Growing the market

Combine engineering ingenuity, the EU CO2 trading market, strategically located power plants and a burgeoning electricity market with the potential to close the fuel cycle ecologically – and the will to survive becomes a recipe to thrive.

Jason Makansi, Pearl Street Inc, Missouri, USA

The conversion of 1950s-vintage boilers from lignite-firing to biomass at two venerable Hungarian power stations, Borsod and Tiszapalkonya, represents a classic power industry version of “think global, act local.” Faced with the imminent closure of these two plants in 2002, the people of AES Borsod, (part of the global power company AES Corp Arlington, VA, USA), which acquired these facilities in 1996, successfully executed a unique turn-around.

The project included the following elements:

  • Sale of CO2 credits to the Dutch government through the Joint Implementation mechanism of Kyoto in the European market to finance part of the project at Borsod.
  • Home-grown bubbling fluidized-bed boiler technology applied at both plants.
  • Addition of an expansive fuel management and fuel preparation strategy that is evolving to include growth and harvesting of energy crops in the area.
  • Programmes and demonstrations to recycle the fly ash from the plants into a compost that exceeds the performance of chemical fertilizers.
  • Contract sales of electricity to meet Hungary’s quota for renewable energy as stipulated by the European Union, as well as sales into the emerging Hungarian electricity market.
  • At around 120 MW total electric output, the two plants now represent one of the largest biomass-to-electricity operations in the world.

Figure 1: Tiszapalkonya power station in Hungary earned a new lease on life by converting to biomass firing
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Both plants are now reaping the rewards of some calculated technical risks taken to survive. The go-ahead for the Borsod turn-around plan was given in 2002, during the depths of the crisis in merchant and global power generation. Visits to AES biomass plants in the US and sharing of technical knowledge among AES business units lent confidence that the turn-around could be achieved.

Old boilers, new technology

Converting 50-year old boilers designed for lignite was the hallmark of the retrofit project. Each plant pursued different strategies to handle a biomass fuel that typically has 33-35 per cent moisture content and 0.5 per cent ash content.

At Borsod, explains plant manager George Vesci, two of the ten boilers, each producing 100 t/h of 75 bar/495à‚°C steam, can now be fired by 100 per cent biomass (Figure 1) and achieve 90 per cent boiler efficiency at 2-3 per cent O2 content. The patented “turbulent-bed” technology was originally developed for coal and adapted for biomass by a consortium of the Hungarian firms Veiki and TransElektro. A small amount of sand, relative to other bubbling bed techniques, is used, along with ash, as bed material. Generally, these units run baseload but operators indicate that they are flexible and can run at 50 per cent capacity with no problems.

Fuel is fed from the top of the boilers from a long bi-directional conveyor system, reconstructed to handle wood chips typically sized to 2.5 cm by 2.5 cm. The conveyor stops over each boiler’s storage bin and discharges woodchips. These chips flow from hold-up storage bins either directly to the furnace or to hammermills which further reduce the size of the fuel to a few millimeters. The hammermills were added after it was determined that the elimination of high bed temperatures would require smaller sized fuel.

Material ultimately entering the furnace is a mixture of approximately 50 per cent woodchips and 50 per cent sized material from the hammermills. This mixture is fed to the furnace from eight fuel injector ports, approximately 9-10 m above the furnace floor, from two levels, separated by 2 m, on opposing furnace walls.

Figure 2: Biomass storage, management and feed systems were installed at both plants
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New forced-draft fans were added to assist the existing induced-draft fans in moving air through the furnace at higher pressure. The bottom of the boiler now includes hundreds of air distribution ports to keep the bed material fluidized. The bed operates at approximately 1000à‚°C.

A third boiler was also converted to burn sawdust. Only minor boiler or furnace modifications were required here as the plant progressively moved from an 80 per cent coal/20 per cent sawdust blend to 100 per cent sawdust firing. This boiler only operates about one week out of the month. The woodchip-fired boilers can also burn sawdust.

The ash is high in potassium, which aggravates deposition on boiler, convective pass, and air heater surfaces. Water washing and air blast systems were added to remove deposits during maintenance outages; some deposits must even be removed by hand. Each boiler generally must come down 1-2 days per month for scheduled maintenance and cleaning. This does not cause any problem since there is much spare capacity and flexibility at the plant.

