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Two Approaches for Converting Papermaking Waste into Fuels: Cellulosic Ethanol and DME

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.



Doug M

Why don't you just co-fire the sludge in a coal-fired boiler? Is the moisture content too high? I realize that by co-firing, you are only offsetting coal cost at $1.50-$2.00/mmBtu (vs. selling diesel fuel at $15/mmBtu) but you avoid the processing/refining/transportation cost of the ethanol/DME


It appears to me these processes can be add-ons for price-reduction, profit-increase. The process can be in-plant. Transportation cost would be reproduced, as is landfill charges. Glad to see this.

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Rafael Seidl

Here's another paper on the subject of black liquor gasification. DME is a clean-burning diesel substitute that becomes liquid under mild pressure (<10 bar). A key consideration is that it can be used even in very cold temperatures. Retrofitting vehicles is quite feasible (cp. LPG for SI engines) but setting up a sufficiently close-knit distribution network is a big hurdle.: DME's gravimetric energy density is 29MJ/kg (vs. 42 for diesel), volumetric energy density in liquid form 19.4MJ/L (vs. ~35MJ/L for diesel). With a tank of the same volume, you have to fill up on DME twice as often.

The base manufacturing costs for DME from black liquor are given as competitive with petroleum distillation if the price of oil is >$30/barrel. That is obviously the case right now and has been for several years. Chances are, it will be for quite a few years to come. The presentation argues that ROI horizons are 2-5 years in Sweden, depending on how the government there assesses the process wrt its CO2 balance.

John S.

To Doug:

Depending upon the paper making process, simply burning the sludge with coal or other fuel could result in the release of dioxins or other harmful pollutants.


I wonder if paper mills are the cellulosic biofuel plants of the future since 1) they have solved the logistics 2) eventually people will want fuel more than paper. I notice a nearby paper mill uses trainloads of coal which doesn't suggest they have any spare fuel feedstocks.



Nice presentation. DME seems like a cool fuel and is quite renouned in Scandinavia.

However, the WW potential of 30 Mton/year equals roughly 236 mb/year of gasoline. That's a drop in the bucket compared to the 30,000 mb/year crude oil currently used.

Clearly there needs to be other sources of feed-stock for DME production other that paperpulp! Coal could be used, but the well-to-wheel CO2 emission would be horrendous.

These are just more reasons why replacing diesel with DME is not currently realistic.

ps: Shifting to DME is probably an order of magnitude easier than going to hydrogen!

Roger Arnold

DME is an attractive energy carrier. It's a high cetane replacement for diesel fuel, well-suited for high efficiency diesel engines. Unlike petroleum-based diesel fuels, it can be stored indefinitely without gumming up. And it's relatively easy to synthesize, compared to heavier hydrocarbons.

Once someone develops cheaper and more efficient electrolysis cells, we can produce hydrogen using "as available" solar and wind power, then synthesize DME from hydrogen and waste CO2 for our transportation needs.

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