Low carbon papermaking

Improving the way that paper mills source and use energy is one of the keys to sustainable papermaking, one of the aims for the global paper industry. As we approach the big sustainability summit in South Africa, this article looks at some of the varied issues involved.

Leslie Webb, Envirocell

The above title is a contradiction in the sense that most papers (including tissue and towel grades) contain about 40-45% carbon. The exception is those papers containing a lot of clay, where the carbon content will be lower - down to about 25% carbon. There's not much that we can do about this when the predominant raw materials for papermaking are cellulose (and many other organic compounds) and calcium carbonate. However when we refer to carbon in this sense, we are usually referring to the carbon content of the energy sources that are used in the manufacture of paper, rather than what's in the paper itself. This interest in carbon, of course, comes from the current hot topic of global warming or, more generally, climate change, which is caused (we think) by the accumulation of various gases in the earth's atmosphere. Our understanding of the forces behind climate change is largely due to the work of the Intergovernmental Panel on Climate Change (IPCC), which was set up by the World Meteorological Office and the UN Environment Programme in 1988. Their third report was published at the beginning of last year and provides the strongest evidence so far that the global warming of the last 50 years is due largely to human activity. Any increase in surface temperatures may be due to natural factors, such as variations in solar irradiation and volcanic activity, but the available evidence points to man-made factors, such as the increase in emissions of greenhouse gases, being the most significant.

Discussions about climate change inevitably throw up a lot of numbers, usually either very big ones (the emissions) or very small ones (the concentrations of atmospheric gases) and some of these are collected together in Figure 1.

IT IS RECKONED THAT THE PAPER INDUSTRY ACCOUNTS FOR ABOUT 4% OF GLOBAL ENERGY CONSUMPTION, BUT FOR ONLY AROUND 1.2% OF FOSSIL FUEL-DERIVED CARBON DIOXIDE (CO2) emissions due to the industry's extensive use of wood for energy generation. The CO2 from combustion of any biomass like wood is not counted in the projections about global warming /climate change as this carbon should be re-incorporated in the new biomass growing to replace the old (provided, of course, that forests are managed sustainably, another key environmental issue for the paper industry).

According to IPCC, the single most significant factor behind climate change is the increase in CO2 concentration caused by fossil fuel combustion - this has increased the CO2 concentration to about 370 ppmv (parts per million by volume) currently compared to a stable 280 ppmv over the hundreds of years leading up to the start of the industrial revolution in the middle of the 19th century.

The main vehicle for limiting emissions of CO2 and other greenhouse gases is the UN Framework Convention on Climate Change set up at the 1992 Earth Summit in Brazil. However, the Convention itself contained little in the way of real targets beyond the requirement for developed countries to return their emissions of greenhouse gases to 1990 levels by 2000.

For targets beyond 2000, the Kyoto Protocol (see summary in Box) was pieced together in 1997, but several of the detailed practical points, particularly about the definition of allowable carbon sinks and the so-called flexible mechanisms, were left for later resolution.

This task took several further meetings and much travelling around the world by participants and journalistic hordes (with its associated CO2 emissions), but was finally resolved in late 2001 to everyone's surprise satisfaction except that of the worlds biggest CO2 emitter, the USA. This last fact, together with concessions made to and some faltering by other important countries, undermines both the credibility and the likely practical achievements of the Protocol. Nevertheless, other countries such as the EU bloc are pressing ahead with plans to ratify the Protocol so that that it can formally enter into force before the next big global event, the Rio+10 conference on Sustainable Development being held in Johannesburg this coming September.

THE BIG QUESTION FOR THE PAPER INDUSTRY IS HOW IT CAN LINK THE NECESSARY MEASURES TO REDUCE CO2 EMISSIONS WITH ALL THE OTHER ENVIRONMENTAL STRANDS OF ITS DRIVE TOWARDS OVERALL SUSTAINABILITY. For most manufacturing industries, reducing CO2 emissions equates to optimising energy supply and management across the whole product life cycle, which requires shifting the management of energy in the following directions:

-maximising the use of all renewable forms of energy such as biomass and hydro;

-switching to combined heat/power (CHP) generation wherever possible;

-moving away from fossil fuels with a high carbon intensity per unit of energy to those with a lower or zero carbon intensity;

-using energy-efficient designs for buildings, process plant and transportation.

The first action area is one where the paper industry can make a decisive contribution through maximising the use of wood residues from its own forestry operations, from pulping/papermaking wastes and/or from non-recyclable used papers. These days, tissue mills tend to be less and less integrated with virgin pulp production, so the possibilities for harnessing wood residues are rather limited. On the other hand, tissue and towel products are increasingly being made from recovered paper, which generates large quantities of sludges/rejects from the normal deinking stage. The sludge stream can carry 30-35% of the materials in recovered paper, so assuming that about 30% of the 20M tonne or so of tissue/towel produced every year is made from recycled fibre, the tissue sector generates something like 3M tonne of deinking sludge solids globally each year. A number of mills (such as the SCA tissue mills at Stembert in Belgium and Mannheim in Germany) exploit the combustible fibre fraction in on-site incinerators equipped with energy recovery. Another SCA tissue mill (Edet in Sweden) has turned to an original way of using waste energy through excavating sludge that had been placed in a local valley over many years for use, with fresh sludge residues, in a new boiler plant. The problem with this technique is that it still leaves large quantities (around 1.5M tonne globally each year from tissue mills) of mineral ash for disposal or to find other uses for. As practised by another SCA tissue mill (Tilburg in the Netherlands), one way round this is to send the sludge to a cement (or brick) works. In this way, the sludges fibre content replaces some of the fossil fuel used in the process, whilst the mineral materials (clay and calcium carbonate) are incorporated beneficially in the product.

