What waste calorific value measures
Waste calorific value is the heat released when a unit mass of the waste is burned completely, expressed in MJ/kg or in kcal/kg. Two figures circulate for every fuel and every waste. The higher heating value counts all of the heat released, including the latent heat of the water vapour that the combustion produces. The lower heating value subtracts that latent heat, and it is the one that matters here, because the flue gas leaves an incinerator above the dew point of its water and the latent heat is never recovered.
The other distinction to settle before any figure is used is the basis. A value quoted on a dry basis describes the combustible material in the waste; a value quoted as received describes the waste that arrives at the gate, water and inerts included, and it is the as-received figure the furnace actually sees. Mixing the two is the commonest error in a heat balance.
For a single chemical or a sorted plastic, calorific value is a number. For municipal refuse, clinical waste or a tank of industrial sludge it is a distribution, and the figure to design on is the low end of the range the feed will actually present rather than its average.
Waste calorific value for incinerator sizing: the heat balance
The heat balance of a combustion chamber is short, and everything else follows from it. Heat in equals throughput multiplied by the lower heating value of the feed. Heat out goes four ways: into heating and evaporating the water in the feed; into heating the flue gas, the largest term by far, whose mass is set by the excess air; into losses through the walls, the doors and the ash; and into holding the chamber and its refractory at the temperature the process needs.
The consequence is that the chamber temperature is an outcome, not a dial. Excess air is the most powerful lever available: air that is not needed for combustion still has to be heated from ambient to chamber temperature by the same heat release, so surplus air dilutes the flame and lowers the temperature, while scarce air leaves carbon monoxide and unburnt material in the gas. That is what the oxygen trim on the combustion control is adjusting.
Water is the second lever, and on wet feeds the dominant one. Every kilogram of water in the feed consumes heat to evaporate, and that heat comes out of the same budget as the chamber temperature. A feed that looks acceptable on a dry analysis can be a cold, smoky burn as received, which is why the moisture content is the first figure to check and the easiest to change.
The calorific value of the common waste streams
The bands below are the ones usually quoted in design guidance, and they should be read as a starting method rather than as a specification: they describe classes of waste, not your waste, and they are wide because moisture and inert content move the figure more than composition does.
- Municipal solid waste calorific value is commonly quoted from roughly 5 to roughly 12 MJ/kg as received, with packaging-rich refuse at the top and food-heavy or monsoon-season refuse at the bottom. Design figures are usually taken below the annual mean.
- Clinical and medical waste is plastic- and textile-rich, so it sits comfortably above household refuse, and a unit rated for it is a different machine from one rated for refuse at the same throughput.
- Sludge incineration calorific value has to be calculated rather than looked up. A dewatered cake is mostly water, and at 20 % dry solids almost the whole heat release goes into evaporating the rest, so the arithmetic belongs on the actual cake rather than on a band.
- Plastics, rubber, oils and solvents sit high, and a stream dominated by them will overheat a furnace sized for mixed refuse rather than underheat it.
Incinerator auxiliary fuel consumption: when the feed cannot hold the temperature
A combustion chamber has to reach its working temperature and stay there. Below a certain calorific value the waste cannot do that unaided, and the plant needs a burner running continuously rather than only at start-up. In municipal-waste practice a design rule of thumb places that threshold in the region of 7 MJ/kg as received; it is a rule of thumb rather than a standard, because the real figure moves with the excess air, the combustion air temperature and the heat losses of the chamber.
A support burner is a running cost, not an emergency provision, and it belongs in the operating budget from the first estimate. Incinerator auxiliary fuel consumption follows from the gap between the heat the feed supplies and the heat the chamber needs, and it rises in wet weather, at low load, and whenever the excess air is set high.
Where the feed is short of heat there are four responses, usually combined: reduce the water, by dewatering, draining or covering the storage bunker; preheat the combustion air; cut the excess air to the lowest value that still gives complete combustion; or accept the auxiliary fuel and size the burner and the fuel store honestly.
