# Chapter IV. Measurement units and conversion factors
## A. Introduction
4.1. Energy products are measured in physical units by their mass, volume, and energy content. The measurement units that are specific to an energy product and employed at the point of measurement of an energy flow are often referred to as "original" or "natural" units. Coal, for example, is generally measured by its mass and crude oil by its volume. On the other hand, cross-fuel tabulations, such as the energy balances, are displayed in a "common" unit to allow comparison across energy products. These "common" units are usually energy units and require the conversion from an original unit through the application of an appropriate conversion factor.[^24]
4.2. When different units are used to measure a product, the compiler is left with the task of converting data that, in the absence of specific information on the products necessary for the conversion between different units (such as density, gravity and calorific value), may lead to discrepancies.
4.3. This chapter reviews the measurement units used for energy statistics, explains the concepts of "original" and "common" units, and presents default conversion factors to use in the absence of country- or region-specific calorific values.
## B. Measurement units
4.4. This section covers "original" or "natural" units, as well as "common" units. It also makes reference to the International System of Units, often abbreviated as SI from the French "Système International d'Unités", which is the modern metric system of measurement established by international agreement. It provides a logical and interconnected framework for all measurements in science, industry and commerce (See box 4.1 for more details on SI).
> **Box 4.1**
> **International System of Units**
>
> The International System of Units (SI) was established by and is defined by the General Conference on Weights and Measures (CGPM). It is the result of work that started in 1948 to make recommendations on the establishment of a practical system of units of measurement suitable for adoption by all signatories to the Convention du Mètre.
>
> In 1954 and 1971, the CGPM adopted as base units the units of the following seven quantities: length, mass, time, electric current, thermodynamic temperature, luminous intensity and amount of substance.
>
> In 1960, the CGPM adopted the name Système International d'Unités, with the international abbreviation SI for this practical system of units and laid down rules for prefixes, derived units, and the former supplementary units; it thus established a comprehensive specification for units of measurement.
>
> Source: Based on the International Bureau of Weights and Measures (BIPM), www.bipm.org/en/measurement-units.
4.5. Standardization in the recording and presentation of original units is a primary task of an energy statistician before quantities can be analysed or compared.
4.6. The base units of SI are a choice of seven well-defined units that by convention are regarded as dimensionally independent. There are seven base units, each of which represents, at least in principle, different kinds of physical quantities.
| Physical quantity | Base unit |
| --- | --- |
| length | metre |
| mass | kilogram |
| time | second |
| electric current | ampere |
| thermodynamic temperature | kelvin |
| luminous intensity | candela |
| amount of substance | mole |
4.7. Derived units of SI are those formed by combining base units according to the algebraic relations linking the corresponding quantities. They are defined as products of powers of the base units. When such product includes no numerical factor other than one, the derived units are called coherent derived units.[^25]
4.8. SI uses a specific set of prefixes known as SI prefixes, which indicate a multiple or fraction of the unit. These prefixes are:
| Factor | Name | Symbol | Factor | Name | Symbol |
| --- | --- | --- | --- | --- | --- |
| 10¹ | deca | da | 10⁻¹ | deci | d |
| 10² | hecto | h | 10⁻² | centi | c |
| 10³ | kilo | k | 10⁻³ | milli | m |
| 10⁶ | mega | M | 10⁻⁶ | micro | μ |
| 10⁹ | giga | G | 10⁻⁹ | nano | n |
| 10¹² | tera | T | 10⁻¹² | pico | p |
| 10¹⁵ | peta | P | 10⁻¹⁵ | femto | f |
| 10¹⁸ | exa | E | 10⁻¹⁸ | atto | a |
| 10²¹ | zetta | Z | 10⁻²¹ | zepto | z |
| 10²⁴ | yotta | Y | 10⁻²⁴ | yocto | y |
### 1. Original units
4.9. As mentioned in para. 4.1 above, original units are the units of measurement employed at the point of measurement of a product flow that are best suited to its physical state (solid, liquid or gas) and that require the simplest measuring instruments.[^26] Typical examples are: mass units (e.g., kilograms or metric tons) for solid fuels;[^27] volume units (e.g., barrels or litres) or mass units (metric tons) for oil; and volume units (e.g., cubic metres) for gases. The actual units used nationally vary according to country and local conditions and reflect historical practice in the country, sometimes adapted to changing fuel supply conditions.[^28]
4.10. It should be noted that in questionnaires utilized for the collection of energy statistics, data may be required to be reported in different units from the original/natural unit. For example, statistics on crude oil and oil products may be requested in a mass or weight basis, since the heating value of oil products by weight displays less variation than the heating value by volume. Statistics on gases, as well as wastes, can be requested in terajoules or other
energy units in order to ensure comparability, since gases (and wastes) are usually defined on the basis of their production processes, rather than their chemical composition, and different compositions of the same type of gas (or waste) entail different energy contents by volume. The collection of statistics on wastes in an energy unit is based on the measured or inferred heat output used directly for heat raising.
