A procedure for estimating the value of forest fuels

Biomass 8 (1985) 283-300

A Procedure for Estimating the Value of Forest Fuels Gerard J. Lyons,t Frank Lunny$ and Hugh P. Pollock§ t Agricultural Institute, Oak Park Research Centre, Carlow, Ireland $ National Board for Science and Technology, Shelbourne House, Shelboume Road, Dublin 4, Ireland § Faculty of Engineering, University College, Cork, Ireland (Received: 3 April, 1985)

A BSTRA CT The gross caloriBc value (or 'higher heating value') of fuelwood is typically 18.6 MJ kg-1 o f dry wood. But the heat energy actually available from combustion o f freshly-harvested fuelwood may be as low as 5.9 MJ kg -1. For this reason, there is a tendency to overestimate the true energy and economic values o f forest biomass with respect to commercial energy sources. This paper interprets the main conventions used in quoting fuel values, and derives simple formulae (and graphs) for estimating energy contents o f fuelwood at any given moisture level The 'usable heat content' approach is developed as an equitable basis for comparing fuel values, and here provides energy and economic equivalents for wood, oil, coal, natural gas and peat. Key words: biomass, forest fuels, heating value, usable heat, fuel comparisons, economics.

INTRODUCTION Each year the world's forests produce about 65 billion tonnes of dry plant biomass. 1 This is equivalent to a fixed wood energy content of 1209 E J - more than four times the current global primary energy demand. Fuelwood accounts for over half (1.76 billion cubic metres) the world's annual timber harvest and is utilised predominantly in lowefficiency applications in developing regions. 2 In industrialised nations 283 Biomass 0144-4565/85/$03.30-© Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

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G. J. Lyons, F. Lunny, H. P. Pollock

forest residues and intensively-cultured tree plantations are being developed as economic energy sources, despite falling real prices for conventional fuels. Central to the appraisal of w o o d as an energy source is an awareness o f its realisable fuel value. But the variety of energy content assumptions, conventions and definitions used in the forest energy literature, is likely to confuse all b u t the experienced fuel technologist. What then are the most appropriate measures o f w o o d e n e r g y - gross or net calorific values, volumetric or unit weight values, or usable (recoverable) heat content? H o w does w o o d compare with commercial fossil fuels? This paper examines the factors affecting w o o d fuel value and interprets the engineering procedures for deriving energy contents. 3-s

WOOD F U E L COMPOSITION In its freshly harvested state, w o o d is a bulky, high moisture and high volatile content fuel. In industrialised countries the principal sources of fuelwood are logging residues, mill residues and purpose-grown energy plantations. R o u n d w o o d from natural forest stands is also harvested for fuel in developing regions. Thus, the fuel is variable and may consist of bark, twigs, shavings, sander dust, or whole-tree chips, and range in size from fine dust to long slabs. Moisture varies from 10 to 60% o f the total weight, but is typically 50-60% on harvest. Most species and components of fuelwood are uniform in their elemental composition. 6'7 Table 1 shows an ultimate chemical breakdown for conifers and hardwoods, expressed as a percentage o f the oven-dry weight. The ultimate analysis is used to calculate the quantity o f oxygen (and thus combustion air) required to sustain the combustion reactions. It also permits the estimation o f the amount of water formed b y burning hydrogen in the fuel. During combustion, heat is absorbed to vapourise and exhaust this moisture (in addition to the inherent fuel moisture), and the recoverable heat energy o f the fuel is consequently reduced. Because fuelwood is sulphur free and low in nitrogen (see Table 1), it produces minimal SO:, and NOx pollutants, but particulate emissions o f unburned carbon in the flue gases can present pollution control problems. Proximate analysis is the standard test method for evaluating solid fuels. It gives the relative amounts of ash, fixed carbon and volatile

285

A procedure for estimating the value or forest fuels TABLE 1

Typical Ultimate Chemical Breakdown of Wood and Bark (Per Cent of Dry Weight)a Conifers

Hydrogen Carbon Oxygen Nitrogen Sulphur Ash

Hardwoods

Wood

Bark

Wood

Bark

6.3 52-9 39-7 0.1 nil 1.0

5.9 53.1 37.9 0.2 nil 2.3

6.4 50.8 41.8 0.4 nil 0-9

6.0 51.2 37.9 0.4 nil 5.2

TABLE 2

Typical Proximate Analysis of Wood and Bark (Per Cent of Dry Weight)a Volatile matter

