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Taking Pulp to the Pump

Gasifying black liquor from pulp mills will accelerate second-generation biofuels.
December 12, 2008

Pulp and paper plants could soon double as biorefineries if financing for a Swedish gasification project is any indication. As gas prices have slumped this fall, threatening to run some biofuels innovators out of business, Swedish company Chemrec has pulled in a stream of grants and investments backing a process for turning the black liquor left over from pulp and paper bleaching into a clean-burning synthetic biofuel.

Black liquor: Chemrec’s gasification plant in New Bern, NC. The paper mill there consumes up to 330 tons of black liquor per day–a mixture of caustic chemicals and dissolved wood left over from the bleaching of pulp for paper production. It currently produces a clean burning gas that provides heat energy to the mill, and recycles the caustic chemicals.

Chemrec received $20 million in venture-capital funding earlier this month, and another $300,000 from the U.S. Department of Energy this week to assess the feasibility of applying its process at a pulp mill in Escanaba, MI. The Stockholm-based firm was already ramping up R&D through a $37 million EU-supported research consortium involving seven European industrial firms that was launched in September.

Part of the attraction is the ecological profile of the biofuel generated with Chemrec’s process, dimethyl ether (DME), which can be used as a replacement for liquefied petroleum gas (LPG) and diesel. Amidst growing angst over the ecological impacts of biofuels production and the disruption caused to food production, recent analyses, such as the EU’s Renew study of second-generation biofuels, have found that DME made from biomass gasification provides the highest greenhouse-gas reduction for the lowest cost.

The heart of Chemrec’s technology is a gasification process that turns black liquor into a mix of carbon monoxide, hydrogen, and CO2 called synthesis gas, or syngas, for short. Gasification of coal is already a booming business in China, where the resulting syngas is converted into chemicals and fuels. And gasification of wood chips is also on the rise. For example, Canada’s Nexterra Energy is one of several developers installing small power plants that gasify wood chips and burn the resulting syngas to generate power and heat for residential developments.

But black liquor is an obvious feedstock for biomass gasification. Pulp mills already take care of gathering loads of biomass, and, as a liquid, the waste liquor is easier to feed into the gasifier than are solid chunks of biomass. In practice, however, this waste has proved tough to gasify. The mixed success to date of black liquor gasification developer ThermoChem Recovery International, based in Baltimore, exemplifies the challenge. Of two large-scale installations using ThermoChem’s technology, one is still running, while the second never operated commercially due to gasifier design flaws.

Chemrec CEO Jonas Rudberg explains that black liquor is particularly difficult to deal with because of the highly caustic inorganic chemicals, such as sodium hydroxide, employed to break down the pulp. In Chemrec’s reactor design, black liquor and pure oxygen injected in from the top feed an 1,800 °C fireball at the center of the reactor. Most of the dissolved wood in the black liquor forms syngas and flows out of the reactor.

The inorganic chemicals, however, form a molten smelt of sodium sulfide and sodium carbonate on the heat-shielding ceramic tiles protecting the reactor walls. As the smelt flows down and out of the reactor, it attacks the ceramics. “In this contact between smelt and ceramic, reactions occur which alter the surface of the refractory,” says Rudberg. “The key trick is to select materials which can withstand this chemical impact.”

Rudberg says that Chemrec worked closely with researchers at Oak Ridge National Laboratory to identify appropriate materials for testing in a gasification plant that has operated at a Weyerhaeuser mill in New Bern, NC, since 1996. This plant can process up to 15 percent of the mill’s black liquor. Rudberg says that the refractory at New Bern has been operating for two years, which he believes is long enough to prove that its commercialization is viable.

That performance is clearly enough to convince Chemrec’s backers to finance the next step: generating biofuel from the syngas. While Weyerhaeuser simply burns the syngas to generate heat at New Bern, Chemrec’s small research plant in Pitea, Sweden, has demonstrated production of syngas pure enough for catalytic fuel synthesis. BioDME, Chemrec’s EU-funded consortium, will turn that syngas into between four and five metric tons of DME per day.

Another BioDME partner, Haldor Topsoe, will build the DME synthesis plant, to start up in 2010. Göteborg-based Volvo Group (not to be confused with the Ford-owned luxury-car division, Volvo Cars) will adapt the fuel systems of 14 long-haul diesel trucks to run on DME. And Swedish oil company Preem is building four fueling stations to distribute the DME across Sweden.

At the same time, Chemrec is doing the engineering for two plants that would be 25 times larger, producing 40,000 tons of DME each year: one at Pitea, and one at the New Page mill in Michigan. Converting every pulp mill in the United States would, according to Rudberg, generate the equivalent of about 7.5 billion gallons of fuel–about one-fifth of the U.S. government’s total target for 2020.

But it remains questionable whether demand would naturally follow. DME is currently used primarily as a substitute in aerosol spray cans, and clearly more than four fueling stations in Sweden will be needed for it to take off as a biofuel. Marc Londo, a senior research and biofuels expert at the Energy Research Center of the Netherlands, in Amsterdam, says that this chicken-and-egg dilemma is a major drawback. He believes that Chemrec’s success would be better assured if it produced synthetic diesel from its syngas–a strategy pursued by German biomass gasification innovator Choren Industries. “The strong advantage of synthetic diesel is you can simply blend it with currently available diesel,” says Londo. “For bio DME, you need dedicated distribution networks.”

Londo says that synthetic diesel has another advantage: while it costs slightly more to produce from syngas than DME does, synthetic diesel has a higher energy density. A tank of diesel will take a long-haul truck twice as far as a tank of DME: “For long-haul trucks, energy density is a critical factor, and synthetic diesel is thus a more valuable fuel,” he says.

BioDME project leader Per Salomonsson, an R&D manager with Volvo Group, says that it comes down to how much fuel an acre of land will produce. Synthetic diesel would be a lot easier for Volvo Group to drop into its vehicles, but according to their estimates, DME will deliver over 65 percent more miles of travel per acre cultivated; compared with conventional biodiesel produced from vegetal oil, the advantage is five to one. “There will be a shortage of biomass in the future,” says Salomonsson. “In the long run, we can’t afford to have anything but the most efficient process.”

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