#### Mass units
4.11. Solid fuels, such as coal and coke, are generally measured in mass units. The SI unit for mass is the kilogram (kg). Metric tons (tons) are most commonly used to measure coal and their derivatives. One metric ton corresponds to 1000 kg. Other units of mass used by countries include the pound (0.4536 kg), short ton (907.185 kg) and long ton (1016.05 kg). Table 1 in annex B presents the equivalent factors for converting different mass units.[^29]
#### Volume units
4.12. Volume units are original units for most liquid and gaseous fuels, as well as some traditional fuels. The SI unit for volume is the cubic metre, which is equivalent to a kilolitre or one thousand litres. Other volume units include the British or Imperial gallon (approximately 4.546 litres), United States gallon (approximately 3.785 litres), the barrel (approximately 159 litres), and the cubic foot, which is also used to measure volumes of gaseous fuels. Given the preference of oil markets for the barrel as a volume unit, the barrel per day is commonly used within the petroleum sector to allow direct data comparison across different time frequencies (e.g., monthly versus annual crude oil production). However, in principle, other units of volume per time can be used for the same purpose. Table 2 in annex B shows the equivalent factors to convert volume units.[^30]
#### Relationship between mass and volume—specific gravity and density
4.13. The relationship between mass and volume is called density and is defined as mass divided by volume. Since liquid fuels are measured either by their mass or volume, it is essential to be able to convert one into the other, and knowing the density allows this:
Density = mass / volume
4.14. Specific gravity is a dimensionless unit defined as the ratio of density of the fuel to the density of water at a specified temperature. This can also be expressed as the ratio of the mass of a given volume of fuel, for instance oil, at 15°C to the mass of the same volume of water at that temperature:
Specific gravity = densityfuel / densitywater = massfuel / masswater
4.15. When using the SI or metric system in order to calculate volume, mass is divided by density. Vice versa, to obtain mass, volume is multiplied by density. When using other measurement systems, one must consult tables of conversion factors to move between mass and volume measurements.
4.16. Another measure for expressing the gravity or density of liquid fuels is API gravity, a standard adopted by the American Petroleum Institute. API gravity is related to specific gravity by the following formula:
API gravity = (141.5 / specific gravity) − 131.5
#### Energy units
4.17. Energy, heat, work and power are four concepts that are often confused. If force is exerted on an object and moves it over a distance, work is done, heat is released (under anything other than unrealistically ideal conditions) and energy is transformed. Energy, heat and work are three facets of the same concept. Energy is the capacity to do (and often the result of doing) work. Heat can be a by-product of work, but is also a form of energy. The coherent derived SI unit of energy, heat and work is the joule (J). The joule is a precise measure of energy and work, defined as the work done when a constant force of 1 Newton is exerted on a body with mass of 1 gram to move it a distance of 1 metre. Common multiples of the joule are the megajoule, gigajoule, terajoule and petajoule.
4.18. Other units include: the kilogram calorie in the metric system, or kilocalorie (kcal) or one of its multiples; the British thermal unit (Btu) or one of its multiples; ton of coal equivalent (tce), ton of oil equivalent (toe); and the kilowatt hour (kWh).
4.19. The International Steam Table Calorie (IT calorie) was originally defined as 1/860 international watt-hour, but was later defined exactly as 4.1868 joules.[^31] This is the definition of the calorie used in the conversion tables in the annex to this chapter. The kilocalorie and the teracalorie are multiples of the calorie that are commonly used in the measurement of energy commodities. In the context of IRES, these are based on the IT calorie. Other definitions of the calorie include the gram calorie, defined by the amount of heat required to raise the temperature of one gram of water 1°C from a reference temperature. With a reference temperature of 14.5°C, the gram calorie equals 4.1855 joules.[^32]
4.20. The British thermal unit is a measure of heat and is equal to the amount of heat required to raise the temperature of 1 pound of water at 60°F by 1°F.[^33] Its most used multiples are the therm (10^5 Btu) and the quad (10^15 Btu). The internationally agreed value for the Btu is currently 1055.06 joules.
4.21. In the past, when coal was the principal commercial fuel, the ton of coal equivalent (tce) was commonly used as an energy unit. However, with the increasing importance of oil, it has been replaced by the ton of oil equivalent (toe). The toe is now defined as 41.868 gigajoules, whereas the tce equals 29.3076 gigajoules. Generally, it should not be assumed that one ton of coal contains one tce or that one ton of oil contains one toe of energy content, since there is a wide spread in calorific values among various types of coals, crude oils and petroleum products.[^34]
4.22. Power is the rate at which work is done (or heat released, or energy converted). The rate of one joule per second is called a watt. As an example, a light bulb might draw 100 joules of electricity per second to emit light and heat (both forms of energy). This light bulb would then draw the power of 100 watts.
4.23. The above definition of watt leads to another commonly used measure of energy, the kilowatt hour (kWh), which refers to the energy equivalent of 1000 watt (joules per second) over a one-hour period. Thus, 1 kilowatt-hour equals 3.6x10^6 joules.
4.24. Electricity is usually measured in kWh. This allows one to perceive the electrical energy in terms of the time an appliance of a specified wattage takes to "consume" this energy. Heat quantities, on the other hand, are usually measured in calories or joules.
