August 1, 2011

Improving the Efficiency of Toronto’s District Heating Plant

Richard Parrish, president of MindShare Communications.

Enwave Energy Corp.’s district heating plants in downtown Toronto will be operating cleaner and more efficiently before the fall 2011 heating season begins when boiler upgrades now under way are completed. Enwave hired Benz Air Engineering (BAE) to design and install upgrades to all eight boilers inside Enwave’s Pearl Street Station. When the $20 million project is completed, the retrofits will produce energy savings exceeding $5 million per year. In addition, the company will receive incentives of $100,000 per boiler from Enbridge, its natural gas provider (Figure 4).

4. Downtown district heating. Enwave’s Pearl Street Station in downtown Toronto, Ontario, produces steam for heating buildings about 170 days each year. Courtesy: Super Radiator Coils

Through Enwave facilities, Toronto also uses deep lake water cooling for economical and environmentally friendly air conditioning in large downtown office buildings (see “Natural Air Conditioning,” POWER, April 2007). Water, at a constant 39F at 272 feet below the surface of Lake Ontario, is used to cool buildings via a separate closed-loop cooling system.

Enwave sells an average of more than 2.5 billion pounds of steam for district heating to 140-plus commercial buildings in downtown Toronto, including residences and a hospital, for a total of 40 million square feet over an average 170-day heating season. In addition to the Pearl Street plant, steam is produced at Enwave’s Walton Street and Queen’s Park facilities and then distributed through the company’s extensive underground pipe network at 375F and 200 psig.

So far, conversion has been completed on two of the eight boilers (boilers 5 and 6) at the Pearl Street Station (Figure 5). The boiler test results are excellent: Boiler efficiency increased from 79% to 96%, and the stack temperature was reduced from 300F to 240F with no precooling. In addition, the unsightly stack plume was eliminated by extracting up to 95% of the water that is formed as a combustion byproduct. NO_x emissions also plummeted from 130 ppm to less than 3 ppm. The remaining six boiler upgrades will be completed before the fall heating season begins.

5. Upgraded boilers. Two of the eight boilers have completed performance and environmental upgrades. The remaining six boilers will complete upgrades before the fall heating season begins. Courtesy: Super Radiator Coils

Less Burning; Less Emissions

“Enwave was facing a shutdown at the Pearl Street Plant, unless emissions were reduced and the visible plume disappeared from the stack,” said Robert Benz, BAE president. Otherwise, future residents in a major high rise condo development across the street would be constantly subjected to unpleasant fumes coming from the plant.

Benz Air was awarded the project in June 2010. Construction began almost immediately to build a see-through, steel-mesh mezzanine level above the huge boilers. The new floor will make it much easier to install, service, and maintain the large stainless steel housings, pipes, and controls for the new emission and recovery systems on top of each boiler.

Constructed of low-carbon steel, each of the boilers stands 14 ft tall, 45 ft long, and 12 ft wide; is coated with refractory brick at the front/burner end and the back end; and weighs approximately 130,000 pounds. Built in the 1960s, five of the units were fabricated by Inglis and featured a double-burner configuration; the remaining three boilers have a single burner and were manufactured by Babcock and Wilcox. All of the boilers are a water-tube design and have a steaming capacity of 100,000 lb/hr at 390F and 200 psig. Combustion temperature of the flame inside the firebox is about 2,200F, and flue gas leaving the boilers is 450F to 550F.

“By reducing fuel that’s burned, which in Enwave’s case is natural gas, we reduce carbon dioxide emissions and lower energy costs simultaneously,” said Benz. The company’s CondiMax flue gas condensing heat exchanger, placed on the stack of each boiler, is a system that extracts far greater heat than either direct or indirect heat exchangers. The system uses a two-stage heat exchange design, recovering the remaining sensible heat without the waste associated with direct contact heat exchangers.