Effects of Moisture and Hydrogen Content on the Heating Value of Fuels

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Effects of Moisture and Hydrogen Content on the Heating Value of Fuels

A. Demirbas ^a

a Department of Chemical Engineering , Selcuk University , Konya, Turkey Published online: 04 Apr 2007.

To cite this article: A. Demirbas (2007) Effects of Moisture and Hydrogen Content on the Heating Value of Fuels, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 29:7, 649-655, DOI: 10.1080/009083190957801 To link to this article: http://dx.doi.org/10.1080/009083190957801

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DOI: 10.1080/009083190957801

Effects of Moisture and Hydrogen Content on the Heating Value of Fuels

A. DEMIRBAS

Department of Chemical Engineering Selcuk University

Konya, Turkey

Abstract In this work, effects of moisture and hydrogen contents on lower heating value (LHV) of fuels were investigated. The LHV at constant pressure measures the enthalpy change of combustion with and without water condensed, respectively. Mois- ture in biomass generally decreases its heating value. Moisture in biomass is stored in spaces within the dead cells and within the cell walls. Higher heating value (HHV) of a fuel decreases with increasing of its moisture content. The LHV of a fuel increases with increasing of its hydrogen content. The LHV of a fuel depends on its oxygen content and the LHV of a fuel decreases with increasing of its oxygen content. The LHV of a fuel increases with increasing the hydrogen content due to cause combustion water. Moisture in a fuel generally decreases its HHV. The LHV of a fuel increases with increasing the sulfur content due to cause SO_xgases absorbed by water.

Keywords biomass, coal, hydrogen, lower heating value, moisture, reaction water

Introduction

Coal and biomass combustion is a series of chemical reactions by which carbon is oxidized to carbon dioxide, and hydrogen is oxidized to water. Oxygen deficiency leads to incomplete combustion and the formation of many products of incomplete combustion (Demirbas, 2000).

The heating value is obtained by the complete combustion of a unit quantity of fuel in an oxygen-bomb colorimeter under carefully defined conditions. The standard measure of the energy content of a fuel is its heating value, sometimes called the calorific value or heat of combustion. The “gross” or “high” heating value is obtained by this method, as the latent heat of moisture in the combustion products is condensed. The results may be expressed on the “as received” or “dry” or “dry and ash free” basis. The higher heating value (HHV) and the lower heating value (LHV) at constant pressure measures the enthalpy change of combustion with and without water condensed, respectively. The enthalpy (heat) of vaporization of water as a function of temperature (Marsh, 1987) is tabulated in Table 1.

The enthalpy of vaporization of water is given as a function of temperature by apply- ing the Clapeyron equation to the Clausius-Clapeyron regression of the data. The relation between the equilibrium pressure and temperature is given exactly by the Clapeyron equation (Smith and Van Ness, 1975).

Address correspondence to Professor Ayhan Demirbas, Department of Chemical Engineering, Selcuk University, Campus, 42031 Konya, Turkey. E-mail: [email protected]

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650 A. Demirbas

Table 1

Enthalpy (heat) of vaporization of water Temperature

◦C K

Hvapor, J/mol

Hvapor, J/g

Hvapor, cal/g

0 273.2 45,054 2,500.78 597.41

20 298.2 43,990 2,441.72 583.31

40 311.2 43,350 2,406.19 574.82

60 331.2 42,482 2,358.13 563.34

80 351.2 41,585 2,308.23 551.42

100 373.2 40,657 2256.72 539.11

140 413.2 38,643 2,144.93 512.41

180 453.2 36,304 2,015.10 481.39

220 493.2 33,468 1,857.68 443.78

260 533.2 29,930 1,661.30 396.87

300 573.2 25,300 1,404.31 335.48

340 613.2 18,502 1,026.98 245.34

360 633.2 12,966 719.69 171.93

374 647.2 2,066 114.68 27.40

The heat content is related to the oxidation state of the natural fuels in which carbon atoms generally dominate and overshadow small variations of hydrogen content. On the basis of literature values for different species of wood, Tillman (1978) also found a linear relationship between HHV and carbon content.

