"Black Liquor" -- The Source For DMSO -- Which Is The Source Of MSM
Introduction to the Technology
Pulping is the process by which the fibres in the wood are separated and treated to produce pulp. The wet pulp is converted into paper (at an integrated pulp and paper mill) or dried and transported from the pulp mill to the paper mill.
Different pulping processes are used depending on the raw material and end product. The major processes are chemical and mechanical pulping. In mechanical pulping the wood is ground to produce fibres, which are used to make weaker paper types (due to the friction in the pulping), especially newsprint paper.
In chemical pulping the lignin in the wood is dissolved in a digester where the woodchips are cooked. The chemicals can be a mixture of sodium hydroxide and sodium sulphide or sulphite. After a few hours the fibres are separated from the spent pulping liquor (so-called black liquor). The pulp yield is equal to 40-50% (bleached) 50-65% (unbleached) of the incoming wood [Nilsson et al.,1995]. The process chemicals and energy from the spent liquor are recovered.
The black liquor is first concentrated, and subsequently incinerated in so-called recovery boilers, recovering the chemicals and generating steam for the plant (used to produce electricity in a steam turbine plant, with an estimated overall efficiency of 23% [Nilsson et al.,1995]. Generally the heat and electricity production ratio matched demand well. High investments of recovery boilers and increasing electricity demand (and decreasing steam use) has increased the interest in other cogeneration options. Options are fluidized bed combustion or gasification with gas turbine based cogeneration. We will concentrate on the latter because it shows a high efficiency and seems therefore very promising.
Gasification produces a fuel gas with heating values of 3-4 MJ/Nm3 (HHV) using air or 89MJ/Nm3 (HHV) using oxygen as gasifying medium [Grace and Timmer,1995]. The gas should be cleaned to recover inorganic chemicals (e.g. alkalines, H2S) to prevent damage to the gas turbine and reduce emissions. In existing pilot plants the gas is not yet burned in a gas turbine but in steam boilers, because gas clean up technologies are still under development. Besides black liquor, a gasifier could also use wood wastes (bark and other residues) to produce fuel gas. Typically, this is around 5-10% of the pulp wood [Nilsson et al.,1995] and would add 4 GJ per tonne of pulp, to the 20-25 GJ/tonne pulp from the black liquor [Nilsson et al.,1995].
Gasification processes can be divided in low temperature (indirect) and high temperature processes. The problem of low temperature gasification is maintaining the right temperature to reduce tar formation and to avoid agglomeration of bed material. The low temperature processes use a fluidized bed. Solid sodium carbonate is used as the bed material and is precipitated out (and re-used). Two manufacturers are leading the developers of fluidized bed processes, i.e. MTCI (USA) and ABB (Sweden). The MTCI design is an indirectly heated fluidized bed gasifier, and a 50 tonne/day (solids) pilot plant is tested in North Carolina (USA). The ABB design is an air blown circulating fluidized bed gasifier. A 2-4 tonne/day pilot plant has been tested in Sweden since 1991, and 25 tonne/day demonstration unit will be tested in the US [Nilsson et al.,1995].
High temperature processes use an entrained bed gasifier, from which the chemicals are recovered in the smelt (comparable to recovery boilers). The high temperatures lead to higher carbon conversion rates, but may lead to more corrosion. Kvaerner Pulping (Sweden) is the leading developer, and a 75 tonne/day unit is operating at a Swedish pulp mill. A pressurised system (6-7 bar) is in operation in Sweden since 1994 (6-7 tonne/day). Kvaerner Pulping has offered the first commercial sized units (250-300 tpd), and one US order has been received. Tampella (Finland) has limited the research activities to basic research.
Current Market Position
There is currently no market deployment of the technology in the European Union. This is also the case outside of the region where the technology is no further advanced. However the industry is very aware of the potential of the technology and is looking actively at ways to utilise excess black liquor. Should the development be economically attractive, it could have an 80% penetration into the market. There could also be a large export potential for the technology. However it is likely that both Russia and the USA could exploit and export the technology in the longer term.
