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Arnot Ogden Medical Center's biomass boiler.

Also included in the project was a fully automated storage system complete with a 7,200-cu-ft fuel-storage bunker and a material-handling system that feeds the biomass boiler. The underground storage bunker is large enough to meet peak-load plant requirements with a maximum of two fuel deliveries per day. Wood chips are fed to the burner by an automated auger system that is metered to deliver the required fuel based on system load. Therefore, the turndown ratio is almost infinite.

Benefits

From the project's onset, a major priority was to make sure that the improvements made economical and environmental sense for the hospital. The biomass boiler's increased efficiency, safety, and cleanliness helped the project meet those needs. High temperature combustion, a 20-1 turndown ratio, refractory heat storage, and controlled air allow the gasifier not only to respond quickly to boiler demand, but to idle efficiently for economical operation in low-load conditions.

Additionally, natural gas recently has experienced cost swings as high as 35 percent annually. Commercial natural gas averaged $10.50 per million British thermal units, while the wood-chip equivalent averaged $3.90 per million British thermal units based on a fuel cost of $36 per ton. (Projected fuel costs were based on a five-year average for natural gas beginning in 2007.)

FIGURE 2. Biomass-boiler load and savings potential.

The biomass boiler helps insulate the hospital from fuel-price volatility by providing alternative-fuel choices, allowing the hospital to switch energy sources if necessary. Also, the hospital's wood-chip supplier is providing fuel at a fixed price for a six-year period, excluding delivery costs, which are dictated by diesel-fuel cost escalation.

The seasonal efficiency of the biomass boiler (73 percent) is similar to that of a fossil-fuel-fired boiler (75 percent). However, the efficiency of the wood-chip system increases to 78 percent when fuel moisture content reaches 35 percent. Therefore, the average seasonal efficiency of the biomass boiler increases as fuel quality improves, which is fairly simple to achieve.

Limitations

Although Arnot Ogden receives fuel on a regular basis, wood-chip availability could be an issue for other facilities, depending on whether lumberyards and mills are located locally. Further, the biomass system's underground bunker design could be complicated by a high water table. Additionally, the system requires a large amount of floor space to house all of the system components, which can include a high initial price compared with a conventional boiler system.

Meeting Current and Future Needs

Arnot Ogden is planning to construct a new patient tower, and the biomass boiler will help handle the addition to the hospital's heating load efficiently and effectively. The system is sized to provide the hospital with the extra capacity needed to serve the new space when construction is complete. Furthermore, when combined with boilers 1 and 2, the new boiler will provide capacity redundancy, so the entire facility still will receive heat, even if one of the units goes down. Figure 2 illustrates the load and savings potential of the biomass system.

Since its installation during the summer of 2008, the biomass system has helped Arnot Ogden realize approximately $538,000 in energy savings.

A performance-contracting engineer for Honeywell, Michael Daskalakis has more than 20 years of energy-engineering experience and has developed energy-saving performance contracts for commercial and institutional clients throughout the Northeast. He can be reached at michael.daskalakis@honeywell.com. Venkat Iyer is a performance-contracting-engineer manager for Honeywell and has more than 35 years of industry experience. In 2008, he was chairman of the company's performance contracting engineer council; since 2005, he has been an honorary council member. He can be reached at venkat.iyer@honeywell.com.