Biomass fuels are composed of biopolymers that consist of various types of cells and the cell walls are built of cellulose, hemicelluloses, and lignin. HHVs of biomass fuels increase as increase lignin contents (Demirbas, 2003a, 2003b). In general, the HHV of fuels increases with increase in their FC contents (Demirbas, 1997). Moisture, ash and HHVs analysis of the biomass fuels are given in Table 2 (Demirbas, 1997, 2002a, 2003c;

Baxter et al., 1998). The moisture content in test samples was determined according to ASTM D (3173-87) standard test method in sartorious infrared moisture meter (ASTM, 1989a). The HHV of biomass materials is determined according to ASTM D (2015- 85) standard test method in a parr microprocessor controlled oxygen bomb calorimeter (Model 1241 EF) (ASTM, 1989b).

In earlier work (UNDP, 2000), formulae were developed for estimating the HHVs of various lignocellulosic materials, using their ultimate analysis data. The relation between the observed HHV and C, H, and O contents of the samples (wt%) was investigated. Thus the HHV (MJ/kg) of lignocellulosic materials including C, H, and O can be calculated from Eq. (1):

HHV=0.335 C+1.423 H−0.154 O (1)

Experimental

Since there are no standard sampling procedures specified for fuel materials, the samples were collected with due care to get the most representative samples. Preparation of

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Table 2

Moisture, ash and higher heating value (HHV) analysis of biomass fuels

Fuel common/scientific name

Moisture, wt% of fuel

Ash, wt% of dry fuel

HHV, MJ/kg, daf

Almond shells/Pranus dulcis 7.5 2.9 19.8

Almond hulls/Pranus dulcis 8.0 5.8 20.0

Beech wood/Fagus orientalis 6.5 0.6 19.6

Hazelnut shell/Corylus avellena 7.2 1.4 19.5

Oak wood/Quersus predunculata 6.0 1.7 19.8

Oak bark/Quersus predunculata 5.6 9.1 22.0

Olive husk/Olea europaea 6.8 2.1 21.8

Rice straw/Oryza sativa 11.2 19.2 18.7

Spruce wood/Picea orientalis 6.7 0.5 20.5

Wheat straw/Triticum aestivum 6.4 8.1 19.3

Sources: Demirbas, 1997, 2002a, 2003c.

samples was carried out in accordance with ASTM D (2013-86) standard test method (ASTM, 1989c).

The higher heating values of extractive free samples were determined by forming the samples into pellets weighing approximately 0.5 g each, and burning the pellets in an adiabatic oxygen bomb calorimeter according to the ASTM D (2015-66) standard test method (ASTM, 1977). Heat of combustion of liquid hydrocarbon by bomb calorimeter was determined according to the ASTM D (240-64) and ASTM D (240-76) standard test methods (ASTM, 1982; ASTM, 1983).

Direct moisture content measurement of wood and wood base materials was determined according to the ASTM D (4442-92) standard test method (ASTM, 1997a). Total sulfur in the analysis sample of coal and coke was determined according to the ASTM D (3177-75) standard test method (ASTM, 1997b). Volatile matter in the analysis sample of particulate wood fuel was determined according to the ASTM D (872-82) standard test method (ASTM, 1998a). Ash in biomass was determined according to the ASTM D (1755-95) standard test method (ASTM, 1998b). The fixed carbon content of the test samples was calculated by difference.

The proximate analysis of samples were carried out in accordance with ASTM D (3172-73) standard test method (ASTM, 1989d).

Results and Discussion

The elemental analysis, ash and moisture contents, and HHVs and LHVs of fuels are tabulated in Table 3. Biomass fuel properties vary significantly more than those of coal do (Liu, 2004). As examples, ash contents vary from less than 1% to over 16%, oxygen contents vary from less 35% to over 43% and fuel nitrogen varies from around 0.2%

to over 1.2% (Table 3). Other notable properties of biomass relative to coal are high moisture content, low heating value, and low bulk density.

Table 4 shows the mathematical equations for calculating lower heating values (LHVs, kJ/kg)) of different types of fuels:

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652 A. Demirbas

Table 3

Elemental analysis, ash and moisture contents, higher and lower heating values (HHV and LHV, MJ/kg) of fuels

C H N O S Ash Moisture HHV LHV

Coal 77.6 3.8 1.1 3.1 2.9 6.7 4.8 31.9 30.8

Lignite 61.5 4.2 1.6 7.8 6.1 12.6 6.2 25.1 23.9

Beech wood 48.4 5.9 0.3 38.7 — 0.6 6.1 18.7 17.2

Spruce wood 49.3 5.8 0.2 37.8 — 0.5 6.4 19.1 17.6

Grass 42.2 5.1 1.1 36.9 — 8.2 6.7 16.4 15.0

Hazelnut shell 46.9 5.3 1.2 39.1 — 1.5 6.0 18.2 16.8

Corncob 44.1 5.6 0.5 42.8 — 1.3 5.7 17.7 16.2

Wheat straw 41.8 4.8 0.5 38.0 — 8.6 6.3 17.0 15.7

Corn stover 41.3 5.3 0.6 40.8 0.1 5.2 6.7 16.2 14.8

Methanol 37.5 12.5 — 50.0 — — — 22.7 20.0

Table 4

Equations for calculating of lower heating values (LHVs) different types of fuels

