Calorific Values

A calorific value (or heating value) expresses the heat obtained from one unit of a fuel. Calorific values are conversion factors: they convert an original mass or volume quantity into energy content, which is what makes the compilation of overall energy balances possible in the first place. Although most often discussed in the context of energy balances, they apply more broadly to any table that aggregates energy across products, or that compares products against each other [IRES, Ch. IV, para. 4.29, PDF p. 58, 2018]. See Measurement Units for the original/common unit distinction that calorific values bridge.

Calorific values are obtained by laboratory measurement, preferably expressed in joules (or a multiple) per original unit — e.g. gigajoule per metric ton (GJ/t) or gigajoule per cubic metre (GJ/m³). Major fuel producers (mining companies, refineries, etc.) normally measure the calorific value and other qualities of the fuels they produce [IRES, Ch. IV, para. 4.30, PDF p. 58, 2018].

Two independent issues recur whenever a compiler works with calorific values: whether the value is measured gross or net of latent heat, and whether it is a specific value (measured for the actual product/flow/ country) or a default value used as a fallback [IRES, Ch. IV, para. 4.31, PDF p. 58, 2018]. Both are covered below.

Gross and net calorific/heating values

The gross calorific value (GCV), or high heat value, measures the total (maximum) heat produced by combustion. Part of this heat, however, is locked up in the latent heat of evaporation of water present in the fuel before combustion (moisture) or generated during combustion — the latter from hydrogen in the fuel combining with atmospheric oxygen to form H₂O, itself a heat-releasing reaction whose heat is partly reabsorbed by evaporating the water it creates [IRES, Ch. IV, para. 4.32, PDF pp. 58–59, 2018].

The net calorific value (NCV), or low heat value, excludes that latent heat: it is the heat actually available in practice for capture and use. The higher a fuel’s moisture or hydrogen content, the larger the gap between GCV and NCV. For fuels with very little or no hydrogen (some cokes, blast furnace gas) the difference is negligible; for fossil fuels generally (coal, oil, oil products, gas) it is typically under 10 per cent, while for biomass energy (fuelwood, bagasse) it is usually over 10 per cent. The combustion technology used can also affect the realized NCV, depending how much latent heat it recovers from exhaust gases [IRES, Ch. IV, para. 4.33, PDF p. 59, 2018].

Annex B, Table 4 — Difference between net and gross calorific values for selected fuels, as a percentage of the gross value:

Fuel	Percentage
Coke	0
Charcoal	0-4
Anthracite	2-3
Bituminous coals	3-5
Sub-Bituminous coals	5-7
Lignite	9-10
Crude oil	5-8
Petroleum products	3-9
Natural gas	9-10
Liquefied natural gas	7-10
Gasworks gas	8-10
Coke-oven gas	10-11
Bagasse (50 per cent moisture content)	21-22
Fuelwood (10 per cent moisture content)	11-12
Fuelwood (20 per cent moisture content)	22-23
Fuelwood (30 per cent moisture content)	34-35
Fuelwood (40 per cent moisture content)	45-46

This table gives the general pattern behind the sentence above: negligible for fuels with little or no hydrogen (coke), a few per cent for most fossil fuels, and much larger (over 20%) for biomass fuels with high moisture content. Source: United Nations (1987) [IRES, Annex B, Table 4, PDF p. 163, 2018].

It is recommended that NCVs be used in preference to GCVs when expressing energy content in a common energy unit — the heat needed to evaporate moisture (always present in fuels, and also produced during combustion) should not be counted as part of a fuel’s energy-providing capability. NCVs are particularly preferred when building an energy balance, since most current technologies cannot recover the latent heat — IRES points to Chapter VIII for further discussion (see Energy Balance). However, where available, it is strongly encouraged to report both gross and net calorific values [IRES, Ch. IV, para. 4.34, footnote 36, PDF p. 59, 2018].

Default vs. specific calorific values

Energy products with exactly the same chemical composition carry the same energy content, but in practice composition varies: “premium” and “regular” gasoline may differ slightly in formulation; natural gas varies in its ethane/methane proportions; LPG may be pure propane, pure butane, or any mix of the two. Only single energy compounds — “pure” methane or ethane, and electricity — have precise, unalterable energy contents [IRES, Ch. IV, para. 4.35, PDF p. 59, 2018].

