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APPLYING BEST SUSTAINABLE PRACTICES TO PULP AND PAPER
Case Study – Mondi Richards Bay Mill Capacity Expansion
Due to market forces, financial objectives and pressure from environmental groups the global Pulp and Paper industry is facing greater pressure to continuously improve the environmental footprint it leaves. A positive outcome of this pressure is that new and better technologies are fostered.
Sound environmental management has become integral to the way process industries operate these days. Climatic events over the last year have supported the view that global warming is a reality and anthropogenic (environmental pollution and pollutants) emissions of greenhouse gasses are contributing to this phenomenon. Furthermore, water, one of the most basic resources, is vital to life. The ever growing demand on the world's water resources, by an ever increasing population is one of the greatest problems that mankind faces.
In an era where high productivity, cost control and focus on the environment is expected, the focus on innovative thinking to compete in the market must be paramount. Mondi has developed its own thinking on how to best manage these challenges.
The impact that the capacity expansion could have on the environment in the Richards Bay area was of major concern when planning the RB720 project. The challenge for this project has been to avoid an increase in fresh water consumption, effluent discharge volumes, atmospheric emissions, and electrical power requirements in spite of the increased production capacity of 25%. The improvements could mainly be achieved by internal process measures utilising the most cost efficient concepts and design. In addition special focus was put on the treatment of malodorous gasses and odour.
Sound environmental management has become part and parcel of the way process industries operates today. In common with a number of Industries pulp and paper makers are obliged to improve its environmental practices. Reducing the emissions into the atmosphere, waste streams and power consumption continues to be a key concern for the pulp and paper industry.
The sustainable future of South Africa's pulp and paper industry is not just a question of products but also a question of access to overseas markets. In the future, the search for new products and new markets will also be accompanied by a dynamic and proactive research effort into better, cleaner and more competitive ways to make pulp and paper products.
Water consumption, condensate reuse, total reduced sulphur (TRS), sulphur dioxide (SO2), Methanol (MeOH), Chemical Oxygen Demand (COD), levels and power consumption are the most important values indicating the environmental friendliness of the process.
This paper focuses on the production technologies selected and the application of Best Available Techniques to address the atmospheric emissions and the final effluent quality in the Richards Bay expansion project.
The environmental target is a pulp mill that minimises odours and conforms to EU IPPC / BAT standard for atmospheric and effluent emissions.
2. INTRODUCTION TO CHEMICAL PULPING – KRAFT PROCESS
Chemical pulping is the process of reducing a fibre source (eg. wood, Bagasse, straw, etc) during a cooking phase to its component parts (fibre and lignin) with chemical liquors such as caustic, sulphate, sulphite etc. The Richards Bay mill uses the alkaline/sulphate (kraft) process.
In the kraft process wood chips are cooked in a solution of sodium hydroxide and sodium sulphide (white liquor). The alkaline causes the lignin molecules (substance that bonds the fibres) to dissolve in the alkaline liquor. The liquor and dissolved lignin (black liquor) is then burnt in the Recovery Boiler and the alkaline chemicals recovered for re-use.
A significant drawback of the kraft process from an environmental point of view is the release of odorous sulphur compounds from a number of the process steps. The threshold values for detection of these compounds by the human nose are very low; it is possible to perceive the odour with only 1.0 ppb of sulphur compounds in the air. It is highly desirable to reduce or eliminate these emissions.
Figure 1. Kraft Process (Courtesy of Kvaerner Pulping)
3. The RB720 PROJECT
The production target of the project was to increase the capacity of the Richards Bay Mill from 570 000 Adt/annum to 720 000 Adt/annum to correspond to the existing recovery boiler capacity.
Key issues considered in the project development were:
The need for pulp and paper production is increasing all the time, which in turn increases the requirements for environment friendly production at every stage of the process.
As the impact of the capacity expansion on the environment was of major concern in the Richards Bay area, the starting point of the design was to avoid an increase in fresh water consumption, effluent discharges, atmospheric emissions and the specific usage of utilities and bleaching chemicals, despite the increased production. In addition special focus has been put on the treatment of malodorous gas and odour.
3.1 Process Concept
During the study phase of the project we investigated the fibre processing technologies and chemical recovery that offered maximum return on investment with minimal environmental impact. A thorough understanding of the interrelationships of the different processes in the mill is essential to selecting the best combination of concepts. Technology combinations that gave the best environmental performance levels corresponding to BAT were selected.
The following process concept, which is a combination of debottlenecking and new processes, was best suited to meet the project objectives.
Figure 2. – Schematic Layout showing Project Scope
3.2 Design Parameters
The focus when modifying the existing mill was on gaseous emissions, odour problems and liquid effluents. A capacity increase when correctly made can decrease emissions significantly.To achieve this target, new and innovative approaches, procedures and techniques were investigated and applied.
The following table compares the existing plant parameters to that expected after the rebuild and modernisation.