4.25. Table 3 in annex B shows the conversion factors between several energy units.
### 2. Common units
4.26. Since the original units in which energy products are measured vary (e.g., metric tons, barrels, kilowatt hours, therm, calories, joules, cubic metres), quantities of energy products need to be converted into a common unit to allow comparisons of fuel quantities and estimate transformation efficiencies. The conversion from different units to a common unit may require specific conversion factors for each product.[^35]
4.27. The only energy unit in the International System of Units is the joule and it is usually used in energy statistics as a common unit, although other energy units are sometimes also applied (e.g., toe, GWh, Btu, calories, etc.). The use of the joule as a common unit is recommended.
4.28. It is further recommended that national and international agencies in charge of energy statistics and any other organizations that advise them or undertake work for them always clearly define the measurement units, as well as the common units used for presentational purposes in various publications and in electronically disseminated data. The conversion factors and the methods used to convert original physical units into the chosen common unit or units should be described in energy statistics metadata and be readily accessible to users. In addition, it should be made clear whether energy units are defined on a gross or net calorific basis (see section C below for details).
## C. Calorific values
4.29. Calorific values or heating values of a fuel express the heat obtained from one unit of the fuel. They are necessary for the compilation of overall energy balances where the original units in which the fuels are measured are converted into a common unit of measurement. Even though calorific values are often considered in the context of the preparation of energy balances, they have a wider application in the preparation of any tables designed to show energy in an aggregated form or in the preparation of inter-fuel comparative analyses.
4.30. Calorific values are obtained by measurements in a laboratory specializing in fuel quality determination. They should preferably be in terms of joules (or any of its multiples) per original unit, for example gigajoule/metric ton (GJ/t) or gigajoule/cubic metre (GJ/m3). Major fuel producers (mining companies, refineries, etc.) normally measure the calorific value and other qualities of the fuels they produce. A calorific value is a conversion factor, in the sense that it can be used to convert mass or volume quantities into energy content.
4.31. There are two main issues with regards to calorific values that an energy compiler should pay attention to: the first one refers to whether they are measured gross or net of the latent heat (that is the heat necessary to evaporate the water formed during combustion and the water previously present in the fuel in the form of moisture); and the second one is related to whether the calorific value used refers to the specific product-flow-country situation or it refers to a default value. These two issues are presented in detail in the next two sections.
### 1. Gross and net calorific/heating values
4.32. Calorific values can be expressed on a gross and net basis. The gross calorific value (GCV), or high heat value, measures the total (maximum) amount of heat that is produced by combustion. However, part of this heat will be locked up in the latent heat of evaporation of any water present in the fuel before combustion (moisture), or generated in the combustion process. The latter comes from the combination of hydrogen present in the fuel with the oxidant oxygen (O2) present in the air to form H2O. This combination itself releases heat but this heat is partly used in the evaporation of the generated water.
4.33. The net calorific value (NCV), or low heat value, excludes the latent heat. The NCV is that amount of heat from the combustion process that is actually available in practice for capture and use. The higher the moisture of a fuel or its hydrogen content, the greater is the difference between GCV and NCV. For some fuels with very little or no hydrogen content (e.g., some types of coke, blast furnace gas) this difference is negligible. In terms of magnitude, the difference between gross and net calorific values of fossil fuels (coal, oil, oil products and gas) is typically less than 10 per cent, while that of biomass energy (fuelwood, bagasse) is usually more than 10 per cent. Examples of differences between gross and net calorific values are presented for selected energy products in table 4 of annex B. It should be noted that the technology used to burn a fuel can also play a role in determining the NCV of that fuel, for instance, depending on how much of the latent heat it can recover from the exhaust gases.
4.34. It is recommended that, when expressing the energy content of energy products in terms of a common energy unit, NCVs be used in preference to GCVs. In other words, the heat required to evaporate moisture, which is present in all fuels and is also produced in the combustion process, should not be treated as part of a fuel's energy, providing capability. In particular, NCVs are to be preferred over GCVs when building an energy balance, since most current technologies are still not able to recover the latent heat, which would thus not be treated as part of a fuel's energy providing capability (see chapter VIII for further discussion).[^36] However, where available, it is strongly encouraged to report both gross and net calorific values.
### 2. Default vs. specific calorific values
4.35. Energy products with exactly the same chemical composition will carry the same energy content. In practice, however, there are variations in the composition of energy products and consequently, their calorific values can vary. For example, "premium" gasoline may have slightly different chemical formulations (and therefore a different energy content) than "regular" gasoline; natural gas may contain variations in the proportions of ethane and methane; liquefied petroleum gas (LPG) may in fact be solely propane or solely butane or any combination of the two. Only those products that are single energy compounds, such as "pure" methane or "pure" ethane, and electricity, have precise and unalterable energy contents.
4.36. Default calorific values refer to the energy content of fuels with specific characteristics that are generally applicable to all circumstances (different countries, different flows, etc.). They are used as default values when specific calorific values are not available. Specific calorific values, on the other hand, are based on the specificity of the fuel in question and are measurable from the original data source. They are particularly important for fuels that present different qualities: coal, for example, displays a range of quality that makes it suitable for different uses. The respective calorific values are thus specific to the fuel and flow in question. However, in using many different specific calorific values, caution should be applied to ensure consistency between the energy content on the supply side and on the consumption side for a particular country and year.