Fixed carbon

Ash

Conifers Wood Bark

77-2 73-3

22.0 23-7

1.6 3.0

Hardwoods Wood Bark

77.3 76.7

19.4 18-6

3.4 4.6

matter in a fuel as a percentage of its oven-dry weight. Proximate analysis thus indicates the percentage of fuel burned in the gaseous and solid states, respectively, and also shows the quantity of non-combustible ash remaining on the fire grates or ash pit, or entrained with flue gases. A typical analysis for some coniferous and hardwood species is presented in Table 2. Such analyses provide the furnace designer with important information for the sizing and location of primary and secondary air supplies, refractory requirements, ash removal and

286

G. Z Lyons, F. Lunny, H . ~ PoHock

exhaust handling equipment. The proximate analysis in Table 2 shows that about 77% (of the dry weight) of wood fuel is burned as a gas and 18-24% as fixed carbon. Assuming that fixed carbon is burned on the furnace grates and that the volatile constituents are burned as flames, it is clear that over 60% of the total calorific content of the fuel resides in the volatiles (the calorific value of wood is taken as 18.6 MJ kg-1 and carbon as 32 MJ kg-1). The importance of efficiency in burning the volatiles and in extracting heat from the flames cannot be too highly stressed.

WOOD COMBUSTION While other conversion techniques are available, direct combustion is the most commonly used and energy efficient means of deriving useful energy from wood. Because of the characteristically high moisture and volatile contents of wood, combustion takes place in three consecutive and overlapping stages? In the first stage heat is absorbed (from previously ignited fuel) to evaporate moisture in the fuel, and continues until the moisture approaches zero; the fuel temperature normally remains below 100°C during this stage. Volatile matter (other than water) is distilled and combusted (at 540-800°C), with the liberation of heat, during the second combustion stage. Finally, in the third stage, the remaining fixed carbon reacts with oxygen at high temperature to produce carbon dioxide and heat is also released. Thus, the main products of combustion are water (H20) and carbon dioxide (CO2). In practice, however, oxidation is not always complete and small amounts of carbon monoxide (CO), hydrocarbons and other gases are also released. Moisture content and particle size are the most critical factors affecting combustibility of wood fuels. Moisture reduces the heat available from fuel combustion in two ways. Firstly, the initial gross calorific value of the wood is lowered by the presence of water, which does not contribute to the heating value. For example, a wood sample whose moisture content is 30% of its total weight, will have a gross calorific value equal to only 70% of the theoretical anhydrous heating value. Secondly, combustion efficiency is reduced, because (i) heat is absorbed in the evaporation of water in the first stage of combustion, and (ii) flame temperature, and consequently radiant heat transfer, is lowered.

A procedure for estimating the value or forest fuels

287

The effects of moisture on combustion efficiency and boiler capacity are illustrated b y Arola, 8 Junge 1° and other authors. In combustion practice, w o o d moisture content is normally expressed on a wet weight basis, that is, as a percentage of the total original weight of a given sample. However, in the forest products field, a dry weight basis is used, where the amount o f moisture is given as a percentage of the dry substance only. It is important to distinguish between these two definitions in calculating fuel values. Moisture content (MC) on either the wet- or dry-weight basis can be converted to the other using the following relationships: % MC (wet basis) =

% MC (dry basis) 100 + % MC (dry basis)

X 100

and % MC (dry basis) =

% MC (wet basis) 1O0 -- % MC (wet basis)

X 100

All moisture contents in this paper are expressed on a wet weight basis. Particle size directly affects the rate o f combustion and heat content per unit bulk volume of the fuel. As w o o d burns principally in the gaseous state, the rate of combustion is proportional to the time it takes /or the required heat to reach and ignite volatile constituents; this in turn is dependent on the exposed surface area per unit volume of fuel. Theoretically, then, the minimum particle size should be chosen, since the total surface area o f a given quantity of fuel is inversely proportional to the square of the average particle diameter. However, the size of voids in the firebed decreases as the particle size is reduced and a point is reached where individual voids become so small that the resistance to passage o f combustion air is unacceptable. Consequently, the volume and velocity of excess air through the furnace must be increased; this results in the loss of a considerable amount of heat energy to raise ambient air to exhaust temperature and the high velocity may cause entrainment of light fuel particles in the flue gases. In practice, the minimum particle size for grate firing of solid fuels (including w o o d ) is about 1 2 mm and should be one-third to one-sixth of the firebed depth. If the firebed is less than three particle diameters deep the combustion air will not have time to react fully with the fuel