CHP PLANTS ALLOW THE PRODUCTION OF ELECTRICITY WITH AN OVERALL FUEL EFFICIENCY GREATER THAN 80% COMPARED TO NO MORE THAN ABOUT 40% EFFICIENCY AT CONVENTIONAL POWER STATIONS. The pulp and paper industry has always been a significant user of CHP as the balance of heat and power required at mills suits the output from most CHP plants. Information put together by a coalition of several national paper associations for one of the Kyoto Protocol meetings showed this clearly (Figure 2), notably for the industries in Japan and the USA. This also demonstrated the importance of renewable energy sources, not so much in Japan (where a lot of wood is imported as chips), but in Europe where there is a strong uptake, at least compared to its position with CHP. According to CEPIs 2001 Energy Profile, over 90% of any electricity produced on-site at European mills is co-generated with some countries (Spain, Sweden) already at the 100% level.

The choice of fossil fuel for on-site production of steam and/or electricity is influenced, like any raw material, by a number of factors such as availability, security of supply and price. Although a single combustion plant can be designed to burn any fuel interchangeably, most plants use either solid fuels (coal, peat) or gaseous/liquid fuels. National paper industries vary widely in their balance of fuel sources with gas accounting for less than 10% of fossil fuel use in Japan up to a level of nearly 90% in Italy. It is now fairly well-known that natural gas is the best fuel from an all-round environmental perspective as it has an energy content per unit of CO2 emissions over twice that of the purest coal and about 50% more than oil. It is also the most popular fuel for modern CHP plants so the best-case conventional paper mill with an on-site gas-fired CHP plant providing all its energy produces less than 20% of the CO2 emissions of the worst-case paper mill reliant on coal for both on-site steam production and for off-site non-CHP generation of electricity, a massive difference in environmental performance.

ANY REFERENCE TO GASEOUS FUELS TODAY IS A REFERENCE TO METHANE (NATURAL GAS), BUT THE FUEL GAS OF THE FUTURE IS MUCH SIMPLER SUBSTANCE - HYDROGEN. Its supreme advantages are its very high energy content per unit weight (about 2.5 times that of methane) and its zero carbon content, but this is counter-balanced by its very low density (about 10% that of methane) meaning that very high pressures have to be used to store it within a reasonable volume.

As well as being burnt to generate heat, hydrogen can also be combined with oxygen in fuel cells to produce electricity directly.

This is currently very expensive, but is already being used experimentally in cars. The key to using hydrogen in a sustainable way, though, is the need to produce it without the use of fossil fuels and this means using renewable energy to generate it electrochemically from water. At the time of writing, the Island of Islay in Scotland is debating proposals for it to become the first hydrogen-powered community through using wave-generated electricity as the primary power source. This hasn't got anything to do with papermaking, but there are a lot of forests in Scotland. This brings us back to the reality of papermaking and the final way that mills can get on top of the CO2 problem, which is simply by choosing processes and plants that minimise energy use in the first place. This has been a common focus of activity in all industries since the oil supply crisis and price escalation of the early 1970s, but one which has been largely driven by cost con- tainment rather than by environmental improvement. Irrespective of the grade being made, the biggest energy consumer at all paper mills is the production of steam for final drying of the paper web, so the development of more efficient pressing techniques has been a priority. The most widely-applied technique for accomplishing this has been the shoe press, which was developed initially by Beloit in the 1970s. It works by extending the length and hence the sheet residence time in the press nip and has been widely exploited over the last 20 years, notably for packaging grades. Inevitably, any increase in pressing tends to densify the sheet so the use of this technology in tissue manufacture is not easy.

HOWEVER, THIS LIMITATION HAS BEEN OVERCOME IN ANDRITZ'S TISSUEFLEX CONCEPT, WHICH WAS DEVELOPED FROM VOITHS NIPCOFLEX SHOE PRESS. This technology can be utilised to give a range of benefits - better tissue quality in terms of bulk and softness; a cheaper furnish at the same tissue quality; or Lower energy inputs for drying at the same tissue quality. The quality of the product does not match tissue produced on a through air-dried (TAD) machine, but the energy input is much lower. Several TissueFLex units have been retrofitted to existing tissue machines, but the first one on a brand new tissue machine is at the rapidly expanding LPC mill at Leicester in the UK. This started up last year and joins a Beloit tissue machine installed in 1998. Based on the performance of the first machine, the LPC mill took the Energy and Environment prize at the 2001 Papex Awards with citations for its low noise and other emissions to the environment. On the energy front, the mill had the lowest specific energy consumption in the UK tissue sector at less than 14 GJ/tonne product, a value that could be beaten once the new machine is fully commissioned. There are many other techniques to reduce energy use within papermaking (high consistency pulping, variable speed pumps/drives, better management of pulp refining, etc.) and these can collectively contribute just as big savings as improvements in the drying section. A good example of what can be achieved with a sustained campaign is provided by a company at the opposite end of the scale to LPC in terms of corporate size.

Kimberly-Clark has made reducing energy use one of its key environmental targets since the mid-1990s, when it challenged itself to achieve a 10% reduction (in terms of specific energy use per tonne of product) across its worldwide operations by 2000.

It exceeded this target by nearly 2%, bringing the company estimated annual cost savings of US$50M, and has now set itself the task of a further 10% reduction in energy use by 2005. Kimberly-Clark and LPC are not alone in these efforts with many other papermakers putting a lot of (human) energy and ingenuity into lessening the industry's energy footprint and hence its CO2 bubble. Its thus possible that we might see the first zero-carbon paper mill sooner than we think, but zero- energy papermaking is best left to our dreams.

Login or Register to publish a comment