4.37. Often there is a problem in energy statistics as the product produced may not be identical in composition to the product in subsequent processes, even though it is referred to by the same name. Natural gas can be enriched with oil products, for example, to meet market specifications. Motor gasoline can be blended with ethanol and sold as motor gasoline, and depending on the country practice, this may be recorded as consumption of only motor gasoline or as consumption of motor gasoline and the blending agent. In this case, flow-specific calorific values would allow for a more accurate energy balance.
4.38. It is recommended that countries collect data in original units together with data on specific calorific values. A country-specific calorific value is generally calculated as a weighted
average of all the calorific values collected for the energy product in question (see next section). For some products (e.g., coal and crude oil), different calorific values may be needed for production, imports, exports and several major uses. Default calorific values should only be used as a last resort in the absence of specific values, acknowledging that this simplification will affect the precision of the published figures.
4.39. It is further recommended that metadata be provided on the methods used in all calculations and conversions undertaken to arrive at the disseminated data in order to ensure transparency and clarity and to enable comparability. In particular, this would include the conversion factors between original and presented units, whether they are on a gross or net calorific basis, and any use of default values.
### 3. How to calculate average calorific values
4.40. The calculation of calorific values is not straightforward. There are two levels involved in this calculation. The first is the actual measurement of the heating value of an energy product. This is done in laboratories specializing in fuel quality determination. In general, major fuel producers (i.e., mining companies, refineries, etc.) measure the quality of the energy product they produce, as this may affect its price and specification. This type of calculation thus pertains to specialists, and it is not covered in IRES: the calorific values are assumed to be available from the data providers (generally companies producing energy).
4.41. The second level in the calculation of the calorific values pertains more to the compilers of energy statistics, as it involves the aggregation of different qualities of a fuel. Coals produced at different mines, for example, often have different qualities. The quality of imported coal may vary according to the origin of the flow. Similarly, the quality of consumed coal may also differ: the case, for example, of imported steam coal for electricity generation, and home-produced lignite for household consumption. Thus, in the preparation of energy balances and in the comparison of the energy content of energy products, it is necessary to take into account the different qualities of the products themselves.
4.42. In general, in order to aggregate different qualities of an energy product, it is necessary to calculate the average calorific value. Consider, for example, the case where the production of lignite comes from two different mines in a country: mine A produces 1.5 thousand metric tons of lignite with a net calorific value of 10.28 TJ/thousand tons, while mine B produces 2.5 thousand metric tons of lignite with a net calorific value of 12.10 TJ/thousand tons. The average net calorific value of the total production of lignite of the country is calculated as a weighted average of the calorific values from the two mines with their production as weights. The calculations are shown in the example below:
| | Production (1000 metric tons) | Calorific value (TJ/1000 metric tons) | Average calorific value (TJ/1000 metric tons) | Production (TJ) |
| --- | --- | --- | --- | --- |
| Mine A | 1.5 | 10.28 | | |
| Mine B | 2.5 | 12.1 | | |
| Total | 4 | | = (1.5 × 10.28 + 2.5 × 12.10) / (1.5 + 2.5) = 11.42 | = 11.42 × 4 = 45.67 |
4.43. The average calorific value calculated as above corresponds to the country-specific calorific values that are generally collected by international organizations in their energy questionnaires and are reported in the disseminated data.
4.44. Since calorific values may change according to the type of flow (e.g., production, imports, exports, consumption by different types of users, etc.), countries are encouraged to collect calorific values at least on production, imports and exports.
### 4. Default calorific values
4.45. The default calorific values are provided in table 4.1 as a reference for countries when no specific calorific values are available. The default calorific values presented below are those used in the 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories (IPCC 2006). For a number of products, no calorific values are available in the 2006 IPCC Guidelines and thus no value is reported in the table below.