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G. J. Lyons, F. Lunny, H. P. Pollock

before it reaches the bed surface. If it is greater than six diameters then it will be consumed before reaching the surface, resulting in poor combustion o f the upper layers o f fuel and a non-radiant firebed top. E N E R G Y V A L U E OF WOOD Three different conventions are c o m m o n l y used in deriving the value of forest fuels: (1) gross calorific value; (2) net calorific value; and (3) usable heat content. Each o f these measures is useful to the engineer or scientist when applied in its correct context. But if misinterpreted, these values can give rise to underestimates o f w o o d fuel requirements, and impute a higher economic value to fuelwood than is practically realisable. Gross calorific value The basic heat of combustion o f a fuel is its gross calorific value (CV), or 'higher heating value'. This is a measure of the total energy embodied in a unit weight or volume of fuel and is determined experimentally by burning a k n o w n quantity o f fuel in a b o m b calorimeter and measuring the heat released. Reported experimental results for w o o d energy values normally refer to the gross CV, as water vapour from the products of combustion is condensed in the calorimeter and heat of condensation released. In the standard test procedures, w o o d samples are usually oven-dried before calorimetric analysis. Values thus derived are gross anhydrous CV's. Published anhydrous CV's for wood range from 18.0 MJ kg -1 to 24-0 MJ kg -1. But CV varies by species and tree c o m p o n e n t and is higher for resinous conifers (typically 20.0-23.0 MJ kg -1) than for hardw o o d (typically 18.5-20-0 MJ kg -1) species. Gross CV's for a wide range o f tree species are contained in the following references. 6' 8, 11-14 An average anhydrous CV for w o o d o f 18.6 MJ kg -1 is cited in fuel handbooks and is used here as a basic reference value. As all forest fuels contain some moisture when harvested their gross CV is lower than the o p t i m u m anhydrous value. The gross calorific value o f a green w o o d sample is the CV of the dry matter it contains. Thus for a fuelwood at 30% moisture content, each kilogram contains 0.7 kg o f dry timber and the gross CV will be 0.7 X gross anhydrous CV = 0.7 X 18.6 -- 13.02 MJ kg -1

A procedure for estimating the value or forest fuels

289

L Gross(Anhyfl.)CV : I 8.6 Md/kg 18 . . . . . . . . . . . . ~ ~ Gross CV I J'"" " ~ Net CV •.,.... \ ~ Usebish,ot .......

io

""'"'"

"-i\

2

0 -2

Fig. 1.

a

I

ZO

i

I

40

l

I'""

60

Moisture content

80 "'..~

(%)

"',.~

Variation of gross CV, net CV, and usable heat content with fuel moisture.

Figure 1 shows the linear relationship between gross CV and wood moisture content (upper diagonal line in Fig. 1). This is based on the general expression: Gross CV at moisture content (m) is given by: Cg = Cga × (1 --rn)

(1)

where Cg = gross CV; Cga = gross anhydrous CV; rn = moisture content, as a fraction of the wet weight. This reduces to Cg = 18.6 × (1 - - m ) (MJ kg -1) when the reference value for Cga is substituted. Net calorific value Gross calorific value overstates the true energy potential of wood fuel because some of the dry matter must be burned to evaporate moisture (in the first stage o f combustion) before any heat is available for external useful purposes. Thus, moisture may be regarded as having a negative heating value.

