New recovery boilers are being installed around the world which are bigger, have higher efficiencies and produce more electricity. Producing green electricity for sale based on renewable resources has put a new focus on the recovery operation. About 80% of the 20 most recent recovery installations have higher pressures (>85 bar) and higher temperatures (>480o C). New recovery boilers have capacities of more than 5000 tds/day, with some evaporation units exceeding 1000 t H2O/h water removal capacity [1].
New recovery capacity is being built in Asia and South America, where half of the new mills produce HW pulp (Eucalyptus). Europe and North America account for less than half of total new recovery capacity. In Europe and North America, there are a lot of old recovery boilers, with an average age of 30 years. Since the life expectancy of a recovery boiler is about 30-40 years, significant investment in older mills is required within the next 10 years. The alternative is mill closure.
Modern recovery boilers fire black liquor at a solids range of 75%-85%, resulting in increased steam generation, combustion stability, lower TRS and SO2 emissions and also reduced fouling and plugging. The amount of power generated from a recovery boiler depends on its steam pressure and temperature. Generally, recovery boilers operate at a steam pressure of 80 bar and a temperature of 480 C to reduce superheater corrosion and avoid the use of expensive materials of construction. However, a number of modern recovery boilers in Japan and Scandinavia operate at more than 100 bar and 500o C. The maximum steam temperature and pressure can be limited by the ash melting point. A mill with a tight recovery cycle will have elevated levels of K and Cl, which lower the ash melting point. In North and South America, modern boilers are also being constructed which have K and Cl removal systems to reduce corrosion problems.
Firing at 75-85% solids requires evaporators with higher efficiencies. Modern evaporators operate at over 1000 t/h H2O, in a 7+ effects unit, with falling film design. These units have better secondary condensate purity, efficient foul condensate stripping and an integrated methanol liquefaction system. The high solids black liquor obtained from these evaporators is very viscous, and is stored under high temperature and pressure (Liquor Heat Treatment, LHT) to reduce viscosity.
Modern recovery boilers are much larger in size than older units; boilers with over 250 m2 hearth area have been built. One such boiler was started up in China in 2004. The unit has a floor area of 268 m2 and a nominal firing capacity of 5000 tds/d, producing 204 kg/s steam rated at 84 bar and 480 C [1]. Many recovery boilers burn sludge from effluent treatment, and some, filtrate from bark presses. New boilers also burn weak NCGs and dissolving tank vent gases. Most new boilers burn strong NCGs with a dedicated burner in the furnace.
Because of the focus on green energy, electrical energy production is a business objective. Apart from the steam generated in the recovery boiler, power boilers using wood residues are also being used to generate more steam from renewable sources. Recently, kraft mills in the US have been able to claim a tax credit for adding diesel fuel to the black liquor, classifying it as a green fuel because it generates power based on biomass [2].
In terms of future tends, black liquor gasification remains challenging as a replacement for recovery boilers. A driving force is the potential for much greater electricity production. However, new high efficiency recovery boilers have considerably reduced this gap. Many materials and gas clean-up issues remain to be resolved for black liquor gasification. Black liquor gasification can also produce syngas as a raw material for making high value chemicals, i.e., the biorefinery concept. However, given the challenges of black liquor gasification, biomass gasification is currently ahead in terms of commercialization [2]. Since the lime kiln is the largest producer of CO2 in a pulp mill, using biofuel in the kiln is a priority [3].
1. E.K. Vakkilainen, “Modern Recovery Practices and State of the Art Technology”, 2007 International Chemical recovery Conference, Quebec City, May 29-June 1.
2. http://foresttalk.com/index.php/2009/03/24/international-paper-receives-71-6-millio
3. Pulp & Paper, p.28, June 2007
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We produce a liquid fuel product from used motor oil, and animal fat. It is different from a product known as RFO (recycled fuel oil) in that it it is rerefined to remove much of the sulphur and ash. It has characteristics similar to diesel, but sells for much less. Is there a process for us to get this product reviewed for use? It should qualify for a biofuel tax credit. Thank you for your time and consideration.
Jeff Johnston
Oil ReRefining Comany
The biofuel tax credit in the pulp and paper industry is for kraft pulp mills mixing diesel fuel with black liquor and firing it into the recovery boiler. If your product costs much less and is similar to diesel fuel, it could, theoretically, replace it. The oil has to be compatible with the recovery boiler operation, i.e., it must have very low levels of chloride and potassium and the heating value should be comparable to diesel or black liquor. I am not aware of a process to review this product. However, if you provide a kraft mill with the relevant data for your product and convince them to do a mill trial running your product instead of diesel, and the trial is successful, this would be a practical review.