**Table 4.1 — Default net calorific values for energy products**
| Section/Division/Group | Class | Title | Default value | Lower value | Upper value |
| --- | --- | --- | --- | --- | --- |
| 0 | | Coal | | | |
| 01 | | Hard coal | | | |
| 011 | 0110 | Anthracite | 26.7 | 21.6 | 32.2 |
| 012 | | Bituminous coal | | | |
| | 0121 | Coking coal | 28.2 | 24.0 | 31.0 |
| | 0129 | Other bituminous coal | 25.8 | 19.9 | 30.5 |
| 02 | | Brown coal | | | |
| 021 | 0210 | Sub-bituminous coal | 18.9 | 11.5 | 26.0 |
| 022 | 0220 | Lignite | 11.9 | 5.5 | 21.6 |
| 03 | | Coal products | | | |
| 031 | | Coal coke | | | |
| | 0311 | Coke oven coke | 28.2 | 25.1 | 30.2 |
| | 0312 | Gas coke | 28.2 | 25.1 | 30.2 |
| | 0313 | Coke breeze | | | |
| | 0314 | Semi cokes | 28.2 | 25.1 | 30.2 |
| 032 | 0320 | Patent fuel | 20.7 | 15.1 | 32.0 |
| 033 | 0330 | Brown coal briquettes (BKB) | 20.7 | 15.1 | 32.0 |
| 034 | 0340 | Coal tar | 28.0 | 14.1 | 55.0 |
| 035 | 0350 | Coke oven gas | 38.7 | 19.6 | 77.0 |
| 036 | 0360 | Gas works gas (and other manuf. gases for distribution) | 38.7 | 19.6 | 77.0 |
| 037 | | Recovered gases | | | |
| | 0371 | Blast furnace gas | 2.47 | 1.20 | 5.00 |
| | 0372 | Basic oxygen steel furnace gas | 7.06 | 3.80 | 15.00 |
| | 0379 | Other recovered gases | | | |
| 039 | 0390 | Other coal products | | | |
| 1 | | Peat and peat products | | | |
| 11 | | Peat | | | |
| 111 | 1110 | Sod peat | 9.76 | 7.80 | 12.5 |
| 112 | 1120 | Milled peat | 9.76 | 7.80 | 12.5 |
| 12 | | Peat products | | | |
| 121 | 1210 | Peat briquettes | 9.76 | 7.80 | 12.5 |
| 129 | 1290 | Other peat products | 9.76 | 7.80 | 12.5 |
**Table 4.1 — Default net calorific values for energy products (continued)**
| Section/Division/Group | Class | Title | Default value | Lower value | Upper value |
| --- | --- | --- | --- | --- | --- |
| 2 | | Oil shale/oil sands | | | |
| 20 | | Oil shale/oil sands | | | |
| 200 | 2000 | Oil shale/oil sands | 8.9 | 7.1 | 11.1 |
| 3 | | Natural gas | | | |
| 30 | | Natural gas | | | |
| 300 | 3000 | Natural gas | 48.0(a) | 46.5 | 50.4 |
| 4 | | Oil | | | |
| 41 | | Conventional crude oil | | | |
| 410 | 4100 | Conventional crude oil | 42.3 | 40.1 | 44.8 |
| 42 | | Natural gas liquids (NGL) | | | |
| 420 | 4200 | Natural gas liquids (NGL) | 44.2 | 40.9 | 46.9 |
| 43 | | Refinery feedstocks | | | |
| 430 | 4300 | Refinery feedstocks | 43.0 | 36.3 | 46.4 |
| 44 | | Additives and oxygenates | | | |
| 440 | 4400 | Additives and oxygenates | | | |
| 45 | | Other hydrocarbons | | | |
| 450 | 4500 | Other hydrocarbons | | | |
| 46 | | Oil products | | | |
| 461 | 4610 | Refinery gas | 49.5 | 47.5 | 50.6 |
| 462 | 4620 | Ethane | 46.4 | 44.9 | 48.8 |
| 463 | 4630 | Liquefied petroleum gases (LPG) | 47.3 | 44.8 | 52.2 |
| 464 | 4640 | Naphtha | 44.5 | 41.8 | 46.5 |
| 465 | | Gasolines | | | |
| | 4651 | Aviation gasoline | 44.3 | 42.5 | 44.8 |
| | 4652 | Motor gasoline | 44.3 | 42.5 | 44.8 |
| | 4653 | Gasoline-type jet fuel | 44.3 | 42.5 | 44.8 |
| 466 | | Kerosenes | | | |
| | 4661 | Kerosene-type jet fuel | 44.1 | 42.0 | 45.0 |
| | 4669 | Other kerosene | 43.8 | 42.4 | 45.2 |
| 467 | | Gas oil/diesel oil and Heavy gas oil | | | |
| | 4671 | Gas oil/diesel oil | 43.0 | 41.4 | 43.3 |
| | 4672 | Heavy gas oil | | | |
| 468 | 4680 | Fuel oil | 40.4 | 39.8 | 41.7 |
| 469 | | Other oil products | | | |
| | 4691 | White spirit and special boiling point industrial spirits | 40.2 | 33.7 | 48.2 |
| | 4692 | Lubricants | 40.2 | 33.5 | 42.3 |
| | 4693 | Paraffin waxes | 40.2 | 33.7 | 48.2 |
| | 4694 | Petroleum coke | 32.5 | 29.7 | 41.9 |
| | 4695 | Bitumen | 40.2 | 33.5 | 41.2 |
| | 4699 | Other oil products n.e.c. | 40.2 | 33.7 | 48.2 |
**Table 4.1 — Default net calorific values for energy products (continued)**
| Section/Division/Group | Class | Title | Default value | Lower value | Upper value |
| --- | --- | --- | --- | --- | --- |
| 5 | | Biofuels | | | |
| 51 | | Solid biofuels | | | |
| 511 | | Fuelwood, wood residues and by-products | 15.6 | 7.9 | 31.0 |
| | 5111 | Wood pellets | 17.3(b) | | |
| | 5119 | Other Fuelwood, wood residues and by-products | 13.9(b) | | |
| 512 | 5120 | Bagasse | | | |
| 513 | 5130 | Animal waste | | | |
| 514 | 5140 | Black liquor | 11.8 | 5.9 | 23.0 |
| 515 | 5150 | Other vegetal material and residues | | | |
| 516 | 5160 | Charcoal | 29.5 | 14.9 | 58.0 |
| 52 | | Liquid biofuels | | | |
| 521 | 5210 | Biogasoline | 26.8(c) | 13.6 | 54.0 |
| 522 | 5220 | Biodiesels | 36.8(c) | 13.6 | 54.0 |
| 523 | 5230 | Bio jet kerosene | | | |
| 529 | 5290 | Other liquid biofuels | 27.4 | 13.8 | 54.0 |
| 53 | | Biogases | | | |
| 531 | | Biogases from anaerobic fermentation | | | |
| | 5311 | Landfill gas | 50.4 | 25.4 | 100.0 |
| | 5312 | Sewage sludge gas | 50.4 | 25.4 | 100.0 |
