Chipping Away At Emissions
esmagazine.com | March 8, 2009
Tighter air regulations and and abundance of trees have paired up to fuel the move toward hybrid biomass boilers in these Canadian hospitals. Read about the upgraded systems' differences and the considerations for trading fossil fuels for sawdust, wood chips, and bark.
Think of Canada, and there's a good chance that what comes to mind is a sparsely populated country with spectacular scenery. Meandering coastlines, dramatic mountain ranges, and pristine lakes are located throughout the provinces, as well as vast expanses of forest, which carpet 402 million hectares — or almost half — of the country.
Given the enormous amount of trees growing in Canada, it is no wonder that the forest industry is extremely important to the country's economy. It contributes up to 3% of the gross domestic product and over $34 billion in exports, which include the largest amount of market pulp and newsprint in the world.
The province of Quebec is an integral part of the country's forest industry, as its millions of acres of forests supply approximately 500 mills with wood for lumber, pulp and paper, and plywood. These mills process countless numbers of trees every day, resulting in massive quantities of byproducts such as sawdust, bark, and wood shavings. The mills would normally have to pay to have these byproducts removed from their sites, but two Quebec hospitals are now utilizing the wood waste to power their new biomass boilers.
Canada has often been in the forefront of environmental legislation, and continued actions by the government support that position. The government recently introduced tough, mandatory regulations for industry to reduce greenhouse gas emissions by 20% by 2020 and 60% to 70% by 2050. The government also plans to cut air pollution in half by 2015, which is the most aggressive plan in Canadian history.
In order to curb its greenhouse gas emissions and comply with the regulations, the government of Quebec asked the mechanical contracting firm of R. Nantel & Associés Inc., Montreal, to survey its northern hospitals to determine if they were candidates for biomass boilers. “Years ago, the government purchased solid fuel boilers, which today are in such bad shape, there was no way to get them legal with the environment, so they had to be replaced. There are a lot of sawmills in northern Quebec that have a lot of solid fuel left all over the place that the hospitals could use, so the biomass boilers made sense," said Roland Fauteux, of R. Nantel & Associés Inc.
These boilers not only utilize the wood waste from the local mills, they also substantially improve the hospitals' energy efficiency while reducing the facilities' emissions of greenhouse gases. Some might question whether or not burning wood is that much cleaner than burning other types of fuel (Sidebar), but modern biomass boilers come equipped with proven emissions control devices that significantly reduce NOx and CO emissions. In addition, particulate emissions are reduced through the use of cyclone separators, bag houses, or electronic precipitators, which catch almost all the ash generated during the combustion process.
Two small hospitals in northern Quebec, La Sarre and Macamic, were just recently retrofitted with new biomass boilers, and there are more on the horizon. “There are close to 11 hospitals to do," said Fauteux. “Usually after one is done, the start-up completed, and the bugs worked out, we go on to the next one. We do one a year, because by the time the operators are at ease, it takes that much time. It also comes down to budget, because there's only so much money to spend."
And, as Fauteux pointed out, once he gets into the boiler room, there's more to do than just replace the boiler: Most of the other equipment needs to be repaired or replaced as well, including headers, valves, pumps, and heat exchangers. Fauteux noted at one hospital, one of the high-pressure valves had a hole in it, and a screw had been put in to hold the steam. “You definitely didn't want to go alongside where the screw was," he chuckled. “You'd get a hole in your head if it let go."
Hybrid Boiler Fills The Bill
La Sarre and Macamic Hospitals were built in the 1940s, and both still had their original boilers, which Fauteux pointed out were riveted, not welded. Most of the original boilers were disassembled and removed, although each hospital kept one existing boiler as a backup. Ironically, these boilers are not allowed to operate due to their excessive emissions, but they will remain in place until facility personnel become comfortable with the new biomass boilers.
Fauteux installed Hurst multi-fuel “Hybrid Series" solid fuel-fired steam boilers at both hospitals, and back-up oil burners were installed on the 200-hp boiler at La Sarre and the 250-hp boiler at Macamic, just in case there is ever a problem in the fuel handling.
The fuel is kept in a storage area that is located 30 ft down in the ground, which keeps the wood from freezing. Every few days, a truck delivers a load of wood waste — usually sawdust, wood chips, and bark — and dumps it into the reserve. Due to rising fuel prices, these fuel deliveries are getting more expensive, but they're still less than the cost of buying oil or gas.
“Years ago, it cost about $25/ton for the wood waste to be delivered," said Fauteux. “Now it's $65/ton undelivered. But the price of oil and gas is going up, too. The wood waste costs 35% to 40% of what it would to buy oil or gas, which is a big savings."
The fuel in the ground rests on a “walking" floor, which allows the bulk biomass to transition from the storage area into the fuel feed circuit as called for by the power system it is supplying. The walking floor moves the fuel to a conveyor belt and then to a feed/transfer conveyor supplying a metering bin. The metering bin maintains a minimal reserve of fuel, however, its primary function is to provide a positive head of fuel above the variable pitch metering screw to ensure a relatively constant delivery rate per revolution.