Default calorific values are generally applicable energy contents for fuels of specified characteristics, used when specific values are unavailable. Specific calorific values, by contrast, are measured from the actual data source and reflect the specificity of the product in question — particularly important for products like coal that span a wide quality range. Where many specific values are in use, care is needed to keep the energy content consistent between the supply side and the consumption side for a given country and year [IRES, Ch. IV, para. 4.36, PDF p. 59, 2018].

A further complication: the product at one stage of the energy chain may not be chemically identical to the product carrying the same name at a later stage. Natural gas may be enriched with oil products to meet market specifications; motor gasoline may be blended with ethanol and, depending on national practice, recorded either as motor gasoline alone or as motor gasoline plus the blending agent. Flow-specific calorific values allow for a more accurate balance in cases like these [IRES, Ch. IV, para. 4.37, PDF p. 59, 2018].

It is recommended that countries collect data in original units together with data on specific calorific values. A country-specific calorific value is generally a weighted average of all the calorific values collected for the product (see the calculation method below). For some products (e.g. coal, crude oil), different calorific values may be needed for production, imports, exports and major uses. Default values should be used only as a last resort, in the absence of specific values, with the understanding that doing so trades away precision in the published figures [IRES, Ch. IV, para. 4.38, PDF pp. 59–60, 2018].

It is further recommended that metadata be provided on the methods used in all calculations and conversions applied to arrive at disseminated data, to ensure transparency, clarity and comparability — in particular, the conversion factors between original and presented units, whether they are on a gross or net calorific basis, and any use of default values [IRES, Ch. IV, para. 4.39, PDF p. 60, 2018].

How to calculate average calorific values

Calculating a calorific value has two distinct levels. The first — laboratory measurement of a fuel’s heating value — is done by specialist laboratories and is normally handled by major fuel producers (mining companies, refineries, etc.) as part of assessing price and specification; this level of calculation is outside IRES’s scope, which takes calorific values as given by data providers [IRES, Ch. IV, para. 4.40, PDF p. 60, 2018].

The second level is the compiler’s task: aggregating the different qualities of a fuel into a single average calorific value. Coal from different mines, imported coal from different origins, and coal for different end uses (e.g. imported steam coal for electricity generation vs. home-produced lignite for households) can all carry different qualities, so preparing an energy balance or comparing energy content across products requires accounting for this variation [IRES, Ch. IV, para. 4.41, PDF p. 60, 2018].

Worked example — production of lignite from two mines in a country: mine A produces 1.5 thousand metric tons at a net calorific value of 10.28 TJ/1000 t; mine B produces 2.5 thousand metric tons at 12.10 TJ/1000 t. The country’s average net calorific value for lignite is the production- weighted average of the two mines’ values:

	Production (1000 t)	Calorific value (TJ/1000 t)	Average calorific value (TJ/1000 t)	Production (TJ)
Mine A	1.5	10.28
Mine B	2.5	12.10
Total	4		(1.5×10.28 + 2.5×12.10) / (1.5+2.5) = 11.42	11.42 × 4 = 45.67

[IRES, Ch. IV, para. 4.42, PDF p. 60, 2018]

This weighted-average calorific value is the country-specific calorific value generally collected by international organizations in their energy questionnaires and carried into disseminated data [IRES, Ch. IV, para. 4.43, PDF p. 60, 2018].

Because calorific values can vary by flow (production, imports, exports, consumption by different user types, etc.), countries are encouraged to collect calorific values at least on production, imports and exports [IRES, Ch. IV, para. 4.44, PDF p. 60, 2018].

Related

• Measurement Units — original and common units, and the joule-as-common-unit recommendation that calorific values feed into
• Default Calorific Values — Table 4.1’s reference default values, plus fuelwood, charcoal and bagasse-specific determination methods that apply the default-vs-specific and weighted-average methods covered here
• Units Recommended for Dissemination — Table 4.4 and the recommendation to report both physical quantities and country/ flow-specific calorific values
• IRES Chapter IV — Measurement Units and Conversion Factors — the chapter digest this page expands on
• IRES Annexes and References — Annex B, Table 4, the source of the gross/net calorific-value difference table above
• SIEC Classification System — the product classification whose GCV thresholds (coal, para 3.19 ff.) are an early, informal use of the gross/net distinction formalized in this chapter
• Coal — hard/brown coal are distinguished by GCV threshold as part of their SIEC definitions
• Oil — API gravity, a related density-based classification measure (see Measurement Units)
• Biofuels — fuelwood, charcoal and bagasse, whose default calorific values and biomass-specific determination methods are covered on Default Calorific Values