| | 5319 | Other biogases from anaerobic fermentation | 50.4 | 25.4 | 100.0 |
| 532 | 5320 | Biogases from thermal processes | | | |
| 6 | | Waste | | | |
| 61 | | Industrial waste | | | |
| 610 | 6100 | Industrial waste | | | |
| 62 | | Municipal waste | | | |
| 620 | 6200 | Municipal waste | 11.6 / 10.0(d) | 6.8 / 7.0(d) | 18.0 / 18.0(d) |
| 7 | | Electricity | | | |
| 70 | | Electricity | | | |
| 700 | 7000 | Electricity | | | |
| 8 | | Heat | | | |
| 80 | | Heat | | | |
| 800 | 8000 | Heat | | | |
| 9 | | Nuclear fuels and other fuels n.e.c. | | | |
| 91 | | Uranium and plutonium | | | |
| 910 | | Uranium and plutonium | | | |
| | 9101 | Uranium ores | | | |
| | 9109 | Other uranium and plutonium | | | |
| 92 | | Other nuclear fuels | | | |
| 920 | 9200 | Other nuclear fuels | | | |
| 99 | | Other fuels n.e.c. | | | |
| 990 | 9900 | Other fuels n.e.c. | | | |
> **Notes to table 4.1:**
> - (a) Whereas the values provided in this table are presented in units of energy per mass, the calorific values for natural gas are often expressed in units of energy per volume. For example, United Nations (1988) provides a NCV of 39.02 GJ/thousand m3 under standard conditions for natural gas. It should be noted, however, that this number is not derived from the value presented in this table.
> - (b) Source: Austrian Energy Agency.
> - (c) Source: IEA.
> - (d) Values refer to the biomass/non-biomass fraction, respectively.
#### Fuelwood
4.46. In rural areas of many developing countries, the principal source of energy for cooking and heating is fuelwood, but statistics on fuelwood in general are poor. This is due largely to the fact that fuelwood is in great part produced by households for their own use and/or traded in the informal sector.
4.47. There is a large variety of wood species and a large variability of moisture and ash content in wood products that highly affect the calorific value of the product. Countries are therefore encouraged to identify typical fuelwood mixes and average water content and to establish country-specific conversion factors between volume and mass. Guidelines for the measurement of fuelwood and the determination of calorific values are provided below.
4.48. Fuelwood can be measured by either volume or weight. If it is measured by volume, it can be either stacked volume or solid volume. Measures of stacked fuelwood are the stere or stacked cubic metre and the cord (128 stacked cubic feet). Solid volume is obtained by the water displacement method, that is the volume of water displaced if the quantity of fuelwood were to be completely submerged. One advantage of measurement by volume is the relatively small influence of the moisture content of the wood on the measurement results. The weight of fuelwood is highly dependent on moisture content, and this is true for all biomass. The more water per unit weight, the less fuelwood. Therefore, it is important that the moisture content be accurately specified when fuelwood is measured by weight.
4.49. There are two ways of measuring moisture content (mc). They are the so-called dry basis and wet basis and are defined below:
Dry basis: mc% = [(wet weight − dry weight) / dry weight] × 100
Wet basis: mc% = [(wet weight − dry weight) / wet weight] × 100
4.50. When biomass is very wet, there is a large difference between the two moisture contents (e.g., 100 per cent mc dry basis is equivalent to 50 per cent mc wet basis), but when the biomass is air-dry the difference is small (15 per cent mc dry basis is equivalent to 13 per cent mc wet basis). It is important to state on which basis the moisture content is measured. In most, but not all cases, fuelwood moisture is measured on a dry basis.
4.51. Another important determinant of the energy content of fuelwood is ash content. While the ash content of fuelwood is generally around one per cent, some species can register an ash content of up to four per cent. This affects the energy value of the wood since the substances that form the ashes generally have no energy value. Thus wood with four per cent ash content will have three per cent less energy content than wood with one percent ash content.
4.52. The default calorific values for fuelwood (converting from mass units to energy units) are presented in table 4.2. The table shows how the calorific values vary with different moisture content of green wood, air-dried wood and oven-dried wood.
4.53. When fuelwood is collected in volume units, a conversion factor has to be used to obtain mass units. Table 4.3 shows the conversion factors for going from volume to mass units. Table 5 in annex B shows how the different moisture contents of fuelwood affect the conversion factors between cubic metres and metric tons.
