Energy in the Pulp and Papermaking Process
07 September 2017 | Web Article Number:
Energy is required in various forms to turn a tree into paper. In some cases, both fossil fuels (petrol, diesel, gas, oil and coal) and renewable fuels (biomass and black liquor) are used to power these processes.
Black liquor - a by-product from digesting pulpwood chips in the chemical pulping process - is a mixture of spent cooking chemicals and dissolved wood solids. This is concentrated during the chemical recovery process to yield a fuel which is rich in organic material.
The black liquor – regarded as renewable and carbon neutral - is then used to produce energy. Some mills generate all or some of their own electricity by way of condensing power generation and co-generation.
Energy is used for direct process heating and the generation of steam which is the main heating medium in the papermaking process. Steam is also used for energy-efficient electricity generation.
Condensing is the term used for the electricity generation process typically employed by conventional coal-fired power stations. As the industry uses a combination of fossil-based and renewable fuels, the energy produced has a lower carbon impact than power production relying exclusively on coal. Co-generation refers to the generation of electricity from steam created as a by-product of the papermaking process.
Co-generation is the industry’s main method of generating electricity. Steam produced by boilers and furnaces is typically at a temperature and pressure which is too high for use in the pulp and papermaking process. This high-pressure steam is passed through a back-pressure turbine where it expands, thereby spinning within the turbine which generates electricity.
Co-generation offers a number of advantages over condensing power generation:
- A greater portion of the input energy from the base fuel can be used in the production process;
- GHG emissions attributed to generated electricity are significantly lower; and
- Water consumption attributed to electricity generation is almost negligible.
Electricity derived through co-generation (using coal only) has been approximated to have less than half the GHG impact of electricity imported from the national grid (mostly from coal-fired sources). Co-generated electricity derived from gas (and not only coal) has an estimated smaller carbon impact of approximately 25% of the impact of electricity imported from the national grid.
Since there is very little water loss attributable to electricity generated via co-generation, this further implies that this method of power generation is a suitable option, within the water scarce context of South Africa. It is important to note however, that although the pulp and papermaking industry does generate a significant portion of its own electricity, it presently remains reliant on the national grid for the balance of its power needs.