July 1, 2010

Drax Offers Model for Cofiring Biomass

By Sam Saimbi and Doug Hart, Alstom Power

The Master Plan

Drax began preparations for burning biomass in 2003 when it first put biomass through its mills with coal. Then in 2005 it started a cofiring pilot project on one of the plant’s six 660-MW generating units.

The success of this pilot led to a decision to install a dedicated cofiring scheme across all six units. This project, which will take 36 months from design to commissioning, has the capacity to process fuel for 400 MW, meaning that 10% of the plant’s generating capacity will be capable of being fueled by about 1.5 million mt/year of biomass.

Drax Power Ltd. made the decision to build a commercial cofiring project in 2008. The project was split into three separate contracts: the injection system; the rail unloading facility; and the processing works to receive, handle, store, and process various biomass materials. Following a detailed design study, the approximately £50 million ($80 million) contract for the processing works was awarded to Alstom Power in early 2008.

As the world’s largest biomass cofiring scheme, Drax requires (with all six units firing) 500 cubic meters of biomass an hour to be unloaded, processed, stored, milled, transported, and fired. Drax had previously been using the existing coal delivery system and co-milling biomass by using the existing milling system. This approach limited the throughput and the type of biomass fuels that could be fired. Instead, a dedicated fuel-handling and preparation system ensures that the biomass does not affect the existing coal delivery plant.

The project has progressed smoothly to date. The first two units at Drax began part operation in December last year, the second two began operating in early February 2010, and in April 2010 all six units became operational. The entire installation is scheduled to be fully operational by mid-year.

Through plant photos and 3-D renderings, figures 2 through 7 illustrate the biomass process flow from collection through firing.

2 Receive the fuel. As a retrofit project, there were demanding construction challenges and constraints due to the existing plant’s footprint. These challenges were overcome by installing the road unloading, storage, and process works in a coal stockyard away from the boiler building. The road unloading area receives around 250 mt/h of biomass, which is unloaded by five or six trucks. Courtesy: Alstom Power

3. Unload the fuel. The biomass is transported from the road unloading building via an enclosed conveyor system that uses an air cushion to reduce fuel agitation and contain dust emissions. Courtesy: Alstom Power

4. Process the fuel. The biomass fuel then enters a processing facility, where a series of processes including agitation and magnetic separation are used to separate out any metallic objects or stones from the biomass. At the processing facility, a sample of the biomass is taken for testing using a sampling system. Courtesy: Alstom Power

5. Convey the fuel. The biomass is then transported by conveyor from the process facility to four bulk storage silos, each with a capacity of 3,000 m3. Courtesy: Alstom Power

6. Store the fuel. From the bulk storage silos, the biomass is transported across the power plant to three smaller (day) silos, located just outside of the boiler building. This methodology ensures minimal disruption to the plant’s day-to-day operations. Each of these day silos can store enough fuel for two of the plant’s six units. Courtesy: Alstom Power

7. Grind and fire the fuel. The final processing and milling follows when the biomass is fed to a series of hammer mills that grind it into a fine powder. An air knife blower is used to remove any remaining particulates and contaminates. The biomass is then fed into an existing pulverized fuel line, where it is mixed with coal and fed to the burners of each boiler. Due to the wall-fired design of the boilers at Drax, no modifications are needed to the existing burners, which reduced the capital cost of the project. Courtesy: Alstom Power

U.S. Activity Picks Up

With plants like Drax, Fiddler’s Ferry, and Amer 8 as references, Alstom is well positioned to help move biomass into the mainstream as a fuel source for power generation. Drax also provides a glimpse into the potential for biomass energy to be used in electricity production in the U.S., a market that is still dominated by coal-fired generation. We find many utilities are exploring the cofiring option at their coal-fired plants. One key decision is whether to use engineered or raw fuel, crops, or forest residues.

There are also a number of boiler-specific technical questions to be answered, such as how to maintain boiler efficiency and steam temperatures for optimum performance. Generally, the type of fuel selected will strongly determine the effect on boiler operation, requiring a holistic approach to plant modifications. Once you know the amount and type of biomass fuel available, the costs associated with maintaining boiler performance—and the impact on emissions such as SO_x, NO_x, CO, CO₂, heavy metals, and particulates—can be determined. Other effects on the boiler, such as the potential for slagging and corrosion, must also be considered in any retrofit design.