**Table 4.2 — Influence of moisture content on net calorific values of standard fuelwood (wood with one per cent ash content)**
| Wood state | Dry basis | Wet basis | Kilocalories per kilogram | Btus per pound | Megajoules per kilogram |
| --- | --- | --- | --- | --- | --- |
| Green wood | 160 | 62 | 1 360 | 2 450 | 5.7 |
| Green wood | 140 | 59 | 1 530 | 2 750 | 6.4 |
| Green wood | 120 | 55 | 1 720 | 3 100 | 7.2 |
| Green wood | 100 | 50 | 1 960 | 3 530 | 8.2 |
| Green wood | 80 | 45 | 2 220 | 4 000 | 9.3 |
| Green wood | 70 | 41 | 2 390 | 4 300 | 10.0 |
| Green wood | 60 | 38 | 2 580 | 4 640 | 10.8 |
| Air-dried wood | 50(a) | 33(a) | 2 790 | 5 030 | 11.7 |
| Air-dried wood | 40 | 29 | 3 030 | 5 460 | 12.7 |
| Air-dried wood | 30 | 23 | 3 300 | 5 930 | 13.8 |
| Air-dried wood | 25(b) | 20(b) | 3 460 | 6 230 | 14.5 |
| Air-dried wood | 20 | 17 | 3 630 | 6 530 | 15.2 |
| Oven-dried wood | 15 | 13 | 3 820 | 6 880 | 16.0 |
| Oven-dried wood | 10 | 9 | 4 010 | 7 220 | 16.8 |
| Oven-dried wood | 5 | 5 | 4 230 | 7 610 | 17.7 |
| Oven-dried wood | 0 | 0 | 4 470 | 8 040 | 18.7 |
> **Notes to table 4.2:**
> - (a) Average of as-received fuelwood on cordwood basis (4-foot lengths).
> - (b) Average of logged fuelwood.
> - Source: United Nations (1987).
**Table 4.3 — Conversion table for fuelwood[^37] (wood with 25 per cent moisture content)**
| Fuelwood | Metric tons per solid cubic metre | Metric tons per cord | Stacked cubic metres (stere) per metric ton |
| --- | --- | --- | --- |
| General | 0.707 | 1.71 | 2.12 |
| Coniferous | 0.570 | 1.38 | 2.63 |
| Non-Coniferous | 0.742 | 1.79 | 2.02 |
#### Charcoal
4.54. The amount of biomass (usually fuelwood) necessary to yield a given quantity of charcoal depends mostly on three factors: density, moisture content and the means of charcoal production.
4.55. The principal factor in determining the yield of charcoal from fuelwood is the parent wood density, since the weight of charcoal can vary by a factor of 2 for equal volumes. The moisture content of the wood also has an appreciable effect on yields as the drier the wood, the greater the yield. The third determinant factor is the means of charcoal production. Charcoal is produced in earth-covered pits, oil drums, brick or steel kilns and retorts. The less sophisticated means of production generally involve loss of powdered charcoal (fines), incomplete carbonization of the fuelwood and combustion of part of the charcoal product, resulting in lower yields.
4.56. There is always an amount of powdered charcoal produced in the manufacture and transport of charcoal. If powdered charcoal undergoes briquetting, then the weight of the briquettes may be 50–100 per cent higher per given volume of un-powdered charcoal due to greater density.
4.57. The three variables that affect the energy value of charcoal are: moisture content, ash content and degree of carbonization. The average moisture content of charcoal is 5 per cent. The average ash content of wood charcoal is 4 per cent, while that of charcoal produced from woody crop residues, such as coffee shrubs, is near 20 per cent. With the assumption of complete carbonization, the average energy value of wood charcoal with 4 per cent ash content and 5 per cent moisture content is approximately 30.8 MJ/kg. The average energy value of crop residue charcoal with 20 per cent ash content and 5 per cent moisture content is 25.7 MJ/kg.
4.58. Two tables pertaining to charcoal production are provided in annex B. In particular, table 6 illustrates the effect of parent wood density and moisture content on charcoal yield. Table 7 provides conversion factors for the production of charcoal by the various kilns for selected percentages of wood moisture content. It assumes some standard hardwood as input to the process.
#### Vegetal and animal wastes
4.59. Agricultural wastes and waste products from food processing is used to replace woody biomass in fuelwood deficient areas. These waste products can be burned as fuels to fulfil heating or cooking requirements.
4.60. There are two important determinants of the energy value of non-woody plant biomass: moisture content and ash content. While the ash content of wood is generally around 1 per cent, that of crop residues can vary from 3 per cent to over 20 per cent, and this affects the energy value. Generally, the substances that form the ashes have no energy value. Thus, biomass with 20 percent ash content will have 19 percent less energy than a similar substance with 1 per cent ash content. Data for these potential sources of energy are rarely collected directly but derived from crop/waste or end-product/waste ratios. Due to this wide variability in composition in ash and moisture content of general animal and vegetal wastes across countries, it is recommended that these products be reported to international organizations in an energy unit (preferably TJ) rather than their natural units. National authorities are, in general, able to assess and determine the energy content of these wastes. Alternatively, measuring the energy content can be accomplished by measuring the heat or electricity output of transformation devices and applying standard efficiency factors.
4.61. Given the importance of the use of bagasse, the fibrous cane residue from the production of sugar from sugar cane, possible estimation procedures are outlined for this case below. Also, singling out this specific vegetal waste allows the reporting of quantities to international organizations in its natural unit (weight basis), since its composition does not allow much variation. This has been done by international organizations that treat bagasse separately from ordinary vegetal waste. Bagasse is used as a fuel mostly for the sugar industry's own energy needs (at times, excess electricity is also fed into the public grid) in many sugar-producing countries. The availability of fuel bagasse can be estimated based on either data on the input of sugar cane into sugar mills, or production data on centrifugal cane sugar.
