Kraft Pulp mill
2.2.2 Consumption and emission levels arising from process units
2.2.2.6 Use of Energy
The major part of heat energy is consumed for heating different fluids and for evaporating water. Heat energy is also used to accelerate or control chemical reactions. Electrical energy is mostly consumed for the transportation of materials (pumping) and for the operation of the paper machine (only in integrated pulp- mills).
The manufacturing of bleached kraft pulp consumes about 10-14 GJ/ADt of heat energy (steam for the production of electrical power not included). The consumption of electrical energy is 600-800 kWh/ADt, including the drying of pulp. The energy consumption for pulp drying is about 25% of the heat energy and 15-20% of the electrical energy. Over 50% of the electrical energy consumption are used for pumping.
The energy consumption depends on the process configuration, process equipment and process control efficiency.
Table 2.23 below summarises reported average energy consumption as heat (excluding that for back-pressure power generation) and electric power for some non-integrated and integrated pulp mills.
Pulp and paper grade Process heat [GJ/t]
Electric power [kWh/t]
Non-integrated production of bleached kraft pulp - whereof external supply
14.4 1.2
760 0 Unbleached kraft pulp production with integrated
production of paperboard - whereof external supply
16.4 1.5
959 388 Bleached kraft pulp with integrated production of
uncoated fine paper - whereof external supply
17.5 3.5
1218 706 Table 2.23: Average energy consumption in Swedish pulp and paper mills in 1995 [SEPA report 4712]
From available data for Swedish mills in Table 2.23 to Table 2.29 [SEPA report 4712] energy consumption and energy balances for a number of different mills are summarised per sub-process. Thus, the main energy consuming process stages can be identified. The figures in Report 472 represent a modernised mill like a mill built in the 1970s and since then modernised.
A newly built or recently modernised existing mill, would most probably have lower values.
Department Process heat
[MJ/t]
Electric power [kWh/t]
Wood handling 200 45
Cooking 1700 64
Washing and screening 0 45
Evaporation 4000 21
Recovery boiler 600 48
Auxiliary boiler 0 23
Causticizing 0 14
Lime kiln 1500 7
Miscellaneous, pulp mill 2600 133
Total pulp mill 10600 400
Stock preparation 0 200
Paper machine 5800 350
Total paper mill 5800 550
Effluent treatment 0 9
Total per tonne of paper 16400 959
Table 2.24: Average energy consumption for the production of 243000 ADt/a unbleached Kraft pulp, and integrated production of 250000 t/a paperboard
Department Process heat [MJ/t]
Electricity [kWh/t]
Pulp mill
Recovery boiler, process steam +14500
Auxiliary boiler, process steam (only own bark)
+2050
Turbine generator -2050 +571
External supply (heat for lime kiln) +1500 0
Consumption (including lime kiln) -10600 -400
Effluent treatment 0 -9
Excess energy from pulp mill +5400 +162
Paper mill
Consumption -5400 -550
External supply 0 +388
Total external supply 1500 388
Table 2.25: Energy balance for the production of 243000 ADt/a unbleached Kraft pulp, and integrated production of 250000 t/a paperboard
Department Process heat
[MJ/ADt]
Electric power [kWh/ADt]
Wood handling 150 55
Cooking 2050 65
Washing and screening 0 55
Oxygen delignification 400 45
Bleaching 500 83
Bleach chemical preparation 70 6
Bleached stock screening 0 40
Pulp drying 2850 105
Evaporation 4100 30
Recovery boiler 610 60
Power boiler 0 30
Causticizing 0 20
Lime kiln (direct heat) 1500 10
Miscellaneous, pulp mill 2170 136
Total pulp mill 14400 740
Effluent treatment 0 20
Total per ADt of pulp 14400 760
Table 2.26: Energy consumption for non-integrated 250000 ADt/a bleached Kraft pulp
Department Process heat
[MJ/ADt]
Electric power, [kWh/ADt]
Recovery boiler, process steam +17500
Power boiler, process steam (only own bark) +3000
Turbine generators -2600 +650
External supply +1200 0
Consumption (including lime kiln) -14400 -630
Effluent treatment 0 -20
Excess energy from pulp mill (including waste heat) +4700 0
Total external supply 1200 0
Department Process heat [MJ/t]
Electric power [kWh/t]
Wood handling 230 46
Cooking 1800 55
Washing and screening 0 46
Oxygen delignification 400 38
Bleaching 500 70
Bleach chemical preparation 70 5
Bleached stock screening 0 34
Pulp drying 0 0
Evaporation 3600 25
Recovery boiler 600 51
Power boiler 0 25
Causticizing 0 17
Lime kiln 1300 8
Miscellaneous. pulp mill 1900 115
Total pulp mill 10400 535
Stock preparation 0 250
Paper machine 7100 420
Total paper mill 7100 670
Effluent treatment 0 13
Total per tonne of paper 17500 1218
Table 2.28: Energy consumption for an integrated bleached Kraft mill with 250000 t/a of surface-sized uncoated fine paper.
