Black liquor gasification

The other alternative for separating lignin is to use it as a raw material for fuel applications, different material solutions and chemicals. As lignin is one of the most energy-rich com-pounds of wood, its use in fuel applications offers an interesting opportunity. The current pulp mill produces most of its energy in the recovery boiler. If lignin is extracted to reduce the load on the recovery boiler and there is no energy surplus in the mill, lignin can be burnt at the mill in, for example, a power boiler, to produce steam and electricity. Another internal utilization method is to replace fossil fuels in the lime kiln for firing lignin. The other fuel application is to export lignin from the mill if there is an energy surplus. Lignin can be co-fired with other fuels, either pellets, wood chips, coal and oil in CHP plants or in oil condensing plants. There is also an opportunity to process kraft lignin into more valu-able fuel sources with fast pyrolysis, pyrolysis or gasification. Material applications that have been researched include the development of polymeric materials using lignin as a raw material, the production of a thermoplastic material consisting of 85% softwood kraft lignin and the production of carbon fibre. (Öhman 2006, 20-21.)

3.2.2 Black liquor gasification

Black liquor is typically burnt in a recovery boiler, the operating principle of which is de-scribed in Section 3.1.1. Black liquor gasification (BLG) is an alternative process to recover chemicals and produce energy. The BLG can be carried out with almost unpressurized steam or with pressurized air or oxygen. The gasification processes can also be divided ac-cording to the operating temperature. The operating temperature determines the form in which most of inorganic compounds leave the reactor. In a high temperature gasifier, the temperature is 950°C or higher. The function is based on dust gasification. The inorganic compounds are in a melted form when leaving the reactor. The low-temperature gasifiers are instead based on fluidized bed gasification. The temperature is 700°C or lower and the inorganic compounds are in solid form when leaving the reactor. (Hepola & Kurkela 2002, 20.)

The BLG has many advantages compared with the conventional recovery boiler. It is possi-ble to improve the power-to-heat ratio (electric power/heat power) of energy production.

The BLG-IGCC process can reach a power-to-heat ratio of 0.70 and in later second-generation plants even a power-to-heat ratio of 0.83. The other advantages of BLG com-pared with a recovery boiler are (Hepola & Kurkela 2002, 21):

- 5-10% higher heat production efficiency - lower NOx, sulphur, SO2 and CO2 emissions - improved safety

- possibility to raise the fibre yield by approximately 2-4% because of better sulphur recovery.

Although the BLG power plant has been estimated to cost 30% more than the ordinary process, the cost of electricity production is lower than for a recovery boiler. The BLG process in the integrated pulp and paper mill may reduce heat production, and this has to be taken into account. The gasification enables intensification of chemical circulation without the investment in a new recovery boiler however. The profitability of gasification therefore has to dissect the view of the missing heat production costs. (Hepola & Kurkela 2002, 21.)

Research into BLG has been under way for decades in Finland, especially in 1989-1992 when BLG was studied at the pilot plant Äänekoski. Since then, the examination has con-tinued on smaller scale pilot plants. (Hepola & Kurkela 2002, 22.) Nowadays, the most promising technology is the Swedish Chemrec’s atmospheric and pressurized BLG tech-nology. A new development plant was started up in 2005 in Piteå, Sweden. Figure 9 shows a drawing of the development plant. The plant consists of the reactor with the quench cooler, gas cooler and heat exchanger for cooling hot green liquor. The black liquor and oxygen are fed into the reactor from the top. The temperature of the reactor is just above 1000°C. The residence time in the reactor is about 5 seconds. The generated gas and melted inorganic salts are quenched by water spray. Salts separated by gravity are felt in the quench bottom and form green liquor. The generated raw gas is led into a countercurrent gas cooler. The cooled raw gas has to be refined in the H₂S-absorption unit because it

con-tains 1.4-2.5% vol of H2S. In a commercial-scale BLG plant, some structural changes would be necessary. (Lindblom & Landälv 2007.)

Figure 9. The principal units of the oxygen-blown, pressurized black liquor gasification development plant 1 (DP1) in Piteå, Sweden. The plant consists of the reactor with the quench cooler, gas cooler and heat

ex-changer for cooling hot green liquor. (Lindblom & Landälv 2007.)

The BLG process produced syngas that can be used for electricity production in a cogenera-tion unit or in the produccogenera-tion of motor fuels. Excess heat, which is produced in both proc-esses, is used in steam production. Part of this steam is used internally in the BLG plant, but both processes have a large steam surplus that can be used at the pulp mill. It has to be taken into account, however, that less steam is produced in the BLG process than in a con-ventional recovery boiler. If the produced steam cannot cover the mill steam demand, the additional steam can be produced in a CHP plant by burning biomass. Figure 10 shows an overview of the main energy and material streams in a possible future pulp mill with BLG.

(Pettersson & Harvey 2009, 2.)

Figure 10. Main energy and material streams in a market pulp mill/integrated pulp and paper mill. The pro-duced steam from BLG plant can be utilized at the pulp mill. (Pettersson & Harvey 2009, 2.)