4.62. Method (a): Studies based on experiences in Central American countries found that the yield of fuel bagasse is approximately 280 kilograms per metric ton of sugar cane processed. Assuming a 50 per cent moisture content at the time of use, 1 metric ton of bagasse yields 7.72 GJ. The energy values for bagasse corresponding to 1 metric ton of processed sugar cane are, therefore, as follows:
2.16 GJ = 0.516 Gcal = 0.074 tce = 0.051 toe
4.63. Method (b): Based on observations, the Economic Commission for Latin America and the Caribbean (ECLAC) proposed the use of 3.26 kg bagasse yield per kilogram of centrifugal sugar produced. Calorific equivalents for bagasse corresponding to the production of 1 metric ton of sugar are as follows:
25.2 GJ = 6 Gcal = 0.86 tce = 0.59 toe
4.64. Animal waste or dung is another important by-product of the agricultural sector. It can be dried and burned directly as a fuel for space heating, cooking or crop drying. When used as an input to biogas digestors, the outputs are gas for cooking, heating and lighting, and a solid residue for use as fertilizer. Another possibility is to use the animal waste as a feedstock to produce biodiesel. It can also be spread with no or minimal treatment in the fields as fertilizer. Table 8 of annex B presents various animal and vegetal wastes and indicates the approximate calorific values recoverable from them when used as fuels.
### 5. Units recommended for dissemination
4.65. No specific measurement unit is recommended for national data collection, thus allowing countries to choose the units most suitable for their circumstances. However, based on common practices, certain units are recommended for data dissemination. If necessary, countries may use other units, as long as appropriate conversion factors are provided.
4.66. For each main category of energy products, the recommended unit for dissemination is provided in table 4.4. Where there is no special mention, the unit applies to primary as well as to secondary energy products.
**Table 4.4 — Recommended units for dissemination**
| Energy products | Dimension | Unit |
| --- | --- | --- |
| Solid fossil fuels | Mass | Thousand metric tons |
| Liquid fossil fuels | Mass | Thousand metric tons |
| (Liquid) Biofuels | Mass/Volume | Thousand metric tons/ Thousand cubic metres |
| Gases | Energy | Terajoules |
| Wastes | Energy | Terajoules |
| Fuelwood | Volume/ Energy | Thousand cubic metres/ Terajoules |
| Charcoal | Mass | Thousand metric tons |
| Electricity | Energy | GWh |
| Heat | Energy | Terajoules |
| Common unit (e.g., balances) | Energy | Terajoules |
| Electricity installed capacity | Power | MW |
| Refinery capacity | Mass/time | Thousand metric tons/year |
4.67. It is recommended that countries report to international organizations both physical quantities of fuels and their country-specific (and where necessary flow-specific) calorific values. In the case of waste that is well-defined by its constitution, rather than only by the process from which it was generated, it can be assumed that there would not be great variation in specific calorific values. Therefore, data can be reported on a weight basis (thousand metric tons). Even so, the specific calorific values should be provided if they are available.
---
[^24]: A detailed description of units of measure was provided in Energy Statistics: Definitions, Units of Measure and Conversion Factors, Studies in Methods, Series F, No. 44, United Nations, New York, 1987, and in the IEA/Eurostat Energy Statistics Manual, Paris, 2004, chapter 1, section 5. The present chapter incorporates and updates material found in both these publications.
[^25]: An example of a coherent derived unit is the Newton (N): 1 N = 1 kg · m/s2.
[^26]: See IEA/Eurostat Energy Statistics Manual, section 5 chapter 1.
[^27]: With some exceptions, for example, fuelwood, which is usually sold in stacks and measured in a local volume unit, then converted to cubic metres.
[^28]: See IEA/Eurostat Energy Statistics Manual, annex 3.
[^29]: All conversion factors for pound, short ton and long ton are approximate.
[^30]: All conversion factors for gallons and barrel are approximate.
[^31]: Defined at the Fifth International Conference on the Properties of Steam (London, July 1956).
[^32]: Other reference temperatures are also encountered, which leads to different values for the gram calorie.
[^33]: °F denotes degrees Fahrenheit.
[^34]: See chapter IV, section C.
[^35]: For example, the factor to convert from m3 to TJ will be different for different types of gaseous or liquid fuels. However, the factor to convert from kWh to TJ is the same for all products.
[^36]: A number of countries are currently able to recuperate a significant part of the latent heat, and thus the use of gross calorific values may reflect more appropriately their circumstances.
[^37]: Note: Cubic metre is measured under bark at 25 per cent moisture content (dry basis). Weight includes bark. The "General" data is weighted on 20 per cent coniferous and 80 per cent non-coniferous wood. Source: Forest Products Conversion Factors for the UNECE Region, Geneva Timber and Forest Discussion Paper 49. UNECE/FAO, 2010 (www.unece.org/fileadmin/DAM/timber/publications/DP49.pdf), updated in 2015.