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Benefits of Black Liquor Gasification

 

An External Benefits Study of Black Liquor Gasification


Background

 

In the process of breaking down wood into fiber for making paper products, approximately half of the mass of the wood is converted into usable fiber. The other half, along with an equal amount of spent caustic cooking chemicals, forms a byproduct known as black liquor. The organic content is sufficient to make it a low-grade fuel. The current practice is to burn the liquor in specialized boilers to recover energy in the form of steam, and to recover the cooking chemicals in the form of molten salt. After a substantial amount of re processing, the salt is returned to a form where it can be reused in the cooking process. The steam can be used elsewhere in the mill or in a steam turbine to produce electricity.

Approximately 240 million tons of black liquor are produced each year in pulp production world wide (a). The value of the recovered caustic (sodium hydroxide) is about US$26 Billion. The fuel value is equivalent to about 460 million barrels of crude oil, which is worth about US$8.3 Billion. The sum of these values is US$34.3 Billion. The corresponding pulp produced is worth about US$74 Billion.

Recovery boilers have been in use since the 1930's. They have become progressively more sophisticated over the decades, but they are still quite thermally inefficient compared to coal or gas fired power producing boilers. Clearly, incremental improvements in efficiency will lead to significant cost savings.

Gasifying the black liquor resolves a number of problems while introducing some synergistic benefits as well. The first is in the generation of electricity. Gasification is effectively partial oxidation of the hydrocarbons in any given fuel. The product gas (or syn gas) is a mixture of CO2, CO, H2O, and H2, which, after suitable polishing, can be burned in a gas turbine to produce electricity. The hot exhaust gas from the turbine is then passed through a heat exchanger to produce steam for a power producing steam turbine. The use of both types of turbines is known as combined cycle operation. Since gas turbines are thermally more efficient than steam turbines, gasification combined cycle operation can generate more electricity than combustion given the same fuel. In the case of black liquor, the increase is potentially sufficient to make an integrated pulp and paper mill into a net exporter of electricity.

Another benefit is in the inherent separation of sulfur and sodium during BLG. In the conventional recovery boiler, all of the sulfur leaves with the molten smelt as Na2S. In BLG, the gas will contain most or all of the sulfur (depending on the particular gasification process chosen), while the sodium leaves with the smelt phase. There are a number of advanced variants to the kraft pulping process which will increase pulp yield but require varying degrees of sulfur/sodium separation. Since this is a natural consequence of BLG, it lends itself to implementation of these high-yield processes:

- Polysulfide/Enhanced Polysulfide offers as much as 4% increase in bleached pulp
- Split Sulfidity offers a 1-2% increase
- Alkaline Sulfite Anthroquinone offers as much as 8% on linerboard and 4% on bleached pulp.

A third benefit of BLG is the potential for in situ causticization within the gasifier. In the conventional recovery cycle, sulfate must be reduced to sulfide, and carbonate must be causticized to hydroxide. The sulfur reduction occurs in the char bed in the boiler, but the caustization is a separate (and costly) step known as the lime cycle. The sodium carbonate is reacted with calcium oxide to produce sodium hydroxide and calcium carbonate. The calcium carbonate is then burned in large rotating kilns to form carbon dioxide and calcium oxide. Lime kilns have been used by the paper industry since the 1920's. A 1000 ton per day pulp mill will use about 100,000 barrels of fuel oil per year to fire its lime kiln. Through novel chemistries it may be possible to carry out the causticization reactions directly within a black liquor gasifier. This would eliminate the need for the lime cycle and the associated fossil fuel to fire it.

Other incentives for BLG include reduced emissions of sulfur dioxide, carbon monoxide, VOC's, and particulates. There is also the safety aspect of eliminating smelt-water explosions through the elimination of the char bed itself.

Several techno-economic and feasibility studies [1]-[8] reach a general consensus that the capital costs of a BLG Combined Cycle (BLGCC) operation are much higher than a conventional recovery boiler yet electric power generation more than doubles. Since more than 125 recovery boilers in North America will exceed useful life in the next 10-15 years, these boilers can be replaced with more profitable gasifiers if well-identified technical hurdles can be overcome in the interim. However these studies have focused on the cost savings and increased power generation potential of BLG. To date there has not been a thorough assessment of the overall benefits and drawbacks of BLG on the community. Such a study is proposed here.

(a) Production estimates for the U.S. alone were not immediately available at the time of this writing.

Goal

The overall goal would be a comprehensive study of the external benefits/impacts of black liquor gasifiers replacing conventional recovery boilers in the pulping industry. Previous studies have focused only on direct cost savings as an accounting exercise - industry feasibility per se. This study would elucidate the broader societal, environmental, labor, and energy related benefits of BLG.

References

[1] Larson, E.D., McDonald, G.W., Yang, W., Frederick, W.J., Iisa, K., Kreutz T.G., Malcolm, E.W., Brown, C.A., "A Cost-Benefit Assessment of Black Liquor Gasifier/Combined Cycle Technology Integrated into a Krafty Pulp Mill", International Chemical Recovery Conference, Tampa, FL, 1998

[2] Larson, E.D., Raymond, D.R., "Commercializing Black Liquor and Biomass Gasifier/Gas Turbine Technology", TAPPI J., 12, 1997 p50

[3] McKeough, P.J., Arpiainen, T., Makinen, T., Solantausta, Y., "Black Liquor Gasification: Down-stream Processes, Plant Performances and Costs", International Chemical Recovery Conference, Toronto, CA 1995

[4] Isaksson, A., Andren, M., Ahlroth, M., Yan, J., Svedberg, G., "Energy Consequences when Integrating a Black Liquor Gasifier into a Pulp Mill"

[5] Brooks, T.B., Marcinek, F.T., "Emerging and Underutilized Technologies to Increase Kraft Mill Production", TAPPI Engineering Conference, 1996

[6] Berglin, N., Andersson, L., "Process Integration of Black Liquor Gasifiers for Incremental Recovery Capacity", International Chemical Recovery Conference, Whistler, BC, 2001

[7] Nasholm, A.S., Westermark, M., "Energy Studies of Different Cogeneration Systems for Black Liquor Gasification", Energy Convers. Mgmt. 38:15-17, p1655, 1997

[8] Grace, T.M., Timmer, W.M., "A Comparison of Alternative Black Liquor Recovery Technologies", International Chemical Recovery Conference, Toronto, Ontario, CA 1995

[9] Ryham, R., "Black Liquor: A Man-Made Fuel", International Chemical Recovery Conference, Whistler, BC, 2001

 
 

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