|
Demands for higher availability, increased reliability, lower emissions and reduced costs, are just some of the challenges facing boiler makers
By Rhiannon James
Putting the heat on boiler suppliers
Traditionally, boilers have been a bit of a headache for pulp and paper producers. Not only are they are costly to replace and maintain, but they also need to meet increasingly strict environmental regulations and are more often than not the source of bottlenecks in the overall production process. Managed properly though, boilers can also be a valuable source of income to companies looking to sell excess energy or to squeeze a few extra tons of capacity from the mills.
Boiler technology is not the fastest moving area in the pulp and paper industry, but there have been a number of improvements over the past few years that have encouraged papermakers to invest in this area. New installations are few and far between though, and upgrades remain the preferred option for most mills.
According to the research and development (R&D) director for recovery boilers at Ahlstrom Machinery Corporation, Kari Saviharju, suppliers are targeting a number of areas in a bid to improve energy efficiency. These include increased availability, combustion of odorous gases in the boiler and lower emissions to the atmosphere. Along with capacity increases, these are the main driving forces behind investments in boiler facilities.
Starting from scratch
One mill that did decide to splash out on a new boiler last year was Metsë-Botnia's Joutseno pulp mill in Finland. As part of a FIM 900 million ($160 million) project, Ahlstrom replaced the mill¹s recovery boiler, which led to an increase in bleached softwood kraft pulp capacity from 320,000 tons/yr to 410,000 tons/yr.
Sepap Steti sees a change for the better with its new CFB boiler
In most mills, low concentration odorous gases are typically collected and then combusted in the furnace of a recovery boiler. But a special feature of the Joutseno boiler is that it has a new system to collect and burn the mill's high concentration odorous gases as well. Although the process is carried out in the boiler along with black liquor, emissions are kept to a minimum. TRS (total reduced sulfur) pollution is less than 1 ppm, sulfur dioxide is lower than 10 ppm and carbon monoxide is about 10 ppm.
According to Saviharju, an increasing number of mills are opting to install these systems in recovery boilers, not only in Scandinavia, but also in North America.
Another major new installation took place in Europe last year at Frantschach's Pöls mill in Austria. The project aimed to boost bleached softwood kraft pulp capacity to 300,000 tons/yr with the installation of a new black liquor recovery boiler. Babcock & Wilcox (B&W) supplied the boiler and evaporator islands at a cost of $70 million on an EPC (engineer, procure and construct) basis. The new equipment included a boiler with a capacity of 1,900 tons/day of dry solids and steam generation at 800 bar and 480°C.
In addition to supplying traditional recovery boilers, B&W has been keen to develop alternative methods for improving facilities. For this purpose, B&W launched a bottom-supported modular recovery boiler (MRB) as a compact alternative to more traditional recovery boilers. According to the supplier, the MRB is an economical way to expand a mill through incremental liquor recovery capacity. With its modular construction, the boiler can be installed and brought on stream with a minimal impact on mill operations, avoiding long and costly outages. The MRB can also help meet strict environmental regulations. The unit's single drum design increases safety and reliability and the three level air system allows complete combustion. B&W believes that the MRB is suitable for small pulp mills that require a recovery boiler or large mills looking for additional capacity.
Model behavior
As new recovery boiler installations, and even rebuilds, can be top heavy on the investment side, the use of modeling is also becoming popular in the pulp and paper industry. This process allows companies to look before they leap into a large investment. B&W's latest advanced combustion modeling tool is the Como-Pr, which allows the recovery boiler owner to graphically visualize existing operations and investigate alternatives to designs, equipment options and operating conditions.
In addition to cost and capacity considerations, producers are becoming increasingly environmentally aware when they consider purchasing a new boiler or upgrading an existing unit. With this aim in mind, Stora Enso Paperboard has chosen Kvaerner Pulping to build a large new recovery boiler plant at its Gruvön mill in Sweden. This new boiler will considerably improve Stora Enso's environmental standing as far as pulp production is concerned and will also allow the company to boost capacity if needed.
The boiler's capacity will be 2,500 tons/day of dry solids and it will replace two existing boilers, which can no longer meet current emission standards. As the project manager of recovery boilers at Kvaerner Pulping, Juhani Viiala, explains, "The new boiler offers environmental advantages in the form of reduced emissions of sulfur, nitrous oxide and particles. The design also enables future operation with a closed production process."
Fluid alternative
Moving away from recovery boilers, there have also been new developments in the fluidized bed boiler (FBB) area, especially in the circulating model. A number of companies are now opting to invest in this type of boiler, both for new units and conversions.
For example, Kvaerner Pulping is set to supply a new FBB for Alholmens Kraft power plant in Pietarsaari, Finland. The supplier claims it will be the world's largest biofuel-fired circulating fluidized bed boiler (CFB) and the order is worth approximately Euro 70 million ($77 million). Alholmens Kraft is a combined heat and power plant, which will supply process steam for the UPM-Kymmene Wisaforest pulp and paper mill. The capacity of the CFB boiler is 550 MWth, of which 240 MW is electricity. The system produces 194 kg/s of high pressure steam at 545°C and 165 bar, while the figures for the reheater are 179 kg/s at 545°C and 40 bar.