Furnace volume: why the same kilograms per hour are not the same duty
The chamber has to hold the gas at the required temperature for the residence time the standard sets — 2.0 seconds for medical and hazardous waste, 2 seconds for municipal waste — and that requirement is a statement about volume against volumetric gas flow. The gas flow comes from the combustion air, which comes from the oxygen the feed needs plus whatever excess is being run.
Two wastes burned at the same kilograms per hour therefore do not produce the same furnace. A high-calorific-value feed releases more heat in the same volume of chamber, so the volumetric heat release rate rises, and the refractory and the ash-fusion behaviour of the waste set a ceiling on it. Where that ceiling would be exceeded, the design either enlarges the chamber, dilutes the feed, or adds a heat sink such as a water wall or a waste-heat boiler. A low-calorific-value feed has the opposite problem: the chamber is easy to keep cool and hard to keep hot.
This is why a capacity figure in kilograms per hour is meaningless without the waste it was rated on. The honest way to compare two offers is to ask for the design calorific value, the moisture content, the excess air and the resulting chamber volume behind each figure. The companion article on how to size a medical waste incinerator takes the same reasoning into a daily throughput and a cycle count.
Measuring the calorific value of your own waste
A band from a table is where the work starts, not where it ends, because the figure that sizes the furnace is a property of the waste the site generates. Three routes get there, in increasing order of effort.
- Sample the waste as received, over a period long enough to cover the variation, in proportion to the mass each sample represents. For a heterogeneous stream this is the hard part: a single grab sample from one skip is worth very little.
- Have the samples analysed. A proximate analysis gives moisture, volatile matter, ash and fixed carbon; an ultimate analysis gives carbon, hydrogen, nitrogen, sulphur, oxygen and chlorine; a bomb calorimeter gives the higher heating value, from which the lower heating value follows.
- Do an energy balance on the plant itself. Weigh the waste and the auxiliary fuel over a period of normal running, log the chamber temperature and the excess air, and back-calculate the effective calorific value the fuel consumption implies. This needs no laboratory and uses the feed as it really arrives.
Low calorific value waste incineration: the mistakes that follow from a wrong figure
- Sizing on a dry-basis or higher-heating-value figure. The furnace sees the waste as received, water and all, and the error is always in the optimistic direction.
- Using an annual average. The plant has to run on the wet week as well, and a furnace sized on the mean is under-sized and over-fuelled when it matters most.
- Buying a capacity figure in kilograms per hour without asking what waste it was rated on. Two offers can quote the same number and describe different machines.
- Treating the auxiliary burner as emergency equipment. On a low-calorific-value feed it is part of the normal operating condition.
- Raising the excess air to chase a clean stack. Extra air lowers the chamber temperature and increases the gas volume the chamber and the gas train have to handle.
- Ignoring moisture in the storage. A bunker open to rain turns a feed that was marginal on paper into one that cannot hold temperature.
Self-check before a furnace is sized
- Is the figure a lower heating value, on an as-received basis, measured on waste from this site over a period that covers the variation?
- Is the moisture content known, and is it known how much it moves across the year?
- Is the low end of the range recorded, and is the furnace sized against it rather than against the mean?
- Has the heat balance been done, including the excess air the plant will actually run?
- Is the auxiliary fuel duty stated as a continuous duty where the feed needs one, with the fuel consumption and the storage that go with it?
- Is the chamber volume checked against the residence time at the highest gas flow the plant will see?
- Is the capacity figure in the offer qualified by the waste it was rated on?
From a waste analysis to a furnace size
A furnace size is the output of a heat balance, and the heat balance is only as good as the calorific value and the moisture content that go into it. Those two figures come from the waste the site generates, not from a general table, and everything else — the chamber volume, the auxiliary burner, the throughput the unit will really take, the gas train behind it — follows. The chart of incinerator emissions sets out the temperature and residence time the chamber has to hold, and the incinerator pages describe the units the chambers are built into. Send us the waste analysis and the daily tonnage, and the chamber volume, the burner duty and the capacity figure for your waste are confirmed with the factory for your waste analysis rather than read off a table.