CONFERENCE ON ECOLOGICALLY SUSTAl NAB LE INDUSTRIAL DEVELOPMENT

Transcription

1 *fmvl UNIDO % & UNITED NATIONS INDUSTRIAL DEVELOPMENT ORGANIZATION CONFERENCE ON ECOLOGICALLY SUSTAl NAB LE INDUSTRIAL DEVELOPMENT Copenhagen, Denmark, October 1991 Distr. LIMITED ID/WG. 516 /4/Su"ary 22 August 1991 ORIGINAL: ENGLISH Pulp and Paper Case study No. 1 Executive S-ry Prepared by the UNIDO Secretariat

2 ~ The designations employed and the presentation of material in this document do not imply the expression of any opinion whatsoever on the part of the Secretariat of the United Nations Industrial Development Organization (UNIDO) concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Mention of firm names and commercial products does not imply the endorsement of UNIDO. This document has not been edited. This document has been printed on environmentally friendly paper.

3 - 3 - INTRODUCTION The Pulp and Paper Industry, Resources and Processes The total world production of paper and board in 1989 was 233 million tonnes and of pulp 164 million tonnes. About 85 per cent of paper and board and 87 per cent of pulp was manufactured in developed countries. The expected increase in the consumption of paper and board will be 3.4 to 3.6 per cent in developed countries and 5.8 to 5.9 per cent in developing countries. The expected annual growth in the pulp production will be 2.9 to 3.1 per cent in developed countries and 4.4 to 4.5 per cent in developing countries. The basic raw materials used in pulp manufacture are renewable lignocellulosic fibrous materials. In developed countries wood is mainly used for pulping. In many developing countries, wood is in short supply and more than 45 per cent of the pulp is manufactured from other fibrous materials, mainly agricultural residues such as straw, bagasse and bamboo. About 55 per cent of the other fibre pulp is manufactured from straw. Chemical and mechanical processes are used in pulp production but pulp is mainly produced by chemical processes. Among these processes, kraft for wood and soda for non-wood pulping predominate. Both operate in alkaline conditions. The basic steps for pulp production are: fibrous raw material preparation, pulping, screening, washing, bleaching and recovery of chemicals. The paper industry uses both virgin fibres produced by pulping of lignocellulosic fibrous raw materials and also a considerable amount of recycled fibres (waste paper) besides non-fibrous materials (chemicals, minerals e.g. china clay). The industry has an impact on air, water and land. The extent of this impact can, in part, be controlled by appropriate technology. There are still technical and economical difficulties in pollution abatement, particularly in collecting and burning black liquor and recovery of chemicals in small pulp mills. In most developed countries forest management is based on the method of sustained yield. It ensures that annual growth is at least equal to cut volumes. In some countries, e.g. Germany, it is general practice to make almost exclusive use of thinning material and industrial wood residues for pulp production. Use of forest resources is harmonized with environmental protection. Cleaner production options Preparation of fibrous raw materials Prior to pulping wood has to be debarked and, since bark has a value as fuel, many mills prefer to debark in the mill and utilize the bark to generate process steam. Logs are debarked either in wet debarking drums or by dry debarking methods. In dry debarking the Biochemical Oxygen Demand (BOD) discharges are lower than in wet debarking.

