At the 234th meeting of the American Chemical Society in Boston this week, two different teams of researchers reported on their approaches for converting the waste from the paper industry into fuels.
Researchers at Dartmouth are engineering xylose-fermenting microbes to simultaneously saccharify and co-ferment paper sludge to ethanol. Researchers at Penn State are co-processing coal with black liquor to produce a syngas for conversion to dimethyl ether (DME).
Paper sludge to ethanol. The Dartmouth team, led by Lee Lynd (one of the co-founders of Mascoma, earlier post), has been researching the conversion of paper sludge for several years.
Paper sludge is the largest solid waste stream produced by pulp and paper industry, and is currently disposed of primarily in landfills. It offers a number of attractive technical and economic attributes as a feedstock for cellulosic ethanol production processes featuring the enzymatic hydrolysis of cellulose, including:
In many cases, it does not require pretreatment, thereby helping to reduce production costs.
Avoiding the cost of sludge disposal also improves the overall economic picture.
Steam, power and wastewater treatment at a paper mill is potentially at incremental cost.
The possibility of additional revenue from recovering mineral components from sludge.
The Dartmouth researchers have developed a bioreactor capable of aseptic, metered feeding of solid paper sludge. Carrying out simultaneous saccharification and co-fermentation (SSCF) in this reactor, they have demonstrated that > 90% hydrolysis yields can be achieved while producing economically-recoverable ethanol concentrations.
At the ACS meeting, the team described work using two promising recombinant microbes engineered to be able to utilize xylose as well as glucose: Z. mobilis 8b and S. cerevisiae RWB222s. The simultaneous saccharification and co-fermentation of glucose and xylose allows ethanol yields from paper sludge to be increased by about 20%.
Coal and black liquor to DME. Black liquor is a combination of lignin from the wood, the chemicals used in papermaking and water. It is normally disposed of by burning in a recovery boiler, after which the mills extract the inorganic chemicals and recycle them. However, notes André L. Boehman, professor of fuel science, black has more energy value as a synthesis gas which is then used to create other fuels.
A potential approach is to combine the black liquor with a coal slurry and gasify that. While other products are possible from the further conversion of the syngas, the Penn State team opted for DME because it is less energy intensive.
Converting black liquor and coal into DME also releases less carbon as carbon dioxide into the atmosphere than if coal alone was used to produce the fuel. Some of the carbon from these sources remains sequestered in solid form and do not add to global warming.
Fan, Z., C. South, K. Lyford, J. Munsie, P. van Walsum, L.R Lynd; “Conversion of paper sludge to ethanol in a semincontinous solids-fed reactor”; Bioproc. Biosystems Eng. 26:93-101.
Lynd, L.R., K. Lyford, C.R. South, G.P. Van Walsum, K. Levenson; “Evaluation of paper sludges for amenability to enzymatic hydrolysis and conversion to ethanol,” TAPPI J., February 2001, 84:50
Jiayi Zhang and Lee R. Lynd, “Simultaneous saccharification and cofermentation of paper sludge to ethanol by recombinant xylose-fermenting microbes” (ACS 234, FUEL 97)
Nicole Reed, Jamie Clark, Gregory K. Lilik, Qixiu Li, Chunmei Wang, and André L. Boehman; “Conversion of coal and biomass to clean burning DME” (ACS 234, FUEL 107)
Penn State Energy Institute