The metering bin is fitted with a level indicating device, so as the load on the energy system is increased (i.e., steam pressure drops), the demand for fuel correspondingly increases, lowering the fuel level in the bin. A metering screw located immediately below the metering bin is used to transport the fuel, at a measured rate, to a rotary air lock device. The air lock device acts as a draft barrier, allowing fuel to pass from the metering screw to the stoker screw while limiting the intrusion of air through the fuel system. The stoker then brings the fuel into the boiler (the underfeed stoker system is used at the Quebec hospitals).
To control particulate emissions, the boilers are equipped with two multi-cyclones, commonly called collectors, in series. The principle of operation of the cyclones is identical: Hot particle laden flue gases enter the unit and are directed down through the outer of two concentric tubes. Access to the outer cast iron tube is through spinner vanes that impart a centrifugal motion to the flue gases. The heavier entrained particles are forced to the outside of the larger outer tube and fall to the bottom of the cyclone hopper. The gases exit the larger tube through the smaller steel center tube.
Approximately 95% of the particulate matter is removed at the primary collector. The ash/particulate drops to the bottom of the tapered hopper and passes through a rotary air lock to a either dumpster or drag chain conveyor for removal, or to a fan-driven re-injection line back into the combustion chamber. The secondary collector contains a larger number of smaller cast iron and steel tube configurations, and approximately 95% of the remaining finer particulate matter is removed here.
Fauteux said he knows the emission controls on the new boilers are working because the hospitals are no longer identified by the big black clouds hovering over the buildings. “It used to be you didn't have to get directions to get to the hospitals — all you had to do was watch for the smoke coming out. Now you have to watch for the signs, because you'll never get there otherwise."
Same System Used Everywhere
The same hybrid boiler system will be installed at all the other Quebec hospitals in which Fauteux is involved for two reasons. First, the government is comfortable with this particular technology, and second, keeping the same system results in fewer spare parts being needed by the hospitals.
“The government wanted to have one central location in the north that doesn't have to supply parts for 20 different kinds of boilers," said Fauteux. “This is a good move for their budget, because they can supply all the hospitals through one warehouse, because all the motors and drives are the same from place to place. There isn't a heavy budget for spare parts in each hospital."
Using the same boiler system at the different locations will also be easier for hospital personnel, who are having to adjust to using a computer to control the equipment. “They had to get used to this new technology. It won't bite. If necessary, all they have to do is shut down the boiler, and then they restart it. It's not that complicated.
They really appreciate that now they can shut down the boiler from their home or office, or make corrections without going over there."
Neither hospital has been through a complete season, so a full energy assessment hasn't been made. However, Fauteux believes each facility is saving 40% to 45% in their energy costs, compared to what they were paying previously. And most importantly, the boilers passed the stringent emissions tests conducted by the government.
“They haven't decided which hospital we will work on next," said Fauteux, “but now that we have the boiler system down, each job will move a little faster." ES
Sidebar: Advantages of Wood Biomass Equipment
In a paper prepared for the USDA Forest Service, State and Private Forestry Technology Marketing Unit, Forest Products Laboratory, Madison, WI, chemical engineer Richard Bergman and wood technologist John Zerbe outlined some of the benefits of using wood biomass equipment:
“Wood fuel has several environmental advantages compared with fossil fuels. Wood can be continually replenished, which leads to a sustainable and dependable supply. However, proper forest management must be practiced to ensure that growing conditions are not degraded during biomass production.
“There is little net production of carbon dioxide from wood combustion, because the CO2 generated during combustion of wood equals the CO2 consumed during the lifecycle of the tree. Transporting the material using petroleum generates excess CO2.
“Wood fuel contains minimal heavy metals and extremely low levels of sulfur; wood fuel is no threat to acid rain pollution. Particulate emissions from wood are controllable through standard emission control devices such as bag houses, cyclone separators, and electronic precipitators. Bottom ash is minimal. Usually, wood ash is less than 1% of the weight of the wood, and sometimes ash may be used as a fertilizer.
“The principal economic advantage of wood-burning systems is that wood fuel is usually less expensive than competing fossil fuels. However, the price of wood for use as fuel can be extremely variable. Sometimes when surplus supplies of wood residues are available at nearby forest product manufacturing plants or municipal solid waste handling facilities, the cost can be very low or even negative. Transportation for delivering from the supply site to the wood combustion or wood processing unit is the primary expense of wood fuel.
“At other times, mostly dependent on location of the wood power facility, the cost of wood fuel can be quite high because large volumes of fuel are needed to have a dependable and consistent supply of wood fuel. Because the market for wood biomass energy may be uncertain or uncommon in a particular area, potential wood biomass users may want to do a brief, informal feasibility study before undertaking a rigorous economic analysis."