290

G. J. Lyons, F. Lunny, H. P. Pollock

Net calorific value, or 'lower heating value', assumes that the moisture products of combustion remain in the vapour phase passing all heat recovery devices and no condensation heat release is gained (as in the gross CV). Net CV is normally quoted at the calorimetric reference temperatrure o f 25°C. a The latent heat o f vaporisation o f steam at 25°C is 2.44 MJ kg -1. This implies that 2.44 MJ o f heat are absorbed in vaporising 1 kg of moisture. So if the wood contains 30% moisture, 0-3 kg of water must be evaporated per kilogram of fuel burned. This results in a heat o f vaporisation loss of 0.3 X 2.44 = 0.732 MJ kg -1 of wet fuel Besides the moisture stored in w o o d cells, additional water is formed by the reaction o f the fuel's hydrogen content with oxygen, during combustion of dry matter. Since hydrogen produces nine times its own weight of water when burned, and w o o d contains about 6% hydrogen (see ultimate analysis, Table 1), the a m o u n t of water formed by the combustion o f 1 kg of oven-dry w o o d is: 0.06 × 9 = 0.54 kg Again, taking 30% MC fuel, the amount of water produced from hydrogen in the dry matter (70%) is: 0.7 X 0.54 = 0.378 kg and the resultant vaporisation heat loss is: 0-378 X 2.44 = 0.922 MJ kg -~ of wet fuel In summary, the net CV is equal to the gross CV (at the given MC) minus the losses associated with vaporisation o f moisture from the fuel. Moisture originates from two sources: (a) the inherent fuel moisture; and (b) water produced b y burning hydrogen. Summing vaporisation losses gives the net CV (Cn) as follows, for 30% MC w o o d ' Cn = gross CV -- (latent heat of steam) [fuel moisture + moisture from H2] = C g - - 2 . 4 4 1 0 . 3 + 0.378] = 13.02--1.65 = l l . 3 7 M J k g -1

A procedure for estimating the value or forest fuels

291

The general formula for calculating net CV is: C n = Cga(1 -- m ) -- (latent heat o f steam)Ira + 9H(1 - - m ) ]

(2)

where Ca = net CV; Cga = gross anhydrous CV; rn = moisture content, as a fraction o f the wet weight; H -- hydrogen content, as a fraction of the dry weight; latent heat of steam = 2.44 MJ kg-1 at 25°C. For wood, with a gross anhydrous CV of 18.6 MJ kg -1 this expression becomes: C n = 18.6(1 - - r n ) - - 2 . 4 4 [ m + 0.54(1 --rn)] which reduces to Cn = 17.28 -- 19.72m (MJ kg -a)

(3)

This expression yields the dashed diagonal line in Fig. 1, which shows the variation of net CV with moisture content. Usable heat content Gross and net calorific values are useful measures of a fuel's energy content. But, in practice, they do not represent the heat energy which can ultimately be recovered when fuel is burned in a combustion chamber. In calculating the net CV it was assumed that the gaseous products of combustion are discharged at ambient temperature (25°C), so that no heat is lost except the latent heat of moisture vapour. In reality, flue gases are exhausted at an elevated temperature (typically 200°C in an efficient furnace), and energy is lost in: ( 1 ) s u p e r h e a t e d moisture vapour; (2) dry flue gases; and (3) any excess air. These losses must be accounted for in determining the usable heat content (or 'recoverable heat energy') of wood fuel. The Losses Method for determining combustion efficiency is described in fuel texts and handbooks; 3 Ince 12 illustrates its application to wood and bark fuels. To evaluate flue gas losses certain characteristics of the combustion system must be known or assumed. These include: Temperature o f air and fuel entering furnace (Ti) Temperature of flue gases (Tf) Excess air expressed as a decimal of stoichiometric air requirements. In this paper the following conditions are assumed: ri = 25°c

Tf = 200°C

Excess air = 0-5

292

G. J. Lyons, F. Lunny, 1t. P. Pollock

These represent typical combustion conditions for efficient industrial scale wood-fired furnaces. 1. Loss due to moisture (L 1) In addition to the heat absorbed in vapourising fuel moisture in the calculation o f net CV, further energy is required to raise the moisture from ambient temperature to flue gas temperature. The average specific heat capacity for water vapour over this temperature range is 2.0 kJ kg -~ °C-~. The quantities o f moisture involved were deduced in the estimation of net CV. These are: Inherent fuel moisture = m Moisture from burning hydrogen = 0.54 X (1 - - m ) Total moisture = 0.46m + 0.54 Thus, the total heat absorbed in elevating water vapour to flue gas temperature is (kJ kg -1 o f wet fuel): L1 --- 2 - 0 [ 0 . 4 6 m + 0.54] (Tf -- T i)

(4)

