Today and even more so in the future, the pressure to reduce energy consumption will come from the twin drivers of improving cost competitiveness and ensuring market acceptance of pulp and paper products from an environmental viewpoint. Awareness of the energy issue has risen significantly since Canada signed the Kyoto accord in December 1997, in which the Canadian government committed to a 6 per cent reduction of greenhouse gas emissions (GHGs), relative to 1990 levels, by 2010 to 2012. Governments and industry in North America and Europe are looking for strategies and types of policies and measures that ought to be adopted to address these emissions. A large part of the answer must lie in fuel substitution by biomass and in the industry making better use of the fossil fuel and electricity it does consume.
In our industry a number of initiatives are already in place. Eco-labelling in Europe has been developed in order to address issues such as the efficient use of resources, including energy and the minimization of greenhouse gas emissions. At home, the Canadian Industry Voluntary Challenge and Registry (VCR) and the Canadian Industry Program for Energy Conservation (CIPEC) aim at reduced greenhouse gas emissions through voluntary action undertaken by individual companies.
Such well publicized initiatives complement the perhaps less visible activities of the Canadian Pulp and Paper Association (CPPA) and the Pulp and Paper Technical Association of Canada (PAPTAC) in encouraging energy efficiency among their member companies. Many of these activities are long-standing, dating from the oil crisis of the 1970s, and include: the issue of quarterly energy consumption questionnaires and compilation of the annual Energy Monitoring Report; the activities of the Energy Committee, which include organizing the Energy Conservation Oppor-tunity awards; and the co-ordination of the Environmental Profile Data Sheets for products of CPPA member companies, which provide audited information on resource consumption including energy.
The convergence of several issues now emerging will shape how mills view and manage energy costs:
- The move toward gas and electric utility deregulation, combined with the possibility of long-term energy prices rising increase uncertainty;
- The advantage of a low energy bill in an increasingly competitive marketplace;
- Control of greenhouse gas emissions whether in the form of regulations, taxes or voluntary action.
For these reasons there seems to be a growing recognition that proactive management of energy costs, minimizing fossil fuel use, and energy efficiency must be part of company strategy over the long term.
This article draws upon the Guide to Energy Savings Opportunities in the Kraft Pulp Industry [1], which PAPTAC published in December 1999 and distributed to mills across Canada. The PAPTAC Energy Committee supported the development of this energy conservation guide limited to kraft pulping, and a contract was awarded in August 1997, with funding provided by Natural Resources Canada.
The Guide is designed to give the mill engineer a practical, step-by-step approach to improving the energy efficiency of kraft pulp mills. The key elements of the recommended approach comprise in-depth energy audits, benchmarking against existing and new mills, gathering energy savings ideas, assessing the degree of thermal integration of the process, evaluating impacts on mill systems, and projecting energy cost savings. A more comprehensive document has recently been completed covering all types of pulp and paper manufacturing processes and mill types.
Recommending A Systematic Approach
Over the years, many energy conservation measures have been carried out in mills without comparing them to other possible opportunities. If the measure can be clearly seen to save purchased fuel or purchased power, and to meet the company's investment hurdle rate, there would seem to be no apparent reason to delay. However, the shortcomings of this piecemeal approach are that better opportunities may be missed, or even foiled, by sub-optimal investments. Taking a comprehensive review of all possible opportunities takes time, personnel with appropriate skills, and management backing, all of which have been in short supply in recent years given the relatively low priority given to energy.
Therefore, the detailed energy audit is an essential first step in any comprehensive and systematic approach to improving energy efficiency. It includes the setting up of computerized heat and mass balances, which are a necessity to efficiently reconcile all the "hard" sources of data, while also considering the less accurate or "soft" information, so that the most reliable baseline of the current mill operation is established. Audits are also an excellent opportunity to get to know the details of the mill and enlist the cooperation and assistance of the operating staff and access their knowledge.
The balance of this article focuses on benchmarking and identification of energy efficiency projects, while information on other important aspects such as evaluating impacts on mill systems and projecting energy cost savings, may be found in the literature.
