Solid Waste & Recycling

Feature

Air Emissions from Waste-to-Energy Plants

As part of their environmental assessment planning process for waste, the Region of Niagara and the City of Hamilton, Ontario have joined forces to assess different diversion and disposal options. The...


As part of their environmental assessment planning process for waste, the Region of Niagara and the City of Hamilton, Ontario have joined forces to assess different diversion and disposal options. The two jurisdictions plan to aggressively divert 60 to 65 per cent of garbage through curbside recycling and organics processing programs and thereby reduce the need for expanded landfill capacity and dependence on waste exports to the United States. The municipalities are evaluating the option of further reducing that portion of waste that can’t reasonably be recycled or composted via utilization in a waste-to-energy (WTE) plant. This would generate energy while greatly extending the life of local landfills.

The municipalities have not committed to a WTE plant, but they want to objectively evaluate the idea. They anticipate any such proposal to be controversial, given public concern about incinerators and potential emissions, and therefore they retained consultants MacViro Consultants Inc. and Earth Tech (Canada) Inc. to conduct a study.

How much waste are they talking about?

Together, Niagara and Hamilton are planning for the management of up to 500,000 tonnes of material each year. If 65 per cent is diverted at source, by 2013 about 150,000 tonnes will need to be treated and disposed in some way. To be conservative, the consultants assumed a slightly lower diversion rate of 60 per cent and a WTE facility processing 200,000 tonnes of waste and converting it to energy annually. About 20,000 to 50,000 tonnes of ash and residue would remain — depending upon the technology employed — and some air emissions. Most of the emissions would be captured by the air pollution control technology, but some would be released into the air.

What the study found

The study compared two things:

(a) emissions from modern high-tech waste incinerators and gasifiers; and

(b) emissions from other sources that people accept around them every day (such as cars, trucks, and home heating systems).

The consultants knew there would be some emissions; the goal was to see exactly which pollutants (and in what quantities) would be emitted, and put those in perspective compared to other sources. The consultants used only verifiable data from credible sources. Such data is available from places like Europe (e.g., Scandinavia, Germany) and Japan where WTE plants are common.

The consultants found that emissions from modern WTE plants are very low. In addition to generating electricity and heat, a state-of-art facility would produce emissions well below Ontario’s strict Guideline A-7, Combustion and Air Pollution Control Requirements for New Municipal Waste Incinerators (last updated in 2002). The A-7 guideline is as strict (in some cases more strict) as anything in place in the United States or Europe. To make the data unbiased, the estimated emissions from a 200,000 tonnes-per-year facility are based on the emissions from commercially available technologies whose vendors submitted detailed emissions data on their combustion and gasification technologies to Niagara Region in 2003.

The consultants evaluated the emissions and broke them down as follows:

* Carbon dioxide (CO2) emissions that represents less than one tenth of one per cent of total Ontario greenhouse gas emissions. This is approximately equivalent to the emissions from 28,000 homes heated with natural gas or 35,000 cars traveling a typical 17,600 km per year.

* Smog and acid rain precursors NOx approximately equal to the emissions from 200 diesel trucks (tractor-trailers) each traveling a typical 91,200 km per year. SO2 equal to 55 such trucks and particulate matter equal to 40 trucks. (Approximately 65,000 such trucks are registered in Ontario.)

* Emissions of the heavy metal cadmium approximately equal to the emissions from 2,700 diesel trucks (tractor trailers) each traveling a typical 91,200 km per year. Emissions of lead equal to 9,000 such trucks and mercury emissions equal to about 4,300 trucks.

From this perspective, the emissions from a thermal waste-treatment plant would equate to those from other sources that people live with and accept every day. It must be noted also that alternatives to thermal treatment such as landfill are not without emissions, either. For instance, landfills release methane, which causes 21 times the global warming effect as CO2.

Power generation

Emissions from a WTE plant can also be compared to other power plants. Nuclear power plants have few emissions, but their nuclear waste material must somehow be disposed. Of the plants that burn fossil fuels, natural gas (not surprisingly) provides the cleanest power, coal the dirtiest. A WTE plant falls somewhere in the middle. Natural gas and coal are non-renewable resources; a WTE plant generates power using materials that would otherwise be buried in a landfill.

An average WTE facility processing 200,000 tonnes per year of municipal waste will generate approximately 14.1 MW of net electricity according to information provided by thermal technology vendors. Although many times smaller than a coal fired power plant, this would serve the electricity needs of approximately 9,000 homes. It could also potentially provide hot water for district heating systems.

Further reductions

The consultants used conservative data in drawing their conclusions. Yet it must be noted that future improvements could further reduce emissions, potentially far beyond what was assumed in the study. Improvements to plant design and efficiency and the addition of more pollution control equipment (such as secondary filters to screen out fine particulate matter) would help. New product stewardship programs are being introduced for such things as batteries, scrap tires and electronics waste (TVs, computers, etc.). Combined with existing household hazardous waste programs, these will reduce pollutants of concern entering the waste stream such as chloride, sulphur, and heavy metals.

