Where does process heating fit in?



Like you, I’ve heard the arguments both sides have made to prove or debunk the relationship between atmospheric CO2 and temperature. Both can cite lots of scientific data and call on experts to make their cases. I’m not going to take sides here -- one more voice in the chorus isn’t going to sway anyone. Instead, I want to explain some of the facts and relationships related to combustion-generated CO2 emissions, so you’re better equipped to reach your own conclusions and act accordingly. Some of these things may be blindingly obvious, but for the sake of completeness, I’ll cover them anyway.

Combustion of fossil fuels produces CO2 . Burning any fuel with carbon in its makeup will create CO2 as a combustion product, and all widely used commercial fuels, from natural gas to gasoline to coal, do contain carbon.

Do different fuels produce different amounts of CO2? In round numbers, burning 1 million BTU worth of natural gas will create about 120 lb of CO2. Generating the same amount of heat from distillate fuel oil will form about 170 lb of CO2, and 1 million BTU heat release from bituminous coal will generate about 210 lb of CO2. In short, the “heavier” the fuel, the more CO2 it produces per unit of energy released. There’s a simple reason for this -- as you move up the hydrocarbon chain from methane to propane, butane, oil and so on, the fuel contains a higher weight percentage of carbon. Thus, greater CO2 emissions and, conversely, lower water vapor. Hydrogen, containing no carbon at all, wouldn’t produce any CO2.

The scale on the left shows the pounds of CO2 produced by each fuel type per million BTU. The scale on the right the weight percent of carbon. For example, Natural Gas produces 120 lb of CO2 per million BTU and has 69 percent carbon by weight.

If you’re wondering why the figure shows coal with less carbon than all the other fuels but natural gas, that’s because the carbon content of coal is diluted by a significant amount of ash. Remove the ash, and the carbon percentages would continue marching upward.

Will tuning up burners lower CO2 emissions? No and yes. If you burn a certain number of BTUs of fuel, you’ll get however much CO2 that particular fuel generates. However, if a combustion system tuneup raises your efficiency, your CO2 emissions will decrease in proportion to your fuel consumption.

If CO2 emissions do become the subject of regulatory limits or taxes, plants will have an added incentive to upgrade their combustion systems with performance enhancers like heat exchangers, mass flow ratio controls, oxygen analyzers and the like. The combined effect of energy savings and CO2 reduction may make these systems easier to justify.

Why not just convert from fuel firing to electricity? That depends on whether you want to look at the micro or macro picture. The micro picture is your plant -- assuming there’s a suitable electric technology that can be cost-justified, you definitely will lower your CO2 emissions. However, if your electricity comes from a fuel-fired power plant, the macro picture (you plus the power plant) will probably show a net increase in CO2. The overall efficiency of generating electricity from coal or gas, transmitting it through the power grid and the transformers, switchgear and heating devices on your ovens and furnaces will probably be lower than the local efficiency of your fuel-fired system. In other words, the overall amount of energy required, and CO2 produced, to heat your process will probably increase. The CO2 disparity gets worse if you replace a natural gas-fired system with electricity generated from coal. On the other hand, hydroelectric, nuclear, wind and solar generators produce no CO2 emissions.

The strategy and cost of dealing with CO2 emissions will depend partly on the focal point of enforcement. If air-quality agencies concentrate on regulating CO2 at the point of use, manufacturers will be under heavy pressure to raise the efficiency of fuel-fired processes or replace them with electric. By contrast, if the government leans more heavily on the power-generating sector, the cost of electricity is likely to rise, making conversion efforts tougher to justify.

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