Substituting the assumed values for Ti (25°C) and Tf (200°C) reduces eqn (4) to: LI = 3 5 0 [ 0 . 4 6 m + 0.54] Then for 30% moisture w o o d , L1 = 237.3 kJ kg -1 of wet fuel 2. Loss due to dry flue gases (L2) Assuming complete combustion, the following reactions are the principal mechanisms involved in the burning of w o o d : 2H2 + 02 ~ 2H20 C + O5 ~ COs Substituting molar masses shows that 8.0 kg of oxygen are required to burn 1.0 kg o f hydrogen, and 9.0 kg o f water are produced (see net CV). Similarly, 2.67 kg of oxygen are used in burning 1.0 kg o f carbon, and 3.67 kg o f carbon dioxide are given off. The approximate composition o f air is 23.2% oxygen and 76-8% nitrogen, by weight. Thus, the quan-

A procedure for estimating the value or forest fuels

293

tities o f air needed to burn 1 kg of hydrogen and 1 kg of carbon are 34.5 kg and 11.5 kg, respectively. Stoichiometric air requirements are derived from the fuel's ultimate analysis. Table 1 shows that oven-dry wood contains approximately 51% carbon, 40% oxygen, and 6% hydrogen. Hence 6-21 kg of air are required to supply sufficient oxygen to completely burn 1 kg of dry wood. The dry flue gases produced during combustion are: CO2 from burning carbon = (0.51 )(3.67) = 1.87 kg N2 from combustion air = (0.768)(6.21) = 4.77 kg Total weight o f dry flue gases = 6.64 kg Assuming an average specific heat capacity for nitrogen and carbon dioxide o f 1.0 kJ kg-1 °C-1 over the temperature range T i to Tf, the energy loss due to the elevated temperature o f dry flue gases is (kJ kg -1 o f wet f u e l ) L2 = 1.0 [(1 -- m) (6.64)] (T~ -- Ti) = [6-64 -- 6.64m1 (Tf -- Ti)

(5)

For the assumed combustion conditions, this reduces to L2 = 175 [ 6 - 6 4 - - 6.64m] which for 30% moisture wood is L2 = 813.4 kJ kg -~ of wet fuel 3. Loss due to excess air (L fl As excess air is normally expressed as a fraction of stoichiometric air requirements, the weight of excess air exhausted with the dry flue gases can be calculated directly from the weight o f stoichiometric air, estimated above (= 6.21 kg kg -1 o f dry wood). The specific heat capacity for air over the temperature range Ti to Tf is 1.005 kJ kg -1 °C-I. The loss due to the elevated temperature of excess air may thus be expressed as (kJ kg -I of wet fuel): L3 = 1.005 [(1 - - m ) (6.21 X excess air)] (Tt -- Ti) = excess air [6.24 -- 6.24m] (Tf -- Ti) where 'excess air' is expressed as a decimal fraction. This becomes La = 87-5 [ 6 . 2 4 - - 6.24m]

(6)

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G. J. Lyons, F. Lunny, H. P. Pollock

for the assumed combustion conditions (excess air = 0.5). For 30% moisture wood L 3 = 382.2 kJ kg -1 of wet fuel •.

LI+L2+L3

= 1432.9 kJ kg-l of wet fuel

Conventional heat losses In practice, combustion may not always be complete and additional losses are associated with the elevated temperature o f refuse, unburned carbon in the refuse or entrained with flue gases and carbon monoxide in the flue gas. Other unaccounted for losses may be due to radiation, conduction and convection heat transfer efficiencies, and humidity o f combustion air. Together these unaccounted for losses are referred to as 'conventional heat losses', and are usually o f the order 3-5% o f the gross CV. 3' 4,12 Assuming 4% conventional heat losses: Conventional heat losses = 0.04Cg

Summary of losses Exhaust losses due to moisture (L1), dry flue gases (L2) and excess air (L3) may be calculated using the following formulae: Loss due to moisture

L 1 = 2.0 [0.46m + 0.54] (Tf -- Ti)

(4)

Loss due to dry flue gases L2 = [6.64 - 6.64m] (Tf - - T i ) Loss due to excess air

(5)

L 3 = excess air [ 6 . 2 4 - 6.24m] (Tf --Ti)

(6) where m = moisture content, as a fraction of the wet weight; Ti = temperature of air and fuel entering furnace; Tf --- temperature of flue gases; excess air = excess air, as a fraction o f stoichiometric air. The usable heat content o f wood fuel may be estimated by deducting the sum of these losses and conventional heat losses from the net calorific value. Usable heat = C, -- [L1 + L2 + La] -- 0.04Cg

(7)

Substituting the given combustion conditions into the loss formulae ( f o r L I , L2 and L3) and expressing these losses in MJ yields the follow-

ing relationship for usable heat in terms of fuel moisture content (see Fig. 1): Usable heat = 1 4 . 6 4 - 17.43m (MJ kg -1)

(8)

A procedure for estimating the value of forest fuels

295

So, for 30% moisture fuel Usable heat = 9.41 MJ kg -1 from eqn (8) or Fig. 1.

