Wood waste has a purpose
Urban wood waste provides heating, cooling and electricity generation for St. Paul, Minnesota.
A historic setting
The building that houses the District Energy Facility sits on the banks of the Mississippi, surrounded by St. Paul’s beautiful urban forest. The structure is an old brick 1906 design that was originally constructed to burn coal for the production of steam to heat downtown St. Paul. Today, the building is still being used to produce heat but in an entirely different manner.
It all stems from wood fuel
The St. Paul District Energy facility burns as much as 250,000 tons of wood waste each year, drawing from the urban tree trimmings, damaged tree removal, habitat restoration activities and the leftovers from forest management activities that occur within 50 miles of the facility. As many as 45 semi-tractor trailer loads of wood chips are delivered from a nearby collection yard each day to keep the combined heating and power system working 24 hours a day.
District Energy - Here’s how it works
A regulated amount of wood is dropped into a combined heating and power (CHP) boiler to combust the wood using a small amount of natural gas (up to 15 percent of the fuel) that adds intensity and consistency to the combustion process. Inside the boiler are pipes filled with water that absorb the heat to generate steam, which is delivered to the turbine to push the internal rotating blades. The blades turn a shaft at nearly 100 revolutions per second, and the mechanical energy produced is converted by the generator to an electrical current. The turbine can produce 37 megawatts of electricity, 25 of which are sold to the grid while the remaining are used for plant operations.
After turning the blades, the steam is then used to create hot water for the District Energy hot water loop, which consists of 106,100 trench feet of underground supply and return piping that can circulate up to 1 million gallons of hot water (180-250 degrees Fahrenheit) to service more than 85 percent of St. Paul’s downtown square footage.
Coal continues to be burned to augment the production of hot water, especially during peak winter months, when the temperature can be as low as -30 degrees F. Plans are underway to eliminate the necessity of coal by the 2020-2021 heating season, replacing it with increased amounts of wood or other combustible materials that can be utilized with minimal modifications to the coal burning portions of the facility.
Cooling buildings with chilled water
The Ever-Green District Energy Facility also produces chilled water to cool as much as 65 percent of the indoor square footage of St. Paul’s downtown area. Water is chilled through a mechanical compression process that effectively removes the heat from the water. The chilled water (42 degrees F) is then circulated to buildings where it absorbs the heat from internal spaces, which cools the air. The water is then returned to the plant to start the chilling process again. The chilled water system consists of 38,800 trench feet of underground supply, circulating 1,050,000 gallons of chilled water.
The combined cold and hot water heating distribution system serves more than 180 buildings in the downtown district and 300 single-family homes in the adjacent area. While St. Paul houses the largest district energy system in the nation, the concept can be replicated on a much more local scale. Visit Michigan’s Wood Energy Team website to read about smaller scale district energy examples.
Have you considered wood? Contact one of the wood energy team representatives to learn about the benefits available from making the switch. Wood is more stable in price, is carbon neutral and, most importantly, will cost less over time than burning carbon based fuels. Consider making the switch today!