Thursday February 14, His Majesty the King Carl XVI Gustaf inaugurated SCA’s pulp mill Östrand in...
The pros and cons of eucalyptus
There are more than seven hundred species of eucalyptus ranging from shrubs to the tallest of trees and all but three or four are endemic to Australia. Eucalyptus species are tolerant to severe moisture stress, low soil fertility as well as attack by fire and insects. Eucalyptus is very productive because it grows quickly and thrives in short rotation plantations. This makes it ideal for the production of fiber for pulp and paper. Commercial production of eucalyptus began in Victoria, Australia in 1860 and spread quickly.
In many countries the introduction of eucalyptus species was very successful. Eucalyptus plantations played a significant role in providing fuel and construction material at the local level and timber and pulp at the industrial level. In Brazil the eucalyptus was introduced over a century ago. The plantations of this species now cover more than three million hectares. Brazil has gone from being an importer to an exporter of cellulose and is the world's leading producer of eucalyptus-based fiber. Environmental groups claim the monoculture of eucalyptus is causing serious harm to Brazilian ecosystems. According to José Augusto Tosato, an agronomist with the Development Research Center of southern Bahía state, in northern Brazil, "monoculture of extensive areas impoverishes the country's biodiversity, reduces the availability of surface water and causes social imbalances because it forces peasant farmers off the land". Tree companies and researchers point to the species' high productivity, Rubens Garlipp, head of the Brazilian Silviculture Association asserts that the species has been farmed in areas that were already degraded by over-logging of the forests, farming and ranching. He also says that with respect to the species grown in Brazil, studies indicate that concerns that the eucalyptus plantations "dry up the soil" are unfounded. Additionally the supply of good eucalyptus wood is claimed by forestry managers to reduce pressure to cut down native forests.
Eucalyptus is a thirsty plant. The main argument against planting them is that they have such a high consumption of ground water that they can dry up water sources such as rivers and springs. In fact they are so effective at this they are often used to drain marshy ground. The trees possess deep sinker roots that have extremely high rates of hydraulic conductivity, making them very effective in conducting water.
On a hot day a fully-grown tree may lose several few cubic meters of water. About 90% of the water that enters a plant's roots is used for this process. This evaporation of water is called transpiration and it allows for the diffusion of carbon dioxide gas from the air for photosynthesis and also to cools the plant and enables the flow of mineral nutrients from the roots.
General speaking, most fast growing species consume more water than slower growing species. But eucalyptus is also highly adaptable to drought. They can reduce their water uptake as the soil dries out, controlling their water loss by regulating the opening of stomata, the microscopic pores on the under side of leaves.
Eucalyptus can produce economically profitable biomass relative to the water they absorb. They will use roughly 785 liters of water of to produce one kilogram of biomass compared for example to the potato which needs 1000 liters of water to create a kilogram of biomass.
A mixed reputation
Interestingly, despite it’s thirst for water the eucalyptus is a fire hazard. The trees contain oil that is flammable. In Australia on hot days there is so much vaporized eucalyptus oil in the air above the forests that it creates a blue haze. There are even recorded instances where trees have exploded into flame. The oil content also slows the decomposition process of tree liter further increasing the potential for fire. The eucalyptus is regarded as a ‘dirty’ tree because its litter piles up on the ground. This litter is made up of bark, leaves, branches and seedpods. Eucalyptus forests in California have been cited as contributory factors in the frequent wildfires in the state. Another negative aspect to eucalyptus is when the trees grow tightly together forming a canopy which blocks the light and inhibits the growth of ground vegetation.
The right species for the right job
Pulp made from Eucalyptus Globulus requires less refining and less dewatering, than other species of eucalyptus. Drainage is easier and consequently its mill runability (how well a paper runs through presses) is improved. At a given tensile strength, pulps made from Eucalyptus Globulus will have greater opacity, porosity and bulk at a lower manufacturing cost. Eucalyptus pulps are used for papermaking because of the specific properties they give to paper, that is, bulk, opacity, formation, softness, porosity, smoothness, absorbency and dimensional stability. This pulp requires less energy to reach any given tensile strength and therefore volume and porosity are improved.
Wire presses are normally used to de-water pulp before thermal drying. The wire presses form a pulp mat in between two traveling wires, which is then pressed through a series of pressure-loaded rolls causing the water to be extruded. The dewatered pulp is conveyed to flash dryers while the filtrate is returned to the process.
Eucalyptus pulps are rich in fibers but also fiber debris, fines and intra-fiber capillaries. The fiber dimensions of length and width are similar for different eucalyptus pulps; it is the thickness of the cell walls that is the crucial difference. Fiber population (fibers, fiber debris, fines and intra-fiber capillaries) and coarseness affect paper-machine operation and runability. Fines are important for bonding because a pulp for bonding ability and strength. However an excess of fines causes problems in dewatering in the press section.
At a typical pulp mill, digesters cook the wood chips in a solution of caustic soda and sodium sulfite, which is called white liquor. After cooking for four to six hours, the wood fibers have been broken down into a brown pulp. Wood fibers are joined by lignin and during the cooking process, this is removed and the fibers are separated.
Search of a better way of dewatering
Manufacturers of many paper grades have long looked for better methods for dewatering a wet web after formation. For many paper grades, high levels of wet pressing are inadequate solutions, and standard dewatering aids such as vacuum boxes are severely limited in practice.
Efforts have been made to improve water removal by vacuum, but these are ultimately limited to a pressure differential across the web of less than one atmosphere. Air jets and nozzles have been used to varying degrees, but these have not proven effective in terms of water removal or energy efficiency. Air must be heated to dewater a wet web with the associated energy costs.
Another method for dewatering is displacement dewatering. This is where a gas phase is used to drive out the liquid phase. Displacement dewatering has been researched for its potential to increase water removal in a moist web at relatively low applied loads without the energy cost of evaporative drying but this process has yet to be proved practical on a large scale.
An Australian approach
It is back in Australia, the home of the eucalyptus, that a major step forward in treating pulp is planned. The Penola Pulp Mill will produce beached chemi-chermo mechanical pulp (BCTMP) from plantation raised Eucalyptus globulus for paper and cardboard. The mill which is due to go into production in 2010 claims it will be the most efficient industrial water user in the world because every drop of waste water will be treated and recycled. They term this, a Zero Liquid Discharge water recycling plant and say it will be the most environmentally sustainable development in the Pulp and Paper industry worldwide.
Whilst conventional pulp mills use on average 35 to 45 mega liters of water per day, the Penola mill will use just five to seven mega liters. The mill believes this will be seen as a global benchmark. Considering the mixed attitudes to eucalyptus, especially in the past, it is interesting to see eucalyptus production so firmly established for the future.