Future Potential
Implementing a gasifier and combined cycle based cogeneration will increase the electricity production of a pulp mill, making a pulp mill an electricity exporter (baseload). We estimate the savings for gasification of the black liquor and wood wastes of the pulp mill. For the reference situation we assume average 1988 energy consumption (electricity consumption of 800 kWh/tonne pulp, and 15 GJ steam/tonne pulp). For the 2000 situation we use the best 1988 pulp mill (electricity consumption of 710 kWh/tonne and steam use of 12.5 GJ/tonne), while for the 2010 data we assume the model 2000 plant (electricity consumption of 570 kWh/tonne and steam use of 8.0 GJ/tonne), as given by Nilsson [Nilsson et al.,1995].
A gasifier and combined cycle will produce approximately 1800 kWh electricity/tonne pulp and 11 GJ-steam/tonne assuming a kraft pulp mill, using black liquor and the wood residues in the pulp mill [Nilsson et al.,1995]. The prospective energy savings (based on the 1990 production volume) are shown here in a table. A second table. summarises the specific data for black liquor gasification, based on [Larson,1992] and [Nilsson et al.,1995]. It should be stressed that no reliable cost data are available yet.
Chemical pulping in EU member states (1990) [FAO,1995] and estimated annual energy savings (TWh-electricity), using black liquor gasification and CHP. Member states not mentioned do not have chemical pulping capacity installed.
![Chemical pulping in EU member states (1990) [FAO,1995] and estimated annual energy savings (TWh-electricity), using black liquor gasification and CHP. Member states not mentioned do not have chemical pulping capacity installed](../../images/ppblg.gif)
Summary of input data for black liquor gasification and combined cycle-based cogeneration for the ATLAS-project. The energy savings are estimated relative to assumed pulp mill efficiencies for the years. Electricity generation efficiency is assumed to be 40%, and steam generation efficiency is assumed to be 90%. The heat generated from the CHP unit (using the production waste) is consumed in the pulp mill (no heat export).
Competitive Position of EU Countries
It is too early in the development of the technology to consider the export potentials of the technology in detail. However the export potential looks good and Sweden looks set to gain from this development in the longer term.
Market Barriers
It is too early in the development of the technology to have reliable information on this topic.
Technical Development Status
The state of the art of the Swedish technology as in other countries is not further than the basic research or pilot plant stage. It is expected that a full sized pilot plant could be in operation by 2010 subject to the right economic and technical conditions.
Two manufacturers are leading the developers of fluidized bed (low temperature) processes: MTCI (USA) and ABB (Sweden). The MTCI design is an indirectly heated fluidized bed gasifier, and a 50 tonne/day (solids) pilot plant is tested in North Carolina (USA). The ABB design is an air blown circulating fluidized bed gasifier. A 2-4 tonne/day pilot plant has been tested in Sweden since 1991, and 25 tonne/day demonstration unit will be tested in the US [Nilsson et al.,1995].
Kvaerner Pulping (Sweden) is the leading developer for high temperature processes, and a 75 tonne/day unit is operating at a Swedish pulp mill. A pressurised system (6-7 bar) is in operation in Sweden since 1994 (6-7 tonne/day). Kvaerner Pulping has offered the first commercial sized units (250-300 tpd), and one US order has been received. Tampella (Finland) has limited the research activities to basic research.
Current RTD
This technology, which could have uses throughout the paper plants of the European Union and beyond is currently only being developed in Sweden. As a result, the current RTD status reflects the state of the Swedish development.
SE
Currently there is basic research which is building up competence and creating co-operation. Universities and RTD-institutes are involved, industry and government are financing the activities, with involved budget of .33 MECU, to be finished in 1997.
Future RTD Needs
The Swedish developments will continue and cooperation is sought for this work from other companies within the European Union. This cooperation could be obtained with assistance from the EC.
However, all other Member states appear to have no plans (or desire) to help in the development of the technology, so it is recommended that at this stage, initiatives are best taken through national government.
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