4 - 4 - Bagasse has to be depithed before pulping and this operation is normally made in two steps. The first step, wet depithing, is made in the sugar mill and material removal is burned in the sugar mill boiler. The second step is made in suspension in the pulp mill before pulping. Solid material, removed from bagasse, can be mechanically separated from the liquid phase which is recirculated. This mechanical separation and recirculation reduces to a large extent discharges of suspended solids. The predominant treatment during bagasse storage, to avoid excessive degradation, involves spray and circulation of biological liquors or water. From the environmental point of view treatment with biological liquors is better than with water as discharges are reduced, although operating costs are higher. With other non-wood raw materials such as straws and kenaf the tendency is to improve harvesting and collection to reduce contaminants carried to the pulp mill. Pulping Wood is usually digested by the kraft process. Some organic substances are volatile and are contained in the digester and evaporator condensates contributing to the BOD load by 12 to 20 kg BOD per tonne of pulp. During the digestion, metoxyl groups are split off producing methanol and also reacting with sodium sulphide forming odorous compounds. The amount of total reduced sulphur (TRS) is about 3.5 to 5 kg per tonne of pulp, the higher value related to hardwoods. Bamboo is usually pulped by the sulphate process. In order to reduce the formation of malodorous compounds during kraft pulping the amount of sodium sulphide (sulphidity) in the white liquor should be reduced. One option is the partial or complete substitution of sodium sulphide by antraquinone and its derivatives. The quality of the pulp will be lower than kraft and the reduction in quality will depend on the extension of the substitution. This option is being commercially applied in some mills in developed countries. Pulping with organic solvents, ethanol, ethanol-caustic soda, methanol, and sulphide-antraquinone-methanol (ASAM), acetic acid etc., is being studied at laboratory, pilot plant and demonstration level. Other recent developments in kraft pulping include: extended delignification in continuous digestors to reduce the residual lignin in the pulp (kappa number) without reduction in the pulp quality and in-digestor displacement, in batch digestor, to reduce steam consumption and improve pulp quality. Agricultural residues are usually digested by the soda process (without sodium sulphide). The discharge of TRS (odorous compounds) is negligible as only traces of sulphur are present from water and raw materials. An alkali-oxygen process for pulping of straw is emerging. This uses sodium carbonate and caustic soda as the cooking agent together with oxygen. Caustic soda should be used only as a make-up chemical, resulting in recovery without causticizing. A similar process is being developed for other non-wood fibrous raw materials.

5 - 5 - Washing and screening An important operation is washing of the pulp and extraction of the black liquor containing the dissolved organic substances. Vacuum and pressure drum washers are commonly used, mostly in three stages. Black liquor washing efficiency is from 96 to 97 per cent. The efficiency of modern belt washers is as high as 99 per cent. With this development the technological limits are most probably reached and, in the future, efforts will be directed to increase capacity and energy saving for drives and vacuum systems. In old mills pulp is screened after washing, leaving shives and knots only partially washed and discharging BOD load in the screening system. By means of closed, hot screening the effluent can be reduced with corresponding decrease of BOD load. A closed system with good spill collection also decreases discharges of suspended solids. Washing of agricultural residue pulp is less effective and more capital intensive as drainage properties of these are poor and the loading capacity of drum filters low. Bagasse pulp is easier to wash than straw pulps. Application of belt washer to straw pulp washing in China gave better results than press or vacuum washer applications. Recovery of chemicals The recovery of the chemicals from soda or kraft black liquor is a vital part of pulp production because it regenerates chemicals to form fresh digestion liquor, supplies heat for the pulp mill, and reduces discharges of wastes to the environment. In the recovery process black liquor is concentrated in multiple effect evaporators frequently followed by a direct contact evaporator using flue gases, and is incinerated in a recovery furnace. In the kraft process, sodium sulphate is added to make up for losses. The smelt generated in the furnace is dissolved in water and caustic soda and regenerated by caustizing with lime. Recovery of chemicals from agricultural black liquors is more difficult and less efficient. Black liquors from agricultural residues are less concentrated, caused by difficulties in washing. The high silica content of non-wood pulps, mainly straw pulps, causes many problems in the recovery cycle including scale formation in evaporator tubes, deposits in the walls of the recovery boilers, slow setting rate of recausticizing white liquor, and lime sludge unsuitable for reburning. Desilication is required for successful recovery of high silica containing black liquor. UNIDO has developed a successful process at the Central Pulp and Paper Research Institute in India, and the Hindustan Newsprint Ltd. mill, Kerala, India has installed a demonstration plant. A high-efficiency recovery system with a sophisticated furance is very expensive and hardly accessible to small mills. A Brazilian mill using bagasse has successfully installed a simplified version called a Broby smelter. China has developed recovery furnaces suitable for non-wood black liquor and installed them in several mills producing 10,000 to 15,000 t/a of pulp. Recently China developed a furnace for a 25 t/d straw pulp mill. The furnace reportedly is operated without auxiliary fuel.

