Page 345 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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7. APPLICATIONS: ESTIMATION OF THERMOPHYSICAL PROPERTIES 325
one compound (i.e., H 2 OorCO 2 ). However, when a hydro-
heating values of other fuels are calculated as follows:
carbon (C x H y ) is burned the only products are H 2 O and CO 2 . HHV = 121 + 0.22 × 100 = 143 kJ/g or 61000 Btu/lb. The --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
Combustion is a reaction in which the enthalpy of products is
less than enthalpy of reactants and as a result the heat of reac- Hydrogen, Methane, Propane, Carbon, Sulfur,
tion (enthalpy of products − enthalpy of reactants) is always Fuel H 2 CH 4 C 3 H 8 C S
negative. This heat of reaction is called heat of combustion LHV, kJ/g 121 50 46.4 32.8 9.3
C
and is shown by H . Heat of combustion depends on the HHV, kJ/g 143 55.5 50.4 32.8 9.3
temperature at which the combustion takes place. The stan-
C
dard temperature at which usually values of H are reported As it can be seen from these calculations, hydrogen has the
is 25 C (298 K). highest heating value and carbon has the lowest heating value.
◦
Amount of heat released by burning one unit mass (i.e., kg, Thus hydrogen is the best, while carbon is considered as the
g, or lb) of a fuel is called heating value or calorific value and worst fuel. Sulfur heating value is even less than carbon but
has the unit of kJ/kg or Btu/lb (1 kJ/kg = 0.42993 Btu/lb). In sulfur is not really considered as a fuel. Some values of HHV
some cases for liquid fuels the heating values are given per for several other fuels as reported by Felder and Rousseau
unit volume (i.e., kJ/L of fuel), which differs from specific [32] are given in Table 7.13. The calculated value of HHV of
(mass unit) heating values by liquid density. If in the combus- C is near the HHV of hard coal (i.e., solid form) as given in
tion process produced H 2 O is considered as liquid, then the Table 7.13. In natural gases since there are some hydrocar-
heat produced is called gross heat of combustion or higher bons heavier than methane, its heating value is somewhat
heating value (HHV). When produced H 2 O is considered as lower than that of pure methane.
vapor (as in the actual cases), then the heat produced is called
lower heating value (LHV). The LHV is also known as the net Example 7.8 shows that the heating value generally in-
heating value (NHV). The difference between HHV and LHV is creases as the hydrogen content of fuel increases and car-
due to the heat required to vaporize produced water from liq- bon content decreases. In other words, as CH weight ratio
uid to vapor form at the standard temperature (43.97 kJ/mol increases the heating value decreases. Furthermore, presence
or 2.443 kJ/g of H 2 O). The amount of H 2 O formed depends on of sulfur further reduces the heating value. For this reason,
the hydrogen content of fuel. If the hydrogen wt% of fuel is some researchers have correlated HHV to wt% of C, H, S, N,
H% then the relation between HHV and LHV is given as [30]: and O content of fuel. For example, Tsonopoulos et al. [18]
proposed the following relation for estimation of HHV of coal
liquids:
(7.61) LHV = HHV − 0.22H%
HHV [kJ/g] = 0.3506 (C%) + 1.1453 (H%) + 0.2054 (S%)
where both LHV and HHV are in kJ/g. The heating values can (7.62) + 0.0617 (N%) − 0.0873 (O%)
f
also be determined from standard heats of formation ( H 298 ).
f
Values of H 298 for any element (i.e., H 2 ,O 2 , C, S, etc.) is S, N, and O are usually found in heavy fuels and aromatic rich
zero and for formed molecules such as H 2 O are given in fuels such as coal liquids. This equation predicts HHV of coal
most thermodynamics references [12, 21, 31]. For example, liquids with %AAD of 0.55 for some 130 fuels. This equation
for H 2 O(g), CO 2 (g), CO(g), SO 2 ,CH 4 (g), C 2 H 6 (g), C 3 H 8 (g), predicts HHV of pure C as 35 kJ/g. However, this equation is
and n-C 10 H 22 the respective values of H f are −241.81, not recommended for light fuels, petroleum fraction, or pure
298
−393.51, −110.53, −296.81, −74.52, −83.82, −104.68, and compounds. There is a simpler relation for calculation of LHV
−249.46 kJ/mol. The following example shows calculation of of heavy fuels and petroleum fractions [30]:
heating values from heats of formations.
(7.63) LHV [kJ/g] = 55.5 − 14.4 × SG − 0.32S%
where S% is the sulfur wt% in the fuel. A very simple but
Example 7.8—Calculate HHV and LHV of hydrogen, me-
thane, propane, carbon, and sulfur from heats of formation. approximate formula for calculation of HHV of crude oils is
[26]:
(7.64) HHV = 51.9 − 8.8 × SG 2
Solution—Here the calculation of heating value of CH 4
is demonstrated and a similar approach can be used for where HHV is in kJ/g (or MJ/kg) and SG is the specific gravity
other fuels. The chemical reaction of combustion of CH 4 is of crude and S% is the sulfur wt% of the crude. Accuracy of
C C
CH 4 (g) + 2O 2 (g) → 2H 2 O(g) + CO 2 (g) + H , where H = these equations is usually about 1%. A typical crude oil has
f
2 H 298 (H 2 O) + H 298 (CO 2 ) − H 298 (CH 4 ) − H 298 (O 2 ) = 2× heating value of about 10 500 cal/g (∼44 kJ/g). Increase in hy-
f
f
f
(−241.835) + (−393.51) − (−74.8936) − (0) =−802.286 kJ/ drogen content of a fuel not only increases the heating value
mol. Since the produced water is assumed to be in gas phase
so the LHV is calculated as 802.286/16.04 = 50.01 kJ/g.
This is equivalent to 11953 cal/g or 21500 Btu/lb. The TABLE 7.13—Heating values of some fuels.
HHV can be calculated by adding heat of vaporization of Taken with permission from Ref. [32].
Higher heating value
water (2 × 43.97 = 87.94 kJ/mol) to the molar LHV. HHV = Fuel kJ/g Btu/lb
802.286 + 87.94 = 890.2 kJ/mol or 55.5 kJ/g of CH 4 . Equa-
7700
tion (7.61) to convert LHV to HHV or vice versa using Wood 18 15 000
Hard coal
35
H% of fuel may also be used. In this case, H% of CH 4 = Crude oil 44 19 000
(4/16) × 100 = 25 wt%. Thus HHV = 50 + 0.22 × 25 = 55.5 Natural gas 54 23 000
kJ/g. Similarly for H 2 , LHV = 241.81/2.0 = 121 kJ/g. The Hydrogen 143 61 000

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