I have been asked by some of the readers of this website to comment on water and energy benchmarking and to provide some data. Benchmarking is the comparison of a mill’s water and energy consumptions with reference to its competitors or to a model mill with modern technology. This provides opportunities for optimizing water and steam consumption in a mill. Water and energy reduction strategies have been studied by the research community and practiced by mills for the past several years. Factors driving these measures are usually economics and/or regulatory compliance.
Water:
A survey done in 1996 for US and Canadian mills provided data on total water consumption for different types of mills, giving a range of 47-142 m3/adt for bleached kraft mills [1]. Since then a lot of measures have been employed to reduce water consumption. In 1999, the approximate flow of effluent from a modern ECF mill was estimated to be between 13 to 22 m3/adt (10-15 m3/adt from bleaching), whereas that of a TCF mill was between 8 to 17 m3/adt (5-10 m3/adt from bleaching). The main difference is that the alkaline filtrates are more frequently recovered from TCF mills. These low effluent flows indicate that the cooling water and process water are segregated in these mills. The first effluent-free bleach plant based on the evaporation of Q-stage filtrate started in 1997 in Sweden. In this case, the recovery boiler was used to burn the concentrate.
A model reference mill (using best available technologies in the year 2000) was projected to have a total effluent flow of 15 m3/adt, of which 11 m3/adt was attributed to the bleach plant [3]. This mill has dry debarking, continuous cooking, 80% black liquor solids to the recovery boiler, two-stage oxygen delignification, extensive washing, a bleaching sequence of Q(OP)(DQ)(PO) (or TCF alternative of Q(OP)(ZQ)(PO)), and alkaline filtrate recycle to brownstock washing. These measures, taken to reduce water consumption in the pulp mill, generally increase the solids loading of the recovery boiler. Future mills, in which the filtrates are evaporated and the condensates are re-used, will have a total effluent flow of 2 to 7 m3/adt.
Energy:
Data regarding energy consumption are collected based on purchased, self generated and sold energy, as well as the mill’s pulp and paper production. The energy intensity of a mill is the total energy consumed divided by the total production. In order to identify opportunities for energy savings, benchmarking should be done for specific process areas in a mill. For example, a mill producing kraft pulp will have process and energy conversion areas. Process areas cover wood room, pulping, bleaching, evaporation, recausticizing, pulp machine and effluent treatment. Energy conversion areas include power and recovery boilers, turbines and deaerators.
Technology descriptors are defined to account for energy use by different technologies. For example, in kraft pulping, the energy use varies with the pulping method (i.e., continuous, batch or M&D digesters). In a 2006 paper [4], data were collected from 49 mills on fibre and energy inputs and outputs. These were then allocated to the process and energy conversion areas. These data showed that, for bleached kraft market pulp, the fuel consumption (including spent pulping liquor and fossil fuel) was between 22-42.5 GJ/adt and the thermal energy (defined as steam used less condensate returned) consumption varied between 13.7-24.1 GJ/adt. For the production of newsprint from TMP, the electricity consumption was about 2.3-3.0 MWh/adt and the net thermal energy consumption was about 1.9-7.4 GJ/adt.
In terms of selected process areas in kraft pulping, the median thermal energy consumption was 2.43 GJ/odt for continuous digesters, 5.03 GJ/odt for indirect contact evaporators and 2.57 GJ/odt for softwood bleaching. Median thermal energy consumption for the paper machine varied according to the product: 5.36 GJ/adt for newsprint, 6.21 GJ/adt for uncoated GW, and 9.10 GJ/adt for kraft papers.
To start benchmarking, a database of water and energy consumption information from similar mills is required. A mill wide water and energy (steam generation and distribution) audit can be used to construct a base case material and energy balance for a specific mill. This is important, as measures taken to reduce water could impact energy balances and vice versa. Once a model is validated with the mill data, it is easy to study what-if scenarios to see, for example, what can be done to decrease water consumption in a specific area, and what the effect may be on steam use. Pinch analysis is used to optimize steam consumption.
References:
1. Bryant, P.S., et al, Pulp and paper mill water use in North America, Tappi 1996 International Environmental Conference, Orlando, FL, Book 2, pp. 451-460, (1996).
2. Henricson, K., et al, Steps towards minimum impact mill – Mill case examples, 27th EUCEPA Conference – Crossing the millennium frontier, emerging technical and scientific challenges, Grenoble, France, pp. 143-148, (1999).
3. Axegard, P., The Ecocyclic pulp mill- Prospects for closure and energy efficiency, 2000 Japan Tappi annual meeting and Pan Pacific Conference Proceedings, Tokyo, pp. 129-134, (2000).
4. Francis, B., et al, Benchmarking energy use in pulp and paper operations, PAPTAC, 92nd Annual meeting reprints, Book A. pp. 55-61, (2006).
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