Benchmarking Energy Consumption
Whether for energy, production costs, or uptime, benchmarking mill operating statistics is conducted with mills of a similar type producing essentially identical products and is fundamental to competitiveness analysis. Regular audit-based benchmarking exercises allow a mill to track progress and establish the gap between it and mills of newer, more modern design or between mills of similar age.
Energy benchmarking data is presented for some newer mills, a group of older mills more typical of the current age of many of North American bleached kraft pulp mills, and a model mill. The newer mills are representative of designs since about 1990. The sources of the information are published data, mill audits conducted by H.A. Simons (now AMEC) or are data received directly from the mills themselves. Typically, these mills use extended cooking, oxygen delignification, ECF bleaching, high solids firing (72 to 76 per cent), and have a total mill water consumption in the 50 to 70m³ adt range.
The data for the group of older mills, taken from an energy consumption survey published in 1981 for 17 Canadian and seven Nordic mills, is labelled Typical 1980s. This data is very similar to that presented for a 1980s-era mill in the United States.
Data on a hypothetical mill published first in Svenska Papperstidning, and subsequently in English, was prepared by AFIPK of Sweden. The mill included no direct fresh steam heating of warm or hot water, three stage flashing of black liquor at the digester, press-based washing, condensate heating with evaporator flash vapour and no steam use in the causticizing or kiln area.
Additional specific steam consumption data for Swedish mills not presented here is found in recent literature.
Steam Energy Consumption
Definitions:
The specific steam consumption of a department or a steam load is defined as the heat absorbed from the steam and condensate system per unit mill production, and is expressed as GJ/adt in metric units. The definition credits the heat in the condensate returned, while the heat lost in the condensate not returned becomes a charge against that department, which is determined by the temperature of the makeup water, e.g., river temperature. This definition has the advantage of allowing direct comparison with the Nordic countries.
Results:
Specific steam consumption data for eight bleached kraft market pulp mills is presented in Table I. This shows that for softwood, the spread among the 1990s-era mills is 100 to 140 per cent of the best mill, while that for the Model Mill is 25% lower. The corresponding value for the 1980s typical North American mill approaches twice that of the best 1990s mill. The spread in the two hardwood data is also large.
An underlying reason for the differences in consumption is mill design and process technology adopted. This is shaped by the economic environment, such as the cost of fuel, power, labour, construction, requirements for return on capital and owner philosophy. Also, the basis for some data was not a detailed audit due to lack of resources.
Most importantly, some audits were conducted before a sustained production plateau had been reached. Accordingly, the results should be treated as preliminary.
Electric power consumption
Some preliminary specific power consumption data for seven mills is presented in Table II. All of the mills were built in the early-1990s, with one exception, a mill built in the mid1980s. Here the spread for softwood mills is 100 to 141 per cent of the best mill, while that for the Model Mill is 6 per cent lower. Hardwood values are somewhat less. Data for 1980-vintage mills are not presented, but would be similar to that for the 1990s mills. Further specific power consumption data for Swedish mills not presented here is found in recent literature.
Identifying Energy Conservation Projects
Types
Energy cost reductions can be achieved by a broad range of measures, from adopting a new procurement policy, or changing the excess air setpoint on a boiler control in the steam plant, to a project requiring significant capital.
In organizing an approach, it is useful to distinguish between four types of improvement measures:
- Energy procurement strategies and policies;
- Operating practices and maintenance initiatives;
- Energy conversion efficiency and fuel substitution projects: fuels and electricity into heat and shaft power and fuel or falling water into shaft power and electricity;
- Increasing the efficiency of the energy used to meet the needs of the process by 1) better thermal integration for the reuse of secondary heat, and 2) minimizing heat rejected or lost to the surroundings
The list can be thought of as a chain of dependency, with measures towards the top being generally independent of the measures lower down. For instance, lowering an excessive furnace 02 setpoint on a boiler control in the steam plant -- an operating measure -- is an action that will result in immediate benefits, and is essentially independent of steam use and, therefore, on whether a steam energy efficiency project proceeds or not.
