ENVIRONMENTAL IMPACTS OF COMBUSTION

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The negative effects of combustion on the environment – particularly greenhouse gas (GHG) emissions released to the atmosphere that contribute to global warming – have received much attention in recent years. This issue is addressed in the Kyoto Protocol (1997). Canada, which signed the Protocol, aims to reduce emissions between 2008 and 2012 by six percent of 1990 levels. Climate change resulting from global warming is one of the greatest challenges facing not only Canada but also the world. Managing combustion processes better and improving the efficiency of energy generation and use are two of the key strategies for reducing atmospheric emissions.

Therefore, this guidebook is being published in concert with Canadas policy on climate change as one of the tools for implementing it. Canadas goal of reducing GHG and acid-rain emissions can be met only with the co-operation of the owners and operators of combustion equipment. It is beyond the scope of this guidebook to describe the emissions in detail. Instead, a brief ove rv i ew is presented (see Table 2 for a list of some emissions from combustion systems and their effects). More complete information can be obtained from An Energy Efficiency and Environment Primer for Boilers and Heaters.

Table 2. Emissions from Combustion Systems and Their Effects
EMISSION SOURCE EFFECT GHG POTENTIAL RELATIVE TO CO2
CO2 (carbon dioxide) Complete combustion of carbon in fuel Global warming 1
CO (carbon monoxide) Incomplete combustion of carbon in fuel Smog  
SO2 (sulphur dioxide) Combustion of sulphur in fuel Smog, acid rain  
NOx (nitrogen oxides) By-product of most combustion processes Acid rain  
N2O (nitrous oxide) By-product of some combustion processes Global warming 310
VOCs (volatile organic compounds) Leakage and evaporation of liquid fuels (from, e.g., vehicles, fuel tanks, fuel pumps, refineries, solvents from paints) Smog  
CH4 (methane) Principal component of natural gas; leakage from gas wells, pipelines and distribution systems Global warming 21
H2O (water vapour) Combustion of hydrogen in fuel Localized fog  
Particulates (dust, soot, fumes) Unburned or partially burned carbon and hydrocarbons; also ash and dirt in fuel Smog  
Trace elements Impurities in fuel Potential carcinogens  
Halogenated compounds Compounds in fuel or combustion air containing halogens (chlorine, fluorine, bromine and iodine) Potential carcinogens, global warming Up to 24 000

 

Table 3. CCME* NOx Emission Guidelines for New Boilers and Heaters
INPUT CAPACITY NOX EMISSION LIMIT, g/GJ** AND PPM (AT 3% O2)***
10.5 TO 105 GJ/h (10 TO 100 MILLION Btu/h) GREATER THAN 105 GJ/h (>100 MILLION Btu/h)
Natural gas 26 (49.6) 40 (76.3)
Distillate oil 40 (72.3) 50 (90.4)
Residual oil with less than 0.35% nitrogen 90 (162.7) 90 (162.7)
Residual oil with 0.35% or more nitrogen 110 (198.9) 125 (226.0)

*      Canadian Council of Ministers of the Environment
**    g/GJ = grams of NOx emitted per gigajoule of fuel input
***  ppm = parts per million by volume, corrected to 3% O2 in the flue gas (10 000 ppm = 1%)

To correct ppm NOx to 3% O2: NOx at 3% O2 = [NOx measured x 17.9] / [20.9 - O2], where O2 is oxygen measured in flue gas, dry basis

To convert ppm NOx at 3% O2 to g/GJ: for natural gas, g/GJ = ppm / 1.907 for fuel oil, g/GJ = ppm / 1.808

Table 4. Typical NOx Emissions Without NOxControl Equipment in Place
FUEL AND BOILER TYPE TYPICAL NOX EMISSIONS (PPM AT 3% O2)
Natural gas Firetube 75-115
Package watertube 40-90
Field-erected watertube 45-105
No. 2 oil Firetube 70-140
Package watertube 90-150
Field-erected watertube 40-115
No. 4 oil Package watertube 160-310
Field-erected watertube 140-190
No. 6 oil Package watertube 200-360
Field-erected watertube 190-330

Although the other GHGs, unit for unit, are much more potent than CO2 in their effects, the latter is the most important GHG because of its volume. In 1997 it represented three-quarters of Canada's total emissions. Most of the CO2 is generated by the combustion of fuels, whether for residential, industrial, transportation or electric power generation purposes. So, applying energy efficiency measures that reduce fuel consumption is crucial to reducing CO2 emissions.

Fuel consumers face a double challenge. One is economic – to get the best value for their fuel budget. The other is environmental – to keep emissions low, at least within legislated limits. Fortunately, what benefits the first objective also benefits the second.

Higher limits are allowed for equipment with a proven higher efficiency than normal and which, therefore, burns less fuel. Provinces and territories are responsible for enforcement and may enact stricter limits. They also have responsibility for determining to what extent the guideline applies to boilers and heaters that are being modified or overhauled.

Emissions of sulphur dioxide (SO2) and nitrogen oxides (NOx) contribute to acid rain and, therefore, are also of concern. SO2 emissions are controlled by limiting the allowable sulphur content of the fuel, but NOx emissions can be reduced by manipulating the combustion process. Guidelines for new boilers and heaters are presented in Table 3, and An Energy Efficiency and Environmental Primer for Boilers and Heaters describes the strategies for complying with NOx regulations.

 


 

 

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