6 - 6 - Condensates from evaporation from a digester contribute substantially to the BOD load. Condensates in the kraft process contain methanol and also malodorous substances (TRS). It is necessary to clean the condensates by stripping and to incinerate them in a suitable furnace e.g. in the bark furnace or lime kiln. For small pulp mills some possibilities for cleaner pulp production are being studied including simplified recovery furnace, recovery using amphoteric oxides, ferric oxide binding alkali during burning, separation of the lignin and inorganic compounds by ultrafiltration, and anaerobic digestion of black liquor. Bleachinq Traditional methods to bleach chemical pulps involve treatment with chlorine and chlorine derivatives (chlorine, chlorine dioxide and hypochlorites). The residual lignin is dissolved as chlorinated compounds some of which are toxic. The tendency is to substantially reduce discharges from the bleaching area. In developed countries, in addition to traditional limits in BOD, Chemical Oxygen Demand (COD) and Total Suspended Solids (TSS) discharges, limitations are being introduced for toxicity and organochlorine compounds (AOX) discharges. The cleaner production options in bleaching, mainly applied to wood pulp, are basically the following: - reduce the black liquor content in washed pulp by using more efficient washers; - reduce the residual lignin content of the brown pulp, This can be done through: extended delignification in continuous cooking operation; additive cooking by adding antraquinone or polysulphide; in-digestor displacement for batch digestor; and pre-bleaching with oxygen (oxygen delignification) reinforced or not with hydrogen peroxide; reduce the chlorine charge by reduction of the chlorine mu1 tiple ; substitution of elementary chlorine by chlorine dioxide in first bleaching stage to reduce formation of the dioxins and furans; utilization of oxygen or oxygenfiydrogen peroxide reinforced alkali extraction; chlorine-free bleaching by using oxygen hydrogen peroxide and ozone. The final brightness of the pulp will be lower, around 80 to 85O ISO, depending on the sequence used and pulp characteristics; activated sludge treatment of bleaching effluents as well as aeration with oxygen;

7 - 7 - All these options are being applied commercially in large mills in countries where discharge limits are strict. For small mills using non-wood raw materials, however, modification of these technologies is necessary. Possibilities being explored are the use of hydrostatic pressure oxygen reactors for bleaching straw pulp, and the utilization of enzimes to make lignin more accessible in bleaching and to reduce chlorine charge. Another way to reduce discharges in bleaching is to reduce the final brightness of the pulp, but this will depend on the acceptance by the consumers of a paper with lower whitness. Mechanical pulping The BOD and COD load generated in mechanical pulping is just a fraction of the load generated in chemical pulping, but equal to or higher than the total discharge from chemical pulping with recovery. Biological treatment especially for Thermo Mechanical Pulping (TMP) and Chemical Thermo Mechanical Pulping (CTMP) is required to reduce pollution. Resins in wastewater from mechanical pulping are toxic and may disturb anaerobic biological treatment. Mechanical pulps are produced mainly from wood, but in countries with shortages of wood pulp it is produced from bagasse by mechanical treatment in disc refiners. This pulp is mainly used for the production of newsprint paper. The pulp yields in mechanical processes are from 80 to 95 per cent depending on the amount of chemicals used for impregnation. This is one advantage from the point of view of wood consumption. However energy consumption in these processes is higher than in chemical processes and, if energy is generated by thermoelectric units, the final balance will have to evaluated. Waste paper Waste paper is an important raw material in paper production and especially paperboard manufacture. Waste paper is one of the most significant parts of post-consumer refuse yet less waste paper is utilized than discarded as garbage. Less than 40 per cent of waste paper is actually collected. The highest waste paper utilization rates in fibre furnish is recorded in the Netherlands, the United Kingdom and Denmark, where it reached around 60 per cent in In Asia many countries are above 50 per cent and in the Republic of Korea about 70 per cent. Utilization of waste paper for papermaking is ecologically desirable, as it reduces refuse and extends the life-cycle of carbons in waste paper fibres. In countries with shortage of other fibrous raw materials waste paper utilization is preventing over-cutting of forests. Replacement of virgin pulp by waste paper also decreases energy demand, as energy consumption in waste paper processing requires approximately 1.3 MWh/t compared to 4.2 MWht/ for bleached kraft pulp and 6.5 MWh/t for thermo mechanical pulp production. Waste paper is used mainly in paperboard packaging and tissue paper manufacture, but utilization in writing and printing papers, especially newsprint after de-inking is increasing.

