2.D.1 Pulp And Paper

Last updated on 10 Dec 2013 11:53 (cf. Authors)

Pulp and paper (2.D.1)

The fibre for paper production is produced, via chemical or mechanical processes, either from fresh fibre or from processed recycled paper. A distinction is made between integrated and non-integrated pulp and paper mills. Non-integrated pulp mills solely produce pulp for sale on the open market. On the other hand, integrated mills produce both pulp and paper, at integrated sites. A paper mill can either produce paper from fibre material produced at other locations or be integrated within complete pulping processes set up at one site.
Sulphate pulp mills normally operate in both integrated and non-integrated modes, whereas sulphite pulp mills are normally only integrated – i.e. part of paper-production chains. Mechanical pulping and recycled fibre processing is usually an integrated part of the papermaking but has become a stand alone activity in a few single cases.

Fibre production processes
In the chemical pulping process the fibres are liberated from the wood matrix as the lignin is removed by dissolving in the cooking chemical solution at high temperature.

Sulphate process
The sulphate or kraft process is the world's most common pulping process, since it yields higher pulp strengths and can be used with all types of wood. In the kraft pulp process the active cooking chemicals (white liquor) are sodium hydroxide (NaOH) and sodium sulphate (Na2S). The term "sulphate" is derived from the make up chemical sodium sulphate which is added in the recovery cycle to compensate for chemical losses. In the two German plants, carbonate is extracted from the circulation of liquor via bonding with calcium (causticising) and then, in a separate lime oven; the burnt lime is then reused for causticising.
This process produces atmospheric emissions in liquor recovery (boilers), in bark combustion, from lime ovens, in wood-chip storage, in pulp digestion, in pulp washing, in bleaching, in bleach-chemical processing, in evaporation, in sorting and washing, in processing of circulating water and in operation of various types of tanks. Such emissions include fugitive emissions that occur at various processing points – primarily in (liquor)-recovery boilers, lime ovens and auxiliary boilers. The main components of emissions include nitrogen oxides, sulphur-containing compounds, such as sulphur dioxide, and foul-smelling reduced sulphur compounds.
The two German sulphate-pulping plants are fitted with a system for post-incineration of foulsmelling sulphur compounds and with systems for NOx-reduced combustion in recovery boilers (>20 % NOx reduction; figures of the German Pulp and Paper Association (VDP, September 2004).

Sulphite process
Sulphite pulp is produced in 4 of 6 installations in Germany. In such plants, pulping is based on the use of aqueous sulphur dioxide (SO2) and a base - calcium, sodium, magnesium or ammonium. The sulphate process and the sulphite process have numerous similarities, including similarities with regard to possibilities for using various internal and external measures to reduce emissions. From the standpoint of environmental protection, the main differences between the two pulp-production processes have to do with chemical aspects of the boiling process, with aspects of preparation and post-processing of chemicals and with bleaching intensity – bleaching in sulphite plants is less intensive, since sulphite pulp is whiter than sulphate pulp.
Atmospheric emissions occur especially in recovery (boilers) and in bark combustion.
Waste-gas emissions with less-concentrated SO2 are released in washing and sorting processes, and they are released by ventilation shafts of evaporators and by various tanks. Such emissions escape – in part, as fugitive emissions – at various points of the process.
They consist primarily of sulphur dioxide, nitrogen oxides and dust. A number of measures are available for reducing consumption of fresh steam and electrical energy and for increasing plant-internal generation of steam and electricity. Sulphite pulp mills can generate their own heat and electricity by using the thermal energy in concentrated liquor, bark and waste wood. Integrated plants require additional amounts of steam and electricity, however; these additional amounts can be generated in either in on-site facilities or at off-site locations. Integrated sulphite pulp and paper mills consume 18 - 24 GJ of process heat, and 1.2 - 1.5 MWh of electrical energy, per tonne of pulp.
All four sulphite pulping plants in Germany are operated with SO2 scrubbers fitted downstream from recovery boilers (>98 % SO2 reduction). One plant is fitted with equipment for NOx-reduced combustion in recovery and auxiliary boilers (total of >40 % NOx reduction, loc. cit.).

Wood pulp
Wood pulp is produced in 8 plants in Germany. In mechanical pulping, wood fibres are separated from each other via mechanical energy applied to the wood matrix. This process is designed to conserve most of the lignin in the wood, in order to maximise yields while ensuring that the pulp has adequate strength and whiteness. Two main processes are differentiated:

  • The wood-grinding process, in which pieces of wood are are wettened and pressed against a rotating grinder, and
  • The refiner process, in which wood chips are broken down into fibres in disk refiners.

Wood-pulp properties can be influenced by increasing the process temperature and, in the case of the refiner process, by chemical pre-treatment of the wood chips. The pulping process in which wood is chemically pre-softened and then broken down into fibres, under pressure, is known as chemical-thermal-mechanical pulping (CTMP).
In most cases, the waste-gas emissions consist of emissions from heat and energy generation in auxiliary boilers and of emissions of volatile organic carbon (VOC). VOC emissions occur in storage of wood chips, in removal of air from containiners for washing wood chips, as well as from other containers. They also occur in connection with condensates that are produced in recovery of steam from refiners and contaminated with volatile wood components. Some of these emissions are released as fugitive emissions, from various parts of mills.

Paper and carton production
Paper is made from fibre materials, water and chemical additives. The entire paper-making process consumes large amounts of energy. Electricity is required primarily for operation of various motors and for grinding of fibres. Process heat is used primarily for heating water, other liquids and air, for evaporating water in the drying process of paper machines and for converting steam into electrical energy (with heat/power cogeneration). Large amounts of water are required as process water and for cooling. Various additives are used in paper production, as process aids and to enhance product properties (paper additives).
Most of the waste-gas emissions produced by non-integrated paper mills are produced by steam-production and energy-generation systems. The boilers used in such systems are standard boilers that do not differ from those of other combustion systems. It is assumed that such systems are operated in the same manner as other auxiliary boilers of the same capacity.
Overall, most product-specific waste-gas emissions are site-dependent (for example, they depend on the type of fuel used, the size and type of the relevant facility, whether the plant is integrated or non-integrated, whether it generates electricity). The auxiliary boilers used in Germany cover a wide spectrum of different sizes (from 10 to more than 200 MW). With smaller boilers, the only useful approach is to use low-sulphur fuels and the pertinent combustion technologies, while secondary reduction measures can also be effective with
larger boilers.

Methodological issues

The pulp and paper industry produces no process-related emissions of climate gases within the meaning of the IPCC Good Practice Guidance (IPCC, 2000). For other gases, the IPCC-Guidelines default emission factors were used until the 2004 report year.
Since 2005 real emission factors from German plants for pulp production are available. (German contribution to revision of the BVT information sheet (Merkblatt) for the pulp and paper industry, 2007).

Source-specific recalculations

A range of measures in sulphite pulp production, carried out on a continual basis, led to reductions of SO2 emissions. For this reason, linear interpolation was carried out between the default value for 1990 and the real, plant-based value for 2005.

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