Taking Pulp to the Pump

Gasifying black liquor from pulp mills will accelerate second-generation biofuels.

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. Credit: Chemrec

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.

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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."