Traditionally, CFB boilers have been used in coal firing. But in recent years, there has also been a lot of interest in using CFB boilers in biofuel or mixed coal/biofuel combustion. The new Alholmens Kraft CFB boiler is a multi-fuel boiler, which will mainly use bark, peat and wood waste. But the unit can also burn coal as a back-up fuel. As Ari Kokko, a product manager at Kvaerner Pulping, explains, "The design is for the boiler to burn 25% bark and wood residue derived from the Wisaforest pulp mill, providing a handy way to dispose of its bark, with the remaining 75% of its fuel being Finnish peat."
As the power plant supplies the neighboring paper mill with process steam, high boiler availability and reliability are crucial. With the exception of a few brief shutdowns for maintenance, the boiler has to be available all year round so that interruptions to paper production are minimized. Kvaerner has added a new feature to the boiler to enhance its performance. As the supplier's senior project manager, Asko Nyrökorpi, explains, "To increase the overall efficiency of the power plant - meaning to get more electricity out of it - the new CFB boiler will be equipped with a reheater. This a solution not often found in power plants connected with paper mills."
Kvaerner has also developed water-cooled and steam-cooled cyclones for different sizes of CFBs and the latest design in steam-cooled cyclones will be supplied for the Alholmens Kraft boiler. Added to that, the fuel-feeding equipment has been designed for high availability. It consists of four independent silos for different types of biofuels and full capacity can be achieved with three of them. The boiler combusts some 900 m/hour of biofuel.
Improving performance
Alternatively, mills can chose to convert older recovery boilers to BFB or CFB technology. According to B&W, some of the advantages of this investment are:
- reduced capital and operating expenses as the plants need less maintenance
- a significant increase in steam production at a fraction of the cost of a new boiler
- fuel flexibility
- low environmental emissions
- high efficiency.
One mill that recently plumped for the conversion option was Sepap Steti in the Czech Republic. Foster Wheeler Energia was chosen as the turnkey supplier for the retrofit project, which involved converting the mill's existing pulverized coal-fired boiler to a CFB. Foster Wheeler supplied a 176 MWth CFB at a cost of $30 million.
The output of the boiler was not affected by the upgrade as the main aim was to slash the NOx and SO2 emissions of the power plant. The unit produces electricity, process steam and heat for the mill. Since the conversion, the CFB is capable of burning biofuels such as bark and sludge, both of which are currently generated in the Sepap Steti mill.
Foster Wheeler Energia's latest project is slightly more ambitious though. This month, the supplier will begin work on what it claims is the world's first recycled plastic gasification plant. The recycling project will take place at Stora Enso's Corenso subsidiary in Finland. Foster Wheeler will supply a bubbling bed gasifier and gas boiler for the project, which is scheduled for completion in October 2000.
The new equipment includes a complete gasification plant and steam boiler designed to burn the gas generated from the recycling of board used for packaging liquids such as juice. The gasifier turns the flammable component of the recycling process into a gas that is burned to fuel a separate gas boiler. The system will have a heat output of 40 MW and generate 12.5 kg/s of steam at 60 bar and 490°C. Among the advantages of the system are "good usability, high efficiency, low maintenance, fuel flexibility and low emissions," according to Markku Ruutiainen, managing director of Foster Wheeler Service in Finland.
Keeping in condition
Once a boiler is installed, the headache is not necessarily over for producers. The upkeep of boilers can often be a cause for concern for many mill managers, but a number of suppliers are looking at ways to cut down costs and boost efficiency in this area. According to Esa Vakkilainen at Ahlstrom, "The cost of a pulp mill shutdown is Euro 20,000-40,000 per hour." So it is in both producers and suppliers interests to limit the maintenance downtime needed.
One area that is worrying producers is the degree of chemical attack on boiler materials. This often results from the mill¹s water closure system which causes non-process elements, or harmful chemicals, to accumulate in the black liquor. Fortunately, help is at hand as a number of steel producers are studying alternative construction materials. At the same time, suppliers, universities and technical institutes are researching new ideas and studying the chemicals' behavior in the char bed of the furnace and on the furnace floor.
Research is also focused on the furnace floor tubes and superheater tubes of the recovery boiler, which are prone to attack. Raimo Paju at Ahlstrom believes that the main problem is the composite tubes. "In the last 10 years, the cracking of the composite tubes on the furnace floor has become a big concern. Today new types of composite tubes have been developed to withstand the more aggressive circumstances, but still the problem of cracking has not been solved completely. In a conventional composite tube, the outer layer of the tube is austenitic stainless steel AISI 304L. But new materials have much higher nickel (and chromium) content."
Joutseno's new recovery boiler from Ahlstrom gives the mill an energy boost
On the boil
Over the last few years, boiler producers have carried out a number of improvements to their equipment. On the recovery boiler side, Vakkilainen claims that the most important progress has been seen in the increase in fired black liquor dry solids to 80%, the rise in design capacity to over 3,000 tons/day of dried solids and the ability to burn additional waste sources in recovery boilers.
In the future, challenges will continue to be thrown in the path of suppliers. These include ever-lower emissions, even more efficient equipment, better protection against wear and tear and, of course, more cost-competitive solutions to grab that extra few tons of capacity. On the producers' side, solutions will be more than welcomed in what is often seen as a headache prone area of the overall process.
|