4. DETAILS OF PROJECT CONCEPT
4.1 The Cooking Plant
To meet the demands of an increasingly competitive market, Mondi selected the technology that made it possible to cost effectively optimise pulp properties to meet the paper mill specifications. Our selection was the continuous cooking technology offered by Kvaerner. The scope of supply also included deknotting and pressure diffuser washers. Some of the process advantages offered by this technology are:
The new cooking plant is expected to improve the pulping yield by about 2 percentage points. Furthermore, the new cooking process will improve the bleachability of the pulp. These two factors will have a direct impact on the manufacturing costs.
4.2 The Bleach Plant
The driving force in the development of kraft pulp production in the past ten years has been the need to reduce or discontinue the use of chlorine chemicals in bleaching. The use of elementary chlorine has largely been displaced by chlorine dioxide and total chlorine free process.
Before deciding on a bleaching concept it is necessary to go through a number of factors that impact on the choice. The importance of the factors often depend on local circumstances, product profile etc. Deciding on a bleaching concept in many cases means tailor making a process for your mill. Some of the factors to be considered include:
Our selected bleaching concept is a first oxygen delignification stage followed by a four stage bleaching, "Dual D" with an EOP stage and followed by two D stages, from Kvaerner. The advantages offered by the Dual D are the reduced chlorine dioxide requirement (which reduced the AOX in the effluent), reduced yellowing in the pulp and possibility for higher brightness ceiling. As an additional benefit of the investment, an improvement in pulp quality can be expected.
The importance of washing in bleaching cannot be over emphasised. A low COD entering a certain stage means a lower chemical consumption. We have selected wash presses as this provides the best liquid lock between stages. This allows the changes in pH and temperature between subsequent stages to be easily handled.
One of the major targets of the rebuild project is to ensure that the total consumption of water will not increase. As the bleach plant in general is the main source of effluent, the new bleach plant will be built to generate as little effluent as possible. The press also meets this criteria as the wash water requirement is the lowest.
In addition to the bleaching concept, special emphasis was placed on the effluents, heat recovery and gaseous emissions:
4.3 Recovery System
4.3.1 Evaporation Plant
The first step in chemical recovery is efficient evaporation. Evaporators concentrate liquor and remove contaminants from the condensate to allow reuse in the fibreline and recausitcising plants. Modern evaporators can actually serve as "water" factories inside the mill.
Our targets for the evaporator concept were:
From a technology view point the concept proposed by Andritz offered the best solution that maximised uptime and minimised the potential to plug. Some of the design benefits included in this concept are:
4.3.2 Recovery Boiler
The recovery boiler plays a central role in the chemical cycle of a modern pulp mill. The spent liquor from the digesting plant goes first to the evaporation plant where the dry solids content in the liquor is increased to 80%.
Increasing the black liquor solids has many favourable effects:
After final concentration the black liquor is burnt in the combustion chamber of the boiler. The fly ash from the electrostatic precipitators is also mixed into the black liquor at the concentrator.
In order to achieve the positive environmental impacts special attention was given to the air arrangement in the recovery boilers and the electrostatic precipitators as dust generation does increase with increase in black liquor solids.
The hot flow (smelt) of the regenerated chemicals is drained from the furnace floor to the dissolving tank. The chemicals are dissolved into weak white liquor (green liquor) and returned to the recausticising plant for further processing.
4.3.3 Recausticising Plant
The recausticising plant takes the "green liquor" from the Recovery Boiler dissolving tanks and combines it with burnt line (CaO) and produces white liquor for the cooking process.
This part of the plant was only an upgrade and the main criteria for equipment selection was separation and treatment systems that provided the best process performance and productivity. The process solutions selected were based on the best technologies available for individual unit operations.
Disc filters (for both white liquor and lime mud) were selected for the capacity increase in the recausticising plant. The main drivers for the decision were:
The causticising process is a closed loop and the lime kiln forms an integral part of the recovery cycle in the pulp mill. The function of the kiln is to convert lime sludge from the recausticising process into lime with a high content of active CaO. The technology supplier for the new Lime Kiln and auxiliary equipment is FFE Minerals.
Our selection criteria for the Kiln system were:
Kiln systems start with the efficient drying of lime mud prior to the calcination and nodulization processes that takes place in the high temperature zone of the kiln. The drying uses flash drying technology that includes a patented gas recirculation system. The burner system is designed to combust natural gas, methanol, turpentine and the LVHC malodorous gases. Heat recovery form the calcined product is via the Compax cooler. Final gas filtration is via electrostatic precipitators. The ESP dust is reintroduced into the process or can also be removed as a purge stream.
4.4 Odour Treatment System
Another environmental target for the project was to decrease the emissions of odour. This will be achieved primarily by the new modern cooking process and by burning HVLC gases and LVHC gases.
For reasons of safety the NCG system is separated into a concentrated gas system and a diluted gas system.
