Black liquor greening the paper industry
Chemical pulping is an important part of the process of turning timber into paper. Wood chips are digested in a pulping liquor of sodium sulphide and sodium hydroxide. Once the wood fibres are separated, the remaining black liquor is concentrated and then incinerated in recovery boilers, generating steam that produces electricity. The amount of electricity produced from the black liquor biofuel depends on several parameters. As the pulping industry has a tradition in Europe, particularly in Scandinavia, it was natural for European researchers to endeavour to improve upon the current state-of-the-art. One problem is the sulphur content of the black liquor. The sulphur not only corrodes the equipment, but also ends up being emitted into the atmosphere, where it is converted into sulphur dioxide, an environmental pollutant. In the framework of an EESD Programme project called EFPRO, a new pulping process was designed to produce sulphur-free black liquor. Analysis of the new black liquor in the laboratory revealed enhanced energy-producing potential. Pilot tests did not reveal any difficulties with the evaporation phase, which concentrates the black liquor prior to combustion. Continuing with a full lifecycle approach, the EFPRO partners played with different recovery boiler configurations. Following on lessons learned in other kinds of boiler applications, the energy efficiency was increased by tweaking the process temperature. Further gains were made by adopting an Integrated Gasification Combined Cycle (IGCC) process. EFPRO demonstrated that energy production from pulp mills could be improved while at the same time reducing the sulphur content of the black liquor. Various parts of Europe could benefit from the increased production of green electricity deriving from these results.