Energy value of wet fuel per unit dry matter On harvest, wood fuel is typically 50-60% moisture, but is usually dried to a lower moisture content before burning. Thus, the fuel weight may vary considerably but the a m o u n t of dry matter in the fuel remains constant. Consequently, forest fuel biomass yields are frequently quoted as dry matter weights. It is therefore convenient, for economic and energy balance studies, to use the energy equivalent of unit dry matter as a fuel value basis; and it is useful to express the heating value of wet wood as the value per unit o f dry matter it contains. This is equal to : Energy value per unit of wet fuel (1 - m ) Thus, Cg per kg dry matter = Cga Cn per kg dry matter =

from eqn (1)

17.28 -- 19.72m (1 - - m )

from eqn (3)

(9)

This is the general expression for the net CV per unit of dry wood burned at moisture content m. Usable heat per kg dry matter =

1 4 . 6 4 - - 17.43m (1 - - m )

from eqn (8)

(10)

This is the general equation for usable heat content per unit o f dry wood burned at moisture m. Figure 2 shows the relationship between energy value per unit of dry matter and the moisture content at which it is burned, based on eqns (9) and (10). These curves demonstrate how energy value, and therefore economic value, vary with moisture content and may thus be used to illustrate the financial efficacy of fuel drying. For instance, if fuel moisture is 50% at harvest and is allowed to dry naturally to 30% MC before combustion, there is a usable heat gain of 1.59 MJ kg -~ of dry

296

G. J. Lyons, F. Lunny, H. P. Pollock

Gross (Anhyd.) CV : 18,6 M d / k g v

E

18

14

o

NetOV \ \~ 0.b,s,s.t , . , , , , i\1, >

2

o -2

Fig. 2.

zo

40

60

Moisture content m (96)

80/ | I

Variation of net CV and usable heat content per Mlogram o f dry matter burned at moisture content m.

matter. It should be noted, however, that the net energy gain due to drying declines as moisture content decreases.

COMPARISON WITH COMMERCIAL FUELS While fuelwood is the principal source o f energy in developing countries, fossil fuels are likely to maintain their dominance o f commercial energy supplies in developed regions well into the next century. Many consumers may thus be faced with a choice between a range o f fossil fuels, and fuelwood as an alternative or replacement energy source. The usable heat content approach, outlined in the previous section, provides an equitable means for comparing fuels in their useful application. The net calorific values (i.e. lower heating values) o f the main commercial fuels are as follows: Oil Coal

42.8 MJ kg -1 29.3 MJ kg -1

A procedure for estimating the value or forest fuels Peat (@ 35% MC) Natural gas Wood (@ 30% MC)

297

12-6 MJ kg -1 34.8 MJ m -3 11-4 MJ kg -1

At equal efficiency, the useful energy recovered (and consequently the delivered price) from the various fuels should be in proportion to these figures. However, it is difficult to burn all fuels at the same efficiency, due mainly to differences in hydrogen and moisture content. But, if it is assumed that they are all burned completely under the same conditions o f exhaust temperature and excess air, the relative usable heating values may be derived. Following the procedures outlined above, usable heat contents for wood, oil, coal, peat and natural gas were calculated from their net calorific values (given above). These estimates provided the fuel equivalent ratios shown in Table 3. These equivalents represent the energy actually recovered by burning the respective fuels, and may thus be used to compare usable heat costs for the various energy sources. In Fig. 3, they have been used to derive comparative price scales for wood and the main commercial fuels. Current (1984) prices for commercial fuels delivered to industry in Ireland are marked (x) to illustrate how wood fuel economic values may be imputed from the prices for competing energy sources. For

TABLE 3

Usable Heat Content Equivalents of Wood and Commercial Fuelsa Commercial fuel

1 tonne oil 1 tonne coal 1 tonne peat 1000 m3 natural gas

30% MC wood {tonnes)

Oven dry wood b {tonnes)

3.90 2.68 1.11 3-16

2.73 1.88 0.78 2.21

a Assuming complete combustion under the following conditions: inlet air and fuel temperature (Ti) = 25°C; flue gas temperature (Tf) = 200°C; excess air = 0.5. b Dry matter burned at 30% moisture content under the above combustion conditions.