4.4.1 HVLC System
HVLC is characterised by very low levels of odorous compounds but the volume flows are very high. The dilute gasses represent more of an odour nuisance in the immediate vicinity of the mill, but precautions must be taken in the design of a collection system to avoid elevated gas concentrations close to the explosion limit.
Typical sources of diluted NCG's are filtrate tanks, evaporation plant tank yard, washer hood venting, chip bins and condensate tanks.
The concentrations of the sulphur compounds are below the lower explosion limits thus the gases are essentially moist air. In order to minimise the vapour amounts the collected gases is normally dehumidified and then superheated prior to burning. The HVLC collection system is of 'closed' type to minimise the gas volumes, therefore the dilution air is controlled to maintain the required pressure in the tanks and piping.
4.4.2 LVHC System
Collection of strong gases is relatively easy because of few sources and low volume flows. The main emission sources are the evaporator plant, stripper (methanol recovery), turpentine recovery and cooking plant.
The concentrated NCG system is designed to work without contact with air, since leakage of air into the system increases the fire hazard. The concentrated NCG system is isolated from the atmosphere by water seals, safety valves and rupture discs. A steam ejector is typically used to transport the strong gases.
Prior to combustion the collected gas is scrubbed with white liquor to remove Hydrogen Sulphide, reduce the content of Methylmerkaptan and condensate.
4.4.3 Oxidation of NCG's
The HVLC gases collected from the causticising plant will be mixed with the secondary air for the new lime kiln. A cooling scrubber will be added to reduce the vapour amount.
The weak malodorous gases collected at the fibre line and evaporation plant will be burned in the Recovery Boiler via the tertiary air ports. The moist gases are first dehumidified to reduce the vapour amount and then superheated prior to mixing.
The dissolving tank vent gases are also burned in the Recovery Boiler. The gases are scrubbed to remove particulate matter and moisture and then superheated prior to mixing into the secondary air ports.
The LVHC will be burned in the lime kiln and the backup system to this is a separate incinerator which will be on hot standby.
4.5 Effluent Treatment
Water is a very scarce resource in South Africa. Large amounts of fresh water are needed in pulp and paper production. The industry has acknowledged this issue and is actively closing process cycles to minimise water consumption by process optimisation and recycling of process water. The water issue will become even more important in the future.
The most common measurements used for characterising effluents include pH, suspended solids, biological oxygen demand, chemical oxygen demand, phosphorous, nitrogen and adsorbable organic halogens. Final effluent loads from pulp and paper making processes depend to a large extent on the effectiveness of wastewater treatment. Primary and secondary wastewater treatment has become an industry standard.
For the Richards Bay Mill we studied two biological processes for the secondary treatment. These were an activated sludge process and aerated lagoon. Test work on the existing effluent showed the process best suited to reduce the final effluent quality to a level that meets the eco-label targets (Paper Profile) was the activated sludge process.
The effluent characteristics and volume will change as a result of the RB720 project, because the pulp production and chemical recovery steps will take place with completely new or upgraded process equipment. In addiction, water, spill and peak load management will improve.
Despite the increase in pulp production, the specific consumption will decrease thereby reducing the effluent load to the treatment plant. Calculations, based on test results and adjusted for the anticipated future concentrations and volume, confirmed that the activated sludge process is the correct choice.
4.6 Heat and Energy
The production of pulp requires large amounts of energy, both heat and electricity. The generation of heat and electricity causes emissions to air. The amount and type depend on the fuel used, the combustion technique and the flue gas purification method.
In the chemical pulp process energy is generated as a by-product in the recovery boiler. Bark and saw dust is also used to generate energy. Today's kraft mill can be totally energy self-sufficient.
Due to good steam economy of modern digester plants utilising low cooking temperatures, no additional power boiler capacity is required. The consumption of coal will decrease from 415 t/d BD to about 202 t/d BD, mainly due to the increased amount of hog fuel and improved energy balance. The emissions will not change markedly, except that CO2 emissions will go down because of the lower use of coal.
4.7 Solid Waste
The solid waste from the mill that need to be disposed are; lime mud, primary clarifier sludge, boiler ash and in the future biological sludge. At present about 149 t/d (wet) of power boiler ash is dumped to land-fill. The ash will decrease to about 110 t/d (wet) as the coal consumption will decrease and also more bio-fuel will be burned.
As a consequence of the interventions in the rebuild, the primary clarifier sludge will also decrease when the project is fully commissioned. The primary clarifier sludge is consumed at our Felixton Mill as a fibre source for fluting paper.
The biological sludge from the secondary treatment plant will be combined with the black liquor and burned in the Recovery Boiler.
The inorganic material is either used by other industries or land-filled mostly in the mill's own landfill sites.
In the pulp and paper industry, the key factors for economic, environmental and social sustainability are closely interlinked. As water scarcity, energy efficiency and chemical use are an increasingly significant concern, it is important to continue the development of the pulp and paper industry towards eco-efficiency while developing the products further.