08.08.2015   ABOUT THE AUTHOR

Enthalpies of Formation

The enthalpies of formation of biomass are quite useful for thermodynamic calculations. The heats of specific reactions that utilize biomass feedstocks can be estimated from the standard enthalpies of formation at 298 К of the combustion products (in kcal/g-mol: C02, -94.05; liquid H20, —68.37; N02, 8.09; S02, —70.95), the elemental analyses of the biomass being examined, and its HHV. The enthalpy of formation of a particular biomass sample is equal to the sum of the heats of formation of the products of combustion

TABLE 3.8 Comparison of the Measured and Calculated Higher Heating Values of Biomass, Coal, and Peat“

Measured higher heating Calculated higher heating

Material

value (MJ/dry kg)

value (MJ/dry kg)

Error (%)

Giant brown kelp

10.01

9.94

-0.70

Cattle feedlot manure

13.37

13.34

-0.19

Water hyacinth

16.00

16.09

+0.54

Pure cellulose

17.51

17.61

+0.59

Kentucky bluegrass

18.73

18.24

-2.64

Primary biosolids

19.86

17.30

-12.90

Reed sedge peat

20.79

21.43

+3.10

Pine wood

21.24

20.98

-1.23

Illinois bituminous coal

28.28

28.84

+ 1.98

“The measured HHVs (bomb calorimeter) and the carbon analyses were determined by the Institute of Gas Technology. The sample of primary biosolids contained an unusually large amount of fatty material. The calculated HHVs are estimated from the formula 0.4571(% C on dry basis) -2.70.

minus the HHV. It is assumed that the ash is inert. For example, a sample of giant brown kelp has the empirical formula C2.6iH4.e3N0.10S0.01O2.23 (dry basis), which is derived from the elemental analysis, and a HHV of 296.1 kcal/g-mol (12.39 MJ/kg) at an assumed molecular weight of 100, including the ash. The stoichiometry for calculating the enthalpy of formation is

2.61C + 2.315H2 + 0.05N2 + 0.01S + 1.11502

—* C2.6iH463No. ioSo. oi02.23Ash267 (AHf — 107.5).

The enthalpy of formation is —107.5 kcal/g-mol (—4.50 MJ/kg) including the ash for this particular biomass sample. An example of the utilization of this information is illustrated by applying it to the biological gasification process under anaerobic conditions. The stoichiometry of the process is

C2.6iH4.63O2.23 (s) + 0.337H2O(l) -> І. З26СН4 (g)

+ 1.283C02 (g) (ДН -13.85).

The enthalpy of the process is estimated to be —13.85 kcal/g-mol (—0.58 MJ/ kg) of kelp reacted (Klass and Ghosh, 1977).

It is assumed in these calculations that the inorganic components are carried through the process unchanged, and that the nitrogen and sulfur can be ignored since their concentrations are small. For each kilogram of kelp reacted, the feedstock energy input is 12.39 MJ, and the energy output is 11.81 MJ as methane (0.8903 MJ/g-mol at 298 K). The calculations indicate the process is slightly exothermic. About 95% of the feed energy resides in product methane, and about 5% is lost as heat of reaction.