298

G. J. Lyons, F. Lunny, H. P. Pollock Fuel price

I0

o

Wood ($ 't) -8o

8

Nat. gas

Oi (S/t)

-300

Coal

(S/t) -200

-70

6

g OA

-60

~5o

-~6 . . . . . . .

~oo

-40

150

t 30

2

2O

Peat lso

(S/t) - r- °

100

100 -50

40 t250 30 0

6 m15o

4

2

10

o o Fig. 3. Cost comparison of wood and commercial fuels. Notes: 1. wood @ 30% moisture content, peat @ 35% moisture content; 2. IS(US) = IlR£ (Irish pound, Oct-Dec 1984) approx.; 3. commercial fuel prices in Ireland (1984) are marked (x) to illustrate application of the diagram.

example, w o o d fuel (30% MC) is worth $52 t -1 against a fuel oil price o f $203 t -I ($29 barrel-i). It should be remembered that fuel cost comparisons enabled by Table 3 and Fig. 3 are based strictly on useful energy equivalence vaues, estimated for specific combustion conditions. In operating a commercial heating plant, other non-fuel costs (capital, operating and maintenance) must also be charged per unit of heat delivered. These may alter the relative fuel values (shown in Fig. 3) considerably, especially for comparisons between solid fuels and oil/gas. For instance, the cost o f storing, handling, and firing equipment for w o o d is so far greater than that for fuel oil that the apparent price advantage of w o o d is substantially less than what is indicated above. How much less depends on the scale of the operation. For all solid fuels, the non-fuel

A procedure for estimating the value of forest fuels

299

costs should be similar, and the adjustment to the relative fuel values given is consequently small.

CONCLUSIONS This paper has outlined the characteristics of forest biomass which affect its value as a direct combustion fuel. It clearly distinguishes between the different conventions used in quoting energy contents and should enable researchers and managers to assess the value of forest biomass resources and to compare these with alternative fuels. While the calorific comparisons set out above are, and will remain, valid, the price marked for commercial fuels may be expected to increase substantially; and, in the long term, the value o f wood as a source of energy will increase correspondingly.

REFERENCES 1. Baumgartner, A. & Kirchner, M. (1980). Impacts due to deforestation. In: Interactions of energy and climate, Proceedings of an international workshop held in Munster, Germany, 3-6 March, 1980, W. Bach, J. Pankrath and J. Williams (eds), D. Reidel Publishing Co., Dordrecht, Holland, pp. 305-16. 2. Food and Agriculture Organisation (FAO) of the United Nations (1983). Yearbook o f forest products 1981, FAO, Rome. 3. Rose, J. W. & Cooper, J. R. (1977). Technical data on fuel, 7th edn, The British National Committee, World Energy Conference, London. 4. Francis, W. & Peters, M. C. (1980). Fuel technology. A summarised manual, 2nd edn, Pergamon Press, London. 5. Pratt, A. D. (1936). Principles of combustion in the steam boiler furnace, Babcock & Wilcox Ltd, London (reprinted 1954). 6. Corder, S. E. (1973). Wood and bark as fuel, Research Bulletin 14. For. Res. Lab. School of Forestry, Oregon State Univ., Corvallis, Oregon 97331. 7. Koch, P. (1972). Utilization of the Southern Pines. USDA Forest Service, Agriculture Handbook No. 420, Vol. H, Chapter 26, USDA, Washington DC. 8. Arola, R. A. (1976). Wood fuels - how do they stack up? In: Energy and the wood products industry, FPRS Proceedings No. P-76-14, For. Prod. Res. Soc., Madison, Wisconsin, pp. 34-45. 9. Reineke, L. H. (1961). Wood fuel combustion practice, For. Prod. Lab. Report No. 1666-18, USDA For. Prod. Lab.. Madison, Wisconsin.

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