8 - a - Waste paper is a very important raw material in developing countries. However, to achieve a high recovery of waste paper, collection has to be improved. Papermaking Discharges from papermaking are related to water consumption, performance of the fibre recovery equipment, carry-over of pollutants from the pulp mill, efficiency of the retention of chemical additives and age of the machinery. Small paper mills, frequently using old second-hand machines, discharge large volumes of waste water as a result of both high water consumption and use of outdated machinery. Discharge of suspended solids is related to water consumption and to the performance of fibre recovery equipment. Closing the mill system does not solve the problem of soluble substances causing BOD load, but reduced water consumption reduces discharge of suspended solids. To implement a closed mill system with reduced water consumption requires good purification of white water and sufficiently large storage tanks. Savings from reduced water consumption arise as a result of the lower expenditure for fresh water collection and reduced costs for secondary effluent treatment. However, extremely low water consumption causes corrosion and disturbances in the paper chemistry system due to increased content of ions and other undesirable substances. New retention and drainage aids may increase the capacity of paper machines, which is at present low especially when using straw pulp. One possibility to improve the final brightness of the paper when using low brightness pulp in the stock preparation is to utilize high brightness filler for example calcium carbonate. This will depend on its local availability. By coating the paper it is also possible to improve the final brightness of the sheet. Coated papers, however, are more difficult to recycle. Energy conservation in pulp and paper manufacture Energy conservation is complex involving aspects such as: - selection of suitable equipment; - proper design of production line e.g. avoiding over-sizing of pumps, electric motors, drives, etc.; - optimization of energy distribution; and - avoiding losses by steam leakage and no-load running of motors, etc. Energy conservation will also result from the development of new processes and techniques with considerably lower energy demand. Energy is consumed in the process of removing water from the fibre slurry and fibre mat by filtration, suction by vacuum pressing and by drying. The bulk of the energy is consumed in drying (70 to 80 per cent). The more water that is removed by hydromechanical processes (filtration, suction, pressing) the lower the heat demand for drying and the total energy demand, even if energy consumption in the wet part of the paper machine slightly increases. A good press section is a very important factor in energy conservation. Energy conservation in the drying section is also important. An open paper machine without a hood, or only partially enclosed and without heat recovery, has a substantially higher energy consumption. Different paper grades also have different energy consumption.

9 - 9 - BARRIERS a) Economic Constraints In a modern large scale pulp mill, plant and equipment for combustion of pulping waste black liquor and recovery of chemicals requires about 30 per cent of total investment cost. Modern greenfield mills spend about 20 per cent of total investment costs for environmental protection. These investments are absent in small pulp mills. To implement these cleaner production options would double investment costs when compared with a mill without recovery or biological treatment of effluents. The pulp and paper industry is capital intensive. A large pulp mill of 200,000 to 300,000 t/a pulp will cost about US $ 340 million to US $ 450 million. Such a mill will meet present environmental norms and will be in a position to adjust to even more stringent ESID norms at the expected level by the year To finance such a greenfield mill in developing countries requires international financing. Such a mill gives less employment opportunities than small mills. These two barriers may prevent implementation of clean production options for the next 10 to 20 years. International lending institutions only finance projects which meet their environmental guidelines. This also could be a barrier for small mills until new economically sound technologies for such mills can be developed. To close down a mill because of poor environmental performance could be a social problem for developing countries. A solution could be to transfer pulping capacity from the small pulp mills to large units with recovery systems, and use the small ex-pulp mills for conversion to paper. Cleaner production options have been developed mainly for large mills and scaling down results in lower efficiency of heat conservation and chemical recovery. It would also increase relative investment cost per tonne of pulp. In China, the budget for a recovery system in a 10,000 t/a mill is estimated to be US $ 4.5 million. In many cases, small mills cannot affort such an investment. In some countries serious economic barriers to ESID include regulations pertaining to duty on imported equipment, which for paper machinery can reach 40 per cent. Shortage of convertible currency is also a limitation as most modern clean production technologies and equipment have to be imported from Europe, North Amercia or Japan. Price fluctuations of chemicals such as caustic soda and chlorine may act against implementation of cleaner production options. As caustic soda prices increase more rapidly than chlorine, some mills have the tendency to reduce caustic soda charges thus increasing residual lignin content and using more chlorine in bleaching. It can be concluded that varying raw material properties in developing countries and high investment costs are the main barriers to implementation of clean production options.