Readers in other jurisdictions may or may not favor waste-to-energy, but, with this study, Niagara and Hamilton have provided credible information for waste management professionals and interested citizens to use in making informed decisions.

The full report is available online in pdf format under Posted Documents at our website,www.solidwastemag.com

Guy Crittenden is editor of this magazine.

Incineration vs. Gasification

The most common thermal waste treatment technology is incineration (one- or two stage combustion), often accompanied by energy recovery. Then there are so-called “gasification” plants.

The waste-to-energy plant in Burnaby, B.C. is a good example of a single-stage combustion facility. Waste is received in a tipping area and dumped into a receiving pit. Any undesirable material is removed, then fed with a grapple crane into the combustion chamber. Once inside, the material moves across a grate where it dries and burns. Bottom ash falls through the grate and may be further processed to recover metals for recycling; bottom ash is usually landfilled or, in some jurisdictions, utilized as a road construction aggregate.

Before going up the stack into the atmosphere, flue gases pass through various treatments such as a spray of urea solution (to reduce NOx), a lime slurry scrubber (to control acid gases), powdered activated carbon (which controls mercury and dioxins/furans) and a baghouse (removes particulate matter). This air pollution control system produces “fly ash” which must be treated to be rendered non-hazardous and/or disposed in a secure landfill.

Two-stage combustion units, such as those used in Peel Region, Ontario, are generally smaller, modular systems. Waste is loaded into the primary combustion chamber with a front-end loader, then “gasified” (i.e., combusted in a starved-air
condition). This process also generates a bottom ash, but the combustible gas is sent to a second chamber where it is completely combusted at a temperature to in excess of 1,000 degrees Celsius. The resulting flue gases are treated with pollution control equipment, similar to that described above.

There are a variety of gasification technologies. In one example, waste is made into cubes that pass through a heated, gas-tight channel on their way to the gasification chamber. The main gasification chamber operates at 1,200 degrees Celsius; oxygen is introduced and waste is turned into a synthetic gas made up primarily of hydrogen and carbon monoxide. The molten residue can be converted into a glass-like aggregate product and metal pellets. The syngas is cooled and washed with water; this cleaning means further air pollution controls are generally not needed, although the wastewater from the scrubbers may require treatment prior to discharge.

Other gasification systems produce a clean syngas from an “alternative fuel” product. This syngas could then be used to fuel a gas engine or turbine, or steam/hot water boiler. The production of the alternative fuel, from the residual waste stream, requires front-end processing, such as material drying, magnetic separation, special screening, and air classification (i.e., use of special equipment to separate light combustible materials like paper and plastic from heavy non-combustible materials like glass). This front-end processing allows for the recovery of recyclable materials in addition to the production of the alternative fuel. The alternative fuel could also be used as a substitute for fossil fuels at an approved facility. Such facilities could include cement kilns or approved utility boilers equipped with air pollution control equipment.

In the end, the emissions from a one- or two-stage combustion facility equipped with air pollution controls and a gasification plant are very similar. Some emissions (such as greenhouse gases) are slightly lower from combustion technologies while others (such as smog precursors) are slightly lower from gasification. Gasification is less proven and likely more expensive, an important consideration if the emissions are similar.

Schematic diagram of the waste-to-energy plant in Burnaby, B.C. that processes 250,000 tonnes of waste per year. British Columbia has among the most progressive waste diversion and product stewardship programs in North America.

A Perspective on Dioxin Emissions

The annual dioxin/furan emissions from a modern waste-to-energy (WTE) facility thermally processing 200,000 tonnes per year of post-diversion residual solid waste are estimated to equal 0.04 grams per year. This is approximately equal to the dioxin/furan emissions from:

* 1,700 diesel trucks (tractor-trailers) each traveling a typical 91,200 km per year. There are approximately 65,000 of these trucks registered in Ontario, or

* the open burning, in an uncontrolled fashion, of approximately 190 tonnes of waste, or the waste from 190 households burning all of their waste in backyard burn barrels or open dumps;

* some research indicates that dioxins/furans are also emitted in comparable quantities from the recycling of ferrous and aluminum materials, as well as residential wood stoves.

Recent research has shown that these compounds:

i) may not be created in a synthetic gas production process;

ii) get destroyed in a well-controlled high temperature combustion process; and

iii) are created in trace amounts through chemical reactions that occur in the post-combustion exhaust gases following the direct combustion of waste, waste derived fuel, syngas produced from waste, or other hydrocarbons such as diesel fuel.

Further research also indicates that the quantity of dioxins and furans in the exhaust gas are not directly related to the quantity of chlorine (from plastics) in the incoming waste stream. State-of-the-art pollution control technology, in the form of carbon injection followed by bag house filtering, has led to the current low level of these emissions.

Possible future initiatives for further reducing the level of these emissions are expected to focus on techniques to reduce the formation of dioxins and furans rather than on reduction of already formed dioxins/furans.


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