The decision by an Oregon pulp company to install a combined heat and power plant to provide process steam and electricity has not only led to fuel and cost savings but also seen it scoop a prestigious industry award.
Mike Moskowitz, CHP Partnership, Washington, D.C., USA
nergy security, reliability, and environmental performance are key issues for governments and industry in the United States and various strategies and tools are being considered and implemented to address these concerns. Combined heat and power (CHP or cogeneration) is a key tool because it benefits the energy user and the environment by reducing energy costs, improving reliability, and reducing overall emissions of pollutants and greenhouse gases. CHP improves fuel conversion efficiency as compared to grid-supplied power and offers the opportunity to capitalize on biomass and opportunity fuels.
The United States Environmental Protection Agency launched the Combined Heat and Power Partnership in 2001 to foster the use of CHP to improve the environmental signature of energy supply in the US. Weyerhaeuser Company is one of the founding partners in the EPA Partnership and an early adopter of CHP.
Weyerhaeuser Company’s containerboard mill in Albany, Oregon was constructed in 1955. Starting in the late 1990s, Weyerhaeuser upgraded the mill to one of the most modern in the world. Using residual wood chips and recycled fibre, the mill annually produces enough high-quality containerboard and grocery bag paper to cover the Interstate Highway from Canada to Mexico 332 times, the equivalent of 748 300 km.
Figure 1. The CHP system installed by Weyerhaeuser won the DOE’s Energy Star CHP award in 2005
In the early 1990s, Weyerhaeuser Company’s paper mill in Albany, Oregon, realized that as a large energy user, a CHP system at the mill could be a great energy and economic strategy. The mill and two adjacent Weyerhaeuser facilities were purchasing 60 MW of power from the local utility and the mill used 226 800 kg per hour of process steam. The steam was generated from a combination of fossil fuel powered boilers, an inefficient hog fuel boiler, and a 25-year-old recovery boiler.
“We realized that by installing a high efficiency CHP system, we could reduce energy costs, have a more reliable system, and be able to sell power when market conditions were favourable,” said mill manager Fred Reed.
Starting in 1995, a CHP system was installed which supplies all the power needs of the paper mill, two adjacent Weyerhaeuser facilities, and surplus power to the US Northwest power grid. During periods of insufficient power supply in the Northwest Region, the CHP plant has provided up to 15 MW of surplus power into the grid. The CHP system also provides all the process steam requirements of the mill. The Albany mill was the first paper mill in Oregon to generate power from clean-burning natural gas. In today’s energy markets, the project provides the facilities with energy flexibility that ensures energy costs remain competitive.
“Part of Weyerhaeuser’s core environmental policy is to continually improve our environmental performance wherever we do business,” Reed said. “This commitment is accomplished through compliance with applicable laws, practicing sustainable forestry, reducing pollution and conserving natural resources. The CHP system is consistent with that policy.”
According to Mark Copeland, the plant’s environmental and technical manager, the mill upgrade significantly reduced its “environmental footprint”.
“To be aligned with our core environmental policy, we reduced the total hazardous air pollutants (HAP) emissions from the mill by 65 per cent and reduced overall mill particulate emissions by more than 50 per cent. Emissions of odorous compounds from the process were nearly eliminated by our state-of-the-art collection and incineration systems. All odorous compounds from the pulping process are collected and incinerated in a new Ahlstrom recovery boiler,” Copeland said.
In 2005 the CHP system was awarded the US Environmental Protection Agency and Department of Energy’s Energy Star CHP Award. The award recognizes CHP projects that achieve at least a five per cent fuel savings over comparable separate heat and power. Separate heat and power is when a plant purchases power from the electric grid and produces thermal energy in-house. The award compares the overall efficiency of the CHP project to the efficiency of an equivalent separate heat and power project producing the same total useful energy and using the same or similar type of fuel. The award also accounts for transmission and distribution losses or savings based on what per cent of the CHP project power output is sold to the US electric grid. CHP projects that have operated for at least a year and 5000 minimum hours, and use five per cent less fuel than the equivalent separate heat and power, qualify for the award. The Albany Mill project operates at about 70 per cent efficiency and uses about 17 per cent less fuel than equivalent separate heat and power. It reduces emissions of greenhouse gases by about 94 000 annual metric tonnes of carbon equivalent.