Big chipper, little chipper

Feeding these boilers 800-1000 tonnes/day of woodchips requires wood receiving and preparation on both sides of the plant, hence the bi-directional conveyor. On one end, a large 160 cm diameter, 980 rpm, 120 tonne/hr chipper, driven by a 1 MW motor, is fed logs from the storage area by two 12 m long, 6 m wide chain conveyors. Chips are subsequently sized through a mesh screen, and large material is recycled. The smaller back-up chipper is used to maintain an inventory, typically 5000-10 000 tonnes, of chipped wood fuel.

Generally, six to eight hours worth of fuel is kept in the powerhouse storage bins, although plant operators indicate that more hold-up is necessary. This could be achieved by activating storage bins at adjacent boiler units. On one side of the plant 30 days-worth of wood chips is stored, and 30 days of raw wood is held on site. The plant maintains long-term fuel contracts with four privately owned foresting companies for 280 000 tonnes/year, delivered by rail and truck, over ten years.

Other aspects of the project include: refurbishment of the electrostatic precipitators and cooling tower, steam turbine and generator overhaul, and addition of a distributed control system (DCS). While the DCS has helped to automate much of the plant’s operation, except for the fuel handling and prep which is very labour intensive, it could use additional built-in diagnostics, report plant officials, to prevent forced outages caused by the instrumentation and electrical systems.

At Tiszapalkonya, two of the eight 125 t/h, three-pass, balanced-draft type boilers were converted for 100 per cent biomass and two others were converted for 80 per cent coal and 20 per cent biomass.

The 100 per cent boiler conversions proceeded differently from Borsod, because the plant was informed that the units were not suitable for the turbulent bed technology. With the Hungarian firm Matuz as advisors, the burners of the original roof-fired boiler were replaced with biomass injection ports and the roof waterwalls were replaced with refractory lining to retain heat and help dry the wood. Refractory lining was also added to the furnace floor to improve carbon burnout. Flue-gas recirculation (FGR) was added to reduce NOx emissions and maintain appropriate steam temperatures.

The original coal mills and classifiers were modified to achieve a 100-micron sized biomass fuel. A new chain conveyor belt with heat-resistant steel was added to remove ash from the bottom of the furnace. Finally, a furnace ash recirculation system was installed to ensure high carbon burnout. No gas firing support is needed at typical operating loads.

Certainly one source of pride for the plant is the 100 t/hr wood chipper purchased for $50,000 from a Slovak paper mill, deconstructed at the mill site, hauled to Tisza, and rebuilt inside a new housing unit. A new chipper would have cost $2-3 million. Every four shifts, the chipper is taken out of service, the knife is replaced by one with a sharpened blade, and the previous knife is taken in for sharpening.

Plant manager Kiss Csaba explains that Tiszapalkonya is now the largest power plant without long-term power purchase agreements selling into the Hungarian liberalized electricity market. The conversion was achieved while the plant continued to supply power to the grid. The entire retrofit work and equipment cost under $2.5 million, with the electrostatic precipitators accounting for 40 per cent.

CO2 emissions from 2003 levels have been cut by a third and are projected to be cut in half by 2007. Fly ash emissions are below the 50 mg/m3 limit, and SO2 emissions are almost negligible. Csaba expects the plant to sell more than 150 GWh of renewable energy into the Hungarian market in 2005.

Maintain fuel supply lines

Presently, both plants can consume more biomass than is available in the region. Plus, other units at the facilities could be converted for biomass firing. Both plants, for example, burn sunflower hulls (a good fuel because they are relatively low in moisture and have an oil residue, according to plant officials).

To plan for the future, according to Istvan Aved, AES project director, the company has cultivated relationships with area universities and others to develop a regional energy-crop industry. Says Istvan: “There is surplus land available to devote to energy crops, at least 20 per cent of the agricultural land, or one million hectares.” Within a radius of 50 km, 40 000-50 000 hectares are available for energy plantations.