10 b) Technical Constraints In wood pulping almost all stringent limits can be met by implementing modern technology. However, discharges of chlorinated organic compounds from bleaching and malodorous sulphur compound emissions from kraft pulping cannot be reduced by available technology to meet stringent ESID norms. It is expected that by the year 2000, bleaching discharges will be at a sustainable level. Implementation of modified cooking and oxygen bleaching has an impact on the entire pulping process. Yield is lower in both cases resulting in a higher loading of the recovery boiler as the effluent from oxygen bleaching is recycled and burned. More steam and electricity is produced, but in oxygen pre-bleaching the heat demand is higher as compared with conventional pulping and bleaching. The additional investment for a new mill or a mill undergoing a major expansion is modest. Oxygen bleaching requires a pressurized reactor and additional washing equipment. It is easier to add on to an existing mill, but it will lead to production losses if the recovery boiler or the recausticizing plant are capacity bottlenecks. The investment costs will be affected by local mill conditions. In a new mill, bleaching can be reduced to three stages instead of five. If enough washing filters are available, costs can be reduced. If 25 to 30 per cent delignification is sufficient a less expensive, hydrostatic, medium consistency, oxygen reactor may be installed. This may be interesting for pre-bleaching of straw and bagasse pulps as these contain less residual lignin and the bleaching sequence may be reduced. Up to this time no straw or bagasse pulp oxygen bleaching is in operation. Another possibility for reducing pollutant discharges from bleaching is adding oxygen to the first alkali extraction stage, which reduces colour and to some extent AOX as well as saving up to 40 per cent of the chlorine dioxide. Oxidative alkaline extraction stages can easily be made as a retrofit installation in existing bleach plants. Installation costs are between US $ 0.5 million to US $ 0.8 million and pay back period is usually six to eighteen months. In agricultural residues pulping (straw, bagasse), clean production options are more limited due to specific properties of these raw materials e.g. high silica content. In the next decade it will not be possible to reduce BOD and suspended solids discharges as drastically as with wood pulping. Industry INITIATIVES Despite the financial and technical difficulties of implementing cleaner production options the industry has improved its environmental record in many developing countries. The industry should be further encouraged to use better pulp washing equipment and oxygen bleaching in straw and bagasse pulping.

11 Government Government has an important role to play in environmental protection and can bring about changes by issuing environmental standards, enforcing environmental measures, and by offering tax incentives or direct subsidies. The environmental guidelines should be realistically attainable to avoid closing down mills and generating social problems. The tax incentives and direct subsidies should preferably support projects with the greatest potential for minimizing environmental impact. Government programmes should adopt a progressive approach for existing mills, giving them sufficient time to attain the limits. Furthermore, they should support research institutions in their search for appropriate effluent disposal technologies. International co-operation UNIDO and FA0 have supported and upgraded research centres and institutes in developing countries. These institutes are conducting research in environmental protection for example: a black liquor desilication process has been developed by the Central Pulp and Paper Research Institute (CPPRI), India, through a UNIDO project; alternatives for black liquor utilization are also being studied in CPPRI through a UNIDO/UNDP project; and a co-operation regional programme (UNIDO/UNDP) is being developed between six Asian Pulp and Paper Research Institutes in order to organize joint research projects, to facilitate technology transfer and set up a network for continuous on-going exchange of information. Assistance from UNIDO in the following areas are suggested: development of recovery system, adaption of new pulping and bleaching technologies to non-wood raw materials; improvement of washing of non-wood pulp; reduction of fibre losses in existing mills; establishment and strengthening of R & D centres for development of appropriate effluent disposal technologies; energy and water savings in existing mills; assistance for the establishment of pollution control, programmes for existing and new mills; transference of technology between countries through Technical Co-operation between Developing Countries (TCDC), North-South co-operation and human resources development.

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