Department Process heat
[MJ/t]
Electric power [kWh/t]
Pulp mill
Recovery boiler, process steam +13800 Power boiler, process steam
(Only own bark)
+2300
Turbine generator -2100 +512
External supply (incl. lime kiln) +1300 +36
Consumption -10400 -535
Effluent treatment 0 -13
Excess energy from pulp mill +4900 0
Paper mill
Consumption -7100 -670
External supply +2200 +670
Total external supply 3500 706
Table 2.29: Energy balance for an integrated bleached Kraft mill with 250000 t/a of surface-sized uncoated fine paper
As shown in the tables above chemical pulping plants are energy-intensive installations that consume high amounts of energy but at the same time produce steam and electrical power on site by use of regenerative fuels. Thus, modern non-integrated kraft pulp mills are energy self-sufficient mainly because of efficient energy recovery by burning 50% of the incoming wood in the recovery boiler (strong black liquor) and the use of bark as auxiliary boiler fuel.
Furthermore, secondary energy from different process steps can be recovered as warm and hot water (40-80o C). Fossil fuels are mainly used as support fuel (e.g. oil in the lime kiln).
The conventional power plant configuration in a non-integrated kraft pulp mill is a recovery boiler and a bark boiler feeding a back-pressure turbine with intermediate steam tapping and, possibly, condensing. The recovery boiler acts as a power station where the strong black liquor is burnt and generated heat is utilised for the generation of high-pressure, superheated steam.
Part of the energy content in the high-pressure steam is utilised for generating power in a back-pressure turbine. Medium-back-pressure steam extracted from the turbine and low-back-pressure exit steam are utilised for covering the heat energy demand in the kraft pulp process. The ratio of electrical effect/heat effect is, usually, 0.2 - 0.3 [Finnish BAT report, 1997].
Malodorous gases are collected and burnt, mainly for environmental protection reasons, as there is no/little net energy generation.
In an integrated pulp and paper mill the excess heat produced by the pulp mill is not quite enough to cover the energy consumption of the paper production. The additional demand for heat has to be produced in wood/bark boilers and auxiliary boilers. Fossil fuel is used as support fuel in bark and sludge boilers and as the main fuel in auxiliary boilers as well.
Peaks in energy consumption are often satisfied by using a small boiler for fossil fuel.
Combined heat power plants (CHP) based on gas turbine in combination with a steam boiler and steam turbine are very efficient and a few installations have been made in Europe. CHP plants are used when the need for electrical energy at a paper mill is high because the electrical effect/heat effect ratio is usually 0.8 - 0.9 [Finnish BAT report, 1997].
In the following some additional figures of energy consumption of single process stages of pulp mills are compiled:
- De-icing will consume about 30 MJ of heat as hot water or steam per m3 of wood processed.
- The total energy consumption in debarking is 7-10 kWh/m3 of wood [Finnish report]
- The energy for pulp drying (only market pulp) can be of the order of 3 GJ/tonne of pulp or some 25% of the total heat requirement for a kraft pulp mill and 15-20% of the electrical energy.
- The average electrical energy consumption in the manufacture of bleaching chemicals is presented in Table 2.30.
Chemical and (bleaching stage code) Electrical energy consumption [kWh/kg chemical]
Chlorine dioxide (D) 10
Oxygen (O) 0.4 Ozone (Z) 10
Peroxide (P) 3.5
Alkali (E) 1.6
Table 2.30: Average electrical energy consumption in the manufacture of bleaching chemicals, [Finnish report, 1997]
- External treatment of wastewater normally consumes energy [the following figures are compiled from Finnish report, 1997]. Anaerobic treatment is an exception if the energy content in the generated biogas is recovered by burning. The consumption of electrical energy in activated sludge treatment is in the range of 1.2-2 kWh/kg of reduced BOD (aeration and pumping), which is the equivalent of 1 -1.5 kWh/ m3. The energy consumed in filtration is dependent on the drop in pressure across the medium. For example, the ultrafiltration of coating wastewater consumes 3 - 5 kWh/m3. Evaporation at low pressure using mechanised vapour
2.2.2.7 Noise (local)
The debarking plant generates noise that can be controlled by carefully insulating the debarking house. There are several other sources of noise like chipping, fans, engines, stacks and steam vents. Trucks and other vehicles used at the plant can cause noise in the vicinity of the mill.
From two examples of Swedish pulp mills it can be assumed that the noise level about 500 m from the middle of the pulp plant can be about 50 dB (A) over night and at about 2 km away it can be about 45 dB (A). The guidelines for all existing industries in Sweden is < 45 dB (A) over night at the closest house where people live. This may be difficult to achieve if houses are located very close to the mill as for example within a radius of < 500 meter.