# 3.9.2: Environment- Heating Values of Various Fuels

We noted previously that chemist's "standard enthalpy (heat) of combustion" may be a misleading measure of a fuel's heating value in real conditions. Standard enthalpies are precisely defined as the heat energy absorbed or released when a process occurs at 25 oC to give substances in their most stable state at that temperature. For example, for propane, water is produced as a liquid:
C3H8(g) + 5 O2(g) → 3 CO2(g)+ 4 H2O (l) ΔHm = –2219.2 kJ[1] (1)

Most of the plume above smokestacks is condensed water droplets, which release heat of condensation to the atmosphere

But boilers are almost always operated with flue temperatures near the combustion temperature, and maintained at over 100 oC to prevent water from condensing to liquid. Since water is produced as a gas, the standard thermodynamic equation (1), does not apply. If it were formed, liquid water would dissolve acidic flue gases like HCl and SO2 to make acidic solutions which corrode the system. These gases are normally removed by limestone "scrubbers",[2] leaving just the water vapor. The white plumes over smokestacks are water droplets forming by condensation of water vapor. They often disappear as the droplets evaporate.

The precision of our definition of standard enthalpy guarantees that our calculations of heat energy will be sound, but it requires that we pay attention to each energy cost and source. We'll see that inattention to such details has led to misinformation and confusion. Since Higher Heating Value (HHV) may be used by some air quality management authorities,[3] while Lower Heating Values are used by many engineers and European power facilities, it is important to know precisely what the terms mean and which is being used. The USDA[4] reports the following heating costs apparently, but not explicitly, stated to be LHVs:

Fuel

Gross

heating value

Efficiency

(%)

Net

heating valueb

Fuel required

for 1 million Btu of usable heat

Average

cost/unit

Total

annual fuel costa

Natural gas 1.03 million Btu/1000 ft3 80 0.82 million Btu/1000 ft3 1,220 ft3 $7/1000 ft3$854
Propane 91,200 Btu/gal 79 72,000 Btu/gal 13.86 gal $1.25/gal$1,730
Fuel oil #2 138,800 Btu/gal 83 115,000 Btu/gal 8.68 gal $1.40/gal$1,220
Seasoned firewood 20 million Btu/cord 77 15.4 million Btu/cord 0.065 cord $115/cord$747
Electricity 3,413 Btu/kWh 98 3,340 Btu/kWh 299 kWh $0.08/kWh$2,390
Premium wood pellets 16.4 million Btu/ton 83 13.6 million Btu/ton 0.073 ton $120/ton$882

a Based on 100 million Btu of energy for the heating season, a typical value for an average sized house.

There are many variables that affect total heating costs (furnace efficiencies, local energy costs, etc.), but one variable that can potentially be understood is the heat available from fuel combustion. Without precise definitions, even this may be obscured. To illustrate this point, let's examine what are known as the Higher Heating Value (HHV)[5] and Lower Heating Value (LHV)[6] are calculated for propane.