Black Liquor as Biofuels' White Knight

Biofuels have really taken a beating this year in the court of public opinion, as concerns mounted over the impact of biofuel production on everything from biodiversity to food prices to water supplies. IEEE Spectrum reader Luc Rolland of Preston, England captured the sense of disillusionment in his letter to the editors this summer: "Valuing biofuel is very controversialâ''it impoverishes developing countries, and the true energy cost of biofuel is not worth it."

But there are low-hanging fruit that respond to all of Rolland's concerns. Today, at MIT's, I take a look at the biofuel that appears to hang the lowest: dimethyl ether produced via gasification of black liquor. That's quite a mouthful, so let's define our terms:

  • black liquor: the blend of caustic inorganic chemicals and dissolved wood generated in pulp and paper mills
  • gasification: subjected to high heat and pressure, organic matter breaks down into a stream of principally hydrogen and carbon monoxide called syngas that can be catalytically reassembled into chemicals and fuels
  • dimethyl ether or DME: a clean-burning substitute for diesel fuel and liquid petroleum gas (LPG) that's easily synthesized from syngas

Note that this is very similar in principle to the synthesis of methanol via coal gasification. In fact methanol is an intermediate in the synthesis of DME. Production of both methanol and DME from coal is exploding in China, and provides substantial local air quality benefits.

Gasifying black liquor to generate DME, or bio-DME for short, adds ecological and climate benefits, which is why the European Renew study released in July called it one of "the most advanced concepts for fuel production." Net conversion efficiency on a lifecycle basis of 69%, the highest for any biofuel, means that for every hectare of agricultural production this fuel delivers the biggest energy bang. Greenhouse gas emissions, meanwhile, are on the order of 1/20th that of conventional diesel.

Bio-DME alone will not render personal transportation sustainable. Gasification of all of the black liquor generated in US mills would supply enough DME to displace just 3% of total fuel demand in the U.S., according to Jonas Rudberg, CEO of Swedish biomass gasification developer Chemrec. (For Canada, which has a higher pulp to car ratio, bio-DME could satisfy 7%).

But bio-DME's potential looks much larger if one views it as a forerunner to broader application of gasification to solid biomass feedstocks such as switchgrass. Solid biomass tends to gum up gasification plants, as Germany's Choren Industries may be learning as it seeks to start up its 18 million liter/year biomass-to-diesel gasification plant. Mass application of black liquor gasification would invariably accelerate the diffusion of the technology.

If it does, biomass gasification could deliver more than a fifth of Europe's transport fuel by 2030 according to Volvo Group, which is coordinating a European bio-DME R&D consortium. Of course, how low all that fruit will hang remains to be seen.

For more on bio-DME, see "Taking Pulp to the Pump".


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