Figure 2. The mill uses 226 800 kg of process steam per hour and was purchasing 60 MW of power
“The Albany Mill CHP project uses our limited natural resources efficiently and capitalizes on biomass fuels to produce reliable energy supply with lower environmental and climate impacts,” said Katrina Pielli from the US EPA Combined Heat and Power Partnership.
The Energy Star CHP award is one of the services of the US EPA Combined Heat and Power Partnership. This organization is a voluntary public-private partnership in which EPA works with industry, energy users, and federal and state governments to foster the use of clean, efficient CHP in the US. CHP helps reduce emissions of greenhouse gases from power generation by reducing the amount of fuel required to produce the energy. About five projects a year receive the Energy Star CHP Award.
The cogeneration system consists of a GE LM6000PC w/Sprint natural gas fired combustion turbine, a GE 51 MW generator and a Babcock & Wilcox heat recovery steam generator (HRSG) fed by the combustion turbine exhaust stream. When natural gas is in short supply the turbine is capable of burning diesel oil. The HRSG is equipped with natural gas fired duct burners to generate additional process steam. The 86 bar steam from the HRSG and from an Ahlstrom biomass fired recovery boiler, feed an Ansaldo steam turbine driving a 45 MW Ansaldo generator. Normally, the steam turbine produces power based on process steam demand with the LM6000 combustion turbine (CT) taking the swings for electrical load-following. Power requirements for internal use typically averages 60 MW, with 43 MW supplied from the combustion turbine and 17 MW from the steam turbine.
The combustion turbine and HRSG were started up in 1995. The original combustion turbine was a GE LM6000PA capable of generating 43 MW. In 2001, when the 50 000 hour engine rebuild was due, the engine was upgraded to a GE LM6000PC w/Sprint that increased power output to 47 MW and improved the heat rate by 357 BTU/kWh. The controls were also upgraded to the Mark V LM package for a faster fuel update rate. The combustion turbine has two 550 tonne chillers to keep the air inlet temperature at 9°C for optimum power generation. The CT runs 354 days per year and averaged 97.6 per cent reliability in 2004. The HRSG is rated to produce 127 000 kg per hour of 86 bar steam at 545°C and 6804 kg per hour of 11 bar steam.
An Ansaldo steam turbine and generator were installed in 2000. The generator is rated to produce 45 MW with 353 800 kg per hour of 86 bar inlet steam flow to the steam turbine. The steam turbine rotor is currently bladed for 249 500 kg per hour of inlet steam which produces 26 MW. Process steam is extracted from an uncontrolled extraction outlet at 33 bar, a controlled extraction outlet at 11 bar, and the remainder is exhausted at 4.5 bar. The steam turbine runs 360 days per year and had 99.8 per cent reliability in 2004. Both the combustion turbine and steam turbine are remotely controlled through an ABB distributive control system.
Figure 3. Basic outline of the Weyerhaeuser cogeneration system
The Ahlstrom boiler was started up in 2000 and is fueled with the lignin (biomass) removed from wood chips during the production of pulp used to make containerboard that is converted into shipping boxes. The boiler also recovers, for reuse, virtually all of the chemicals required in that process. It is rated to process 1.6 million kg of dry solids per day and produce 244 000 kg per hr of 86 bar steam at 545°C. Two load burners, that normally burn natural gas, are used to produce any steam demand not satisfied by the biomass fuel. The load burners and six startup burners are also capable of using diesel oil. The Ahlstrom boiler is nearly 61 m tall, has a single 1829 mm diameter steam drum, an economizer section, and is equipped with electrostatic precipitators for particulate removal from the flue gases.
According to Reed, the CHP system has performed above expectations and has increased energy efficiency for Weyerhaeuser as well as providing the opportunity to sell power into the power grid that generates an additional revenue stream.