The last category, the efficiency of energy use, is linked to, and therefore generally depends on, the categories that precede it, because they determine the cost of energy saved. Thus, to justify spending capital to reduce energy consumption somewhere in the process depends on the price of fuel or power, those operating and maintenance practices which affect energy conversion efficiency, and the boiler or motor efficiency.
Although this dependency is generally true, it is not a hard and fast rule, particularly when the amount of energy saved is large, such as one that may result in the shutting down of a boiler. The interdependence of energy efficiency projects can be counter-intuitive because of linkages that may exist among the foregoing categories of measures. This makes the role of validated computer models of the mill an essential tool in preventing unintended consequences.
Sources of ideas
Almost inevitably the first projects identified are those found during the detailed energy audit process itself. The projects are generally operational in nature, and though small, many have the merit of not involving any capital investment at all. Often, the idea has its origins with alert operating staff. Savings on such initiatives can justify the cost of the audit itself.
A second source is through the review of technical literature including anthologies and compilations of energy savings projects by CPPA, PAPTAC and TAPPI.
The Guide [1] facilitates the accessibility of kraft pulping-related energy conservation opportunity (ECO) projects from 1982 to 1995 by presenting them departmentally. Project distribution is as follows:
- 65 in the kraft pulping fibreline;
- 98 in the evaporator, recovery and power boiler, steam plant, and power generation areas;
- 11 in the condensate system mill-wide.
An example for the recovery area is shown in Table III, where HI indicates heat integration and PC process change.
Most of the ECOs are applicable to older mills. While some are mill specific with limited broad application, even these are useful in documenting mill ingenuity in surmounting difficult, long-standing inefficiencies. It is obvious, as well, that some energy savings concepts are most easily justified when applied to new mill designs, and are not economical in retrofit situations unless implemented for other process reasons. Examples include increasing steam generation pressure, use of low pressure instead of medium pressure steam at the pulp dryer or adding evaporator effects to increase steam economy.
A new route to identifying potential projects is through evaluation of the degree of process thermal integration using Pinch Analysis -- a procedure that allows determination of the theoretical minimum or "target" energy consumption required for a defined process. This allows the gap with the existing consumption to be quantified, and is a great motivator for improvement. Pinch analysis is not effectively summarized in a few words without introducing examples, but some very good literature is available.
A study that assessed the potential of pinch analysis to reduce greenhouse gas emissions, if it were applied to the whole of the market kraft pulp sector of the Canadian pulp and paper industry, conservatively estimated that the total process thermal energy would be reduced by 6.8 per cent.
This was based on a sampling of mills to determine actual projects completed and put into operation some time after pinch studies were completed. In general, pinch studies initially show much greater reductions.
Small is often best: A surprising aspect of implementing energy savings projects in existing mills is that small projects tend to outperform large ones. As we have seen, a number are operating and maintenance measures which involve no capital expenditure at all. Table IV shows results of the analysis of 11 projects from two separate kraft pulp mill studies. This means that the focus should be on small projects, because chances are that they will be the easiest to justify --particularly during times of low fossil-fuel prices. A similar trend in energy-savings projects was reported in the literature.
Conclusions
Proactive energy management is consistent with ensuring environmental acceptability of products in an increasingly competitive marketplace, deregulation of energy markets combined with the possibility of long-term energy prices rising, and the need to reduce the emissions of greenhouse gases.
Recent published guides recommend a comprehensive~ systematic and ongoing approach. which includes in-depth energy audits, benchmarking against existing and new mills, assessing the degree of thermal integration, gathering energy savings ideas, evaluating impacts on mill systems, and projecting energy cost savings.
References
1. CPPA. A Guide to Energy Savings Opportunities in the Kraft Pulp Industry Prepared by AGRA Simons
D. M Bruce is with AMEC (formerly AGRA Simons Ltd.) based in Vancouver, B.C. This article is based on a paper presented at the 84th Annual Meeting of the Pulp and Paper Technical Association of Canada. ET