LHV of fuel Higher heating value (HHV)−HVapor (9H+W)

LHVCoal HHV −2,851 (9H+W)

LHVLignite HHV −2,803 (9H+W)

LHVBiomass HHV −2,535 (9H+W)

LHVMethanol HHV −2,437 (9H+W)

For coal:

LHVCoal =HHV−2,851 (9H+W) (2)

For lignite:

LHVLignite =HHV−2,803 (9H+W) (3)

For biomass:

LHVBiomass =HHV−2,535 (9H+W) (4)

For methanol:

LHVMethanol=HHV−2,437 (9H+W) (5)

in which Eqs. (2)–(5), HHV (kJ/kg) is higher heating value of fuel, H and W are hydrogen and water (hygroscopic moisture) contents (% by weight) of fuel, respectively.

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Figure 1. Plot of vaporization heat of water versus oxygen content of fuel.

Figure 1 shows the plot of vaporization heat of water versus oxygen content of fuel.

The LHV of a fuel depends on its oxygen content. As seen from Figure 1, the LHV of a fuel decreases with increasing of its oxygen content. The LHV of a fuel increases with increasing the hydrogen content due to cause combustion water. The LHV of a fuel increases with increasing the sulfur content due to cause SOx gases absorbed by water.

The carbon content of fuel also contributes to increase the LHV due to release CO2after combustion.

Moisture in a fuel generally decreases its HHV (Demirbas, 2002b). Moisture in biomass is stored in spaces within the dead cells and within the cell walls. When the fuel is dried, the stored moisture equilibrates with the ambient relative humidity. Equilibrium is usually about 20% in air dried fuel.

Moisture percentage of the wood species originally varied from 41.27 to 70.20%

(Demirbas, 2003c). The HHV of a wood fuel decreases with increasing of moisture content of the wood. Moisture content varies from one tree part to another. It is often the lowest in the stem and increases toward the roots and the crown.

The presence of water in biomass influences its behavior during pyrolysis and affects the physical properties and quality of the pyrolysis liquid: qualitative observations show that dry feed material led to the production of very viscous oil, particularly at higher reaction temperatures. The results obtained show that for higher initial moisture contents the maximum liquid yield on a dry feed basis occurs at lower pyrolysis temperatures between 691 K and 702 K (Demirbas, 2004).

The most important characteristic temperatures of a burning profile are ignition temperature and peak temperature for fuels (Haykırı-Açma et al., 2001). The ignition temperature corresponds to the point at which the burning profile underwent a sudden rise. The ignition temperatures of samples were determined from their burning profiles.

After releasing the moisture, some small losses in the mass of the sample occurred due to desorption of the adsorbed gases. A sudden loss in the mass of the samples started at the temperatures between 450–500 K, representing the release of some volatiles and

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654 A. Demirbas

their ignition. In the rapid burning region, the rate of mass loss proceeded so rapidly that it reached to its maximum value. Rapid loss of mass immediately slowed down at the temperatures between 600 K and 700 K. After then, burning rate apparently decreased and consequently some small losses in the mass of the sample continuously went on as long as temperature was increased up to 1,273 K, indicating the slow burning of the partly carbonized residue. At the end of hold time at 1,273 K, samples reached to the constant weight after given periods (Haykırı-Açma, 2003).

Conclusion

The effects of moisture and hydrogen contents on lower heating value of fuels were investigated. Moisture in biomass generally decreases its heating value. Higher heating value of a fuel decreases with increasing of its moisture content. The LHV of a fuel increases with increasing of its hydrogen content. The LHV of a fuel depends on its oxygen content and the LHV of a fuel decreases with increasing of its oxygen content. The LHV of a fuel increases with increasing the hydrogen content due to cause combustion water. Moisture in a fuel generally decreases its HHV.

The LHV of a fuel increases with increasing the sulfur content due to cause SOx

gases absorbed by water. The carbon content of fuel also contributes to increase the LHV due to release CO2 after combustion.

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