Lignin is one of the most common organic compounds on earth, comprising about 30% of all organic carbon and about 30% of wood on a dry basis. Only cellulose is more abundant. If lignin could be isolated from the black-liquor by-product stream of a pulp and paper mill in a dry, low-ash (< 0.5% ash) state, the result would be an excellent biofuel with essentially the same energy content as coal. Recently, Clemson and LLC2 researchers have discovered that lignin can be precipitated from kraft mill black liquor in the form of a low-viscosity, easy-to-handle liquid phase by contacting the black liquor with gaseous CO2 at moderate temperatures and pressures. Furthermore, this liquid-lignin phase, which has reduced ash content vs. the starting black liquor, can subsequently be further reduced in ash content by acidification with organic, renewable acids. Based on this discovery, the team envisions a liquid-lignin low-ash purification (LLLAP) process for recovering a clean, low-ash (< 0.5%), biofuel lignin from the black-liquor stream of a pulp and paper mill. If 50 of the 100+ large pulp and paper mills in the United States implemented this change, 3 million tons of biomass fuel would be generated a year as a coal replacement for electric power production, enabling a reduction of 15 million tons of CO2/yr. If in the longer term all pulp and paper mills sent all of their black liquor to the LLLAP process, there would be a 2% reduction in the use of coal for U.S. electric power production. These are significant numbers.
The objectives of the project are as follows:
a. Design and construct a continuous-process, research-scale unit of the LLLAP process and use to investigate the effect of LLLAP operating parameters on several measurements of lignin product quality, including yield, ash purity, and oligomeric composition.
b. Determining the effect of LLLAP operating conditions on product quality will be difficult unless one can better quantify the molecular nature of kraft black liquor in general and of liquid lignin in particular. Thus narrow fractions of liquid lignin will be isolated via a modification of the LLLAP process, that is, by successive incremental pH reduction with compressed CO2 down to a pH of ~9. Then the bulk and molecular properties of this lignin will be characterized. For example, for absolute mass and oligomer information, MALDI mass spectrometry will be employed.
c. Fractionate the liquid lignin for molecular characterization in the acidified, salt-free form, where a more well-behaved relationship between properties such as molecular weight and propensity for precipitation should exist. Previous work with other oligomeric systems has demonstrated that multi-stage, supercritical extraction followed sequentially by high-temperature gel permeation chromatography gives excellent selectivities, and individual species can even sometimes be isolated. Characterize resulting micro-fractions in terms of their oligomeric composition via MALDI mass spectrometry. Use the results with those from the above ?pH? fractionation step to establish a relationship between the salt content and oligomeric composition of liquid lignin cuts, and their propensity for CO2-induced precipitation, and use these results to modify LLLAP so as to achieve a lower-ash product.
More than 20 paper and pulp companies around the world have expressed interest in the LLLAP process and liquid lignin technology, so there will be significant licensing opportunities if our low-ash targets can indeed be achieved. Finally, from an outreach standpoint, the research team will disseminate results to the general public through the SC ETV Radio program Your Day.
