better alternatives or synergy with petrochemical feedstocks. Recent exam­ples of the latter have been reported in the literature which illustrates this concept. For instance, recently Berlin (2011) showed that the replacement of methylene diphenyl diisocyanate (MDI) in engineered wood diiso­cyanate adhesives by organosolv (OS) lignin deriva­tives can lead to substantial improvements of the adhesive binding properties (increased modulus of rupture and modulus of elasticity) when applying the lignin—MDI adhesives in engineered wood composite construction materials such as Oriented Strand Boards (OSB) while still meeting the industry standard require­ments for these adhesives. A similar observation was documented when phenol in phenol—formaldehyde resins was replaced by OS lignin derivatives which resulted in a significant increase of the resin normalized bond strength (Berlin, 2012b). These two examples are important because they illustrate a fact often overseen which is the evidence that lignin derivatives can techni­cally outperform petrochemicals when used in conjunc­tion with the latter in certain chemical formulations. This observation hints at the possibility of not needing to completely depolymerize lignin, a longstanding un­resolved challenging technical problem, into the equiv­alent petrochemical monomers in order to achieve similar or better performance of the lignin-derived chemicals in formulated products. On the contrary, further research efforts could be directed toward valo­rization strategies of technical lignins with preserve natural backbone structures to produce viable novel polymeric precursors alternative to petrochemicals.

The recent resurgence of interest in lignin as a renew­able raw material feedstock is evidenced by the growing number of patent applications containing the word "lignin" which have been filed between 2003 and 2012 via the World Intellectual Property Organization (WIPO; Figure 18.1). It is interesting to note the fact

FIGURE 18.1 Number of WIPO patent applica­tions containing the word "lignin" found in May 2013 by using the WIPO search tool Patentscope for the period 2003-2012.

that 25% of all these patent applications were filed by major chemical, pharmaceutical, and energy companies such as BASF, Bayer, Ciba-Geigy, Monsanto, Sumitomo, and Shell with the German chemical giants Bayer (10% lignin filings) and BASF (8% lignin filings) leading the group. The patents found in May 2013 by using the WIPO search tool Patentscope for the period 2003­2012 (25,974 documents) represent ca. 50% of all the pat­ents registered in the WIPO which contain the word "lignin" on the front page (52,895 documents).

Today’s global lignin market is dominated by the Norwegian company Borregaard Lignotech (Norway) followed by Tembec (North America-France) and MeadWestvaco (USA). There are a number of smaller players such as Domsjo (Finland-India), Granit SA (Switzerland), and CIMV (France), among others.