Already, AES has contracted with third parties for 160 000 tonnes of energy crops annually, and is negotiating for wheat and barley straw contracts which could amount to 10 000 to 50 000 tonnes/year. “In 2008 energy crops could represent 40 per cent of Borsod’s fuel requirements,” notes Istvan.

Figure 3: The Hungarian power plants of Borsod (left) and Tiszapalkonya (right) were both built in the 1950s
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One of these energy crops is energy grass, a form of straw. It grows to a height of about 2 m and has been found to be relatively insensitive to weather, rainfall, and soil conditions. The grass would have to be baled to appropriate sizes to fit the lorries that transport fuel to the plants.

Another option under investigation is poplar plantations. Poplars can grow to 6-7 m high with 4-5 cm stump sizes on a one-year rotation. This material would likely be chipped, stored, and dried at the growing sites. The poplars would be around 50 per cent moisture and, ideally, they should be around 35 per cent moisture when fired in the boilers.

Long term, AES may require more than just forest residues, agricultural wastes, and energy crops. Therefore, it is also investigating refuse-derived fuel (RDF) made from a process developed in Italy. If pilot tests with RDF that AES will soon conduct are successful, then the Italian firm will consider building an RDF processing plant in Hungary. The RDF would be delivered to the plant as compact bales.

No resting on laurels

Technology in the service of social responsibility is a hallmark of AES’s facilities around the world. But you can’t let your guard down in this industry. As Peter Lithgow, AES Hungary country manager, remarked, “These conversions are great projects, but still the long-term fate of these plants must be considered.” Should subsidies for renewable-generated electricity retreat, or the CO2 credits market falter, AES is already considering options such as biomass gasification or a high-temperature high-pressure circulating fluidized bed coupled to a highly efficient gas turbine combined cycle. The larger point, concludes Peter, is that “these facilities are strategically located in terms of rail lines, the European grid, fuel sources, etc.”

The evidence is that the new leases on life at Borsod and Tiszapalkonya should ensure their successful operation extending far into the future.

Social responsibility

Social responsibility can manifest itself in many ways. One indicator is called E-Equity, the holistic value a power station posesses when all its outputs-economic and community development, emissions and discharges, electricity, and even national security-are optimized and considered within a framework of industrial ecology and sustainability.

From this perspective, AES Borsod and Tiszapalkonya present an interesting case study over the long-term. First, both plants still cogenerate steam (Borsod produces 60 t/hr) and produce demineralized water for neighbouring industrial facilities, maximizing the productive use of biomass fuel.

Second, low-grade lignite and coal firing has been greatly reduced, with obvious but substantial reductions in SO2, NOx, and flyash emissions.

Importantly, the plants now generate net CO2 credits. In fact, this is how more than 20 per cent of the total conversion costs were financed at Borsod. The Dutch government contracted with AES Hungary to buy 740 000 tonnes/year of CO2 for the period 2008-2012. Half of the contract value came as a prepayment, based on achieving milestones in the conversion project and first-month’s operation.

At the same time, the Hungarian government must meet an EU mandate of 3.6 per cent electricity generation from renewable sources by 2010. Therefore, a compulsory off-take was instituted at a price that is typically double that of the current market prices.

Growth, harvesting, and preparation of wood and energy crops could eventually make up for coal mining losses to the local economy.

Biomass growth (which sequesters CO2) and biomass combustion (which releases CO2) net out as reduced CO2 emissions overall. AES, along with local academic institutions, is even demonstrating that a material consisting of 25 per cent wood combustion ash, bark, and dewatered sewage sludge makes a compost that is superior in quality and lower cost than ammonium-based fertilizers. AES Borsod expects to receive a licence for this beneficial use of flyash soon.

Such examples of industrial ecology-in which discharges are beneficially recycled into another industry-have a net positive impact on the environment. In this case, the impacts caused by drilling, transporting, and processing natural gas into fertilizer are avoided, as are the potential impacts of agricultural run-off when too much fertilizer is applied to the land.

Thus, such power stations come tantalizing close to a true “zero impact” facility. Combine this with the role the power station plays in the community, which even includes partnerships with schools to provide IT and computer education and training in area high schools, and perhaps the ultimate goals of social responsibility are not only met, but surpassed.

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