Why high temperature combustion of landfill gas?
Due to the accumulation of knowledge and the tightening of environmental law, the landfill technology has increasingly become concerned with the management of trace compounds in landfill gas.
With landfill gas, contamination is through sulphur, chlorine, fluorine, halogenated hydrocarbons and heavy metals. Other trace elements such as volatile organic compounds (VOC), impose environmental loading factors.
High temperature combustion is designed to safely and completely eliminate environmental risk.
The 'clean' landfill gas
Under anaerobic conditions, methanogenic bacteria in household waste landfill sites produce gas that consists mainly of methane, carbon dioxide and water vapour.
Without a degassing plant, this gas forces its way to the surface and mixes with air or migrates through the soil to collect in cavities, hollows and shafts.
With a degassing plant, the landfill gas is recovered. The sub-ambient pressure allows air to penetrate the fill and the landfill will have a different composition.
I = natural escape of gas from the landfill
II = with an average gas extraction system and well sealed landfill surface
III = excess suction and insufficient landfill cover
The combustion of main components of landfill gas does not pose a problem as exhaust gas limit values are not exceeded even at normal combustion temperatures.
The real landfill gas
The modern landfill site contains a range of wastes, apart from the usual biodegradable wastes. Modern society contributes propellant gases, refrigerants, printed plastic packaging, metal tubes, batteries as well as everything else that belongs to today's 'modern' household.
The composition of landfill gas under different management strategies varies and typical values of the trace compounds are reported in the table below. These values are based on comprehensive measurements.
I = natural escape of gas from the landfill
II = with an average gas extraction system and well sealed landfill surface
* When depositing gypsiferous waste, the concentration of sulphur can multiply
** Most important odour components in landfill gas
*** Proved aromatic hydrocarbons in landfill gas are benzene, toluene, xylene, etc.
**** The concentrations of heavy metals, cadmium, mercury, led, zinc, depend very much on the deposited waste
The following table shows some organic polluting compounds (VOCs) usually found in landfill gas:
Classification:
Group A1: carcinogenic in humans
Group A2: carcinogenic in experiments with animals
Group B: justified suspicion of cancer causing potential
The combustion in flare stacks with open flames and temperatures of around 800 °C does not completely ensure the destruction of the VOCs. The exhaust gas will be poisonous.
Environmental specifications
In the industrialised Nations, the dangers of these processes are known. Binding limit values are laid down by the authorities and specialised institutes, for proper combustion:
- TA-Luft Technical Directive for Air Pollution Abatement (Germany)
- EA-UK Environmental Agency (United Kingdom)
- The limit values are related to a residual oxygen content 02-residual of 3 Vol. % in dry exhaust gas, applicable for burner plants
- For utilisation in gas engines and -turbines, other higher values have to be taken into consideration
*) The limit value for Dioxin/Furane in the combustion of landfill gas is based on the directive in respect of incineration plants for waste and similar combustible material 17.BImSchV (Germany). The limit value of the sum equivalence is 0,1 ng/m3 exhaust gas at 11 Vol. % O2-residual respective 0,18 ng/m3 at 3 Vol. % O2-residual
The table reflects the direction of development for setting the limit values: the latest directives (TA-Luft 2'002 and EA-UK 2'003) are the most stringent and set a standard for practically all substances found in exhaust gas.
Therefore todays practised high temperature combustion is advantageously orientated to the stringent specifications. The limit values can definitely be observed.
The definition of high temperature process for landfill gas
High temperature combustion destroys the high molecular weight compounds - including the polycyclic aromatics and other complex HC compounds.
The actual temperature achieved, the temperature distribution and the residence time in this temperature zone are all vitally important in the achievement of environmentally safe combustion process.
High temperature combustion, related to landfill gas, can be defined as:
- combustion at a minimum of 1'000 °C
- minimum 0,3 seconds retention time
- no 'cold' zones in combustion chamber
These specifications ensure that the limit values in the exhaust gases are never exceeded, even fallen short of.
Requirement for low emission flare and burner plants
In order to ensure that the limit values are maintained under all operational conditions, the physical requirement is defined as follows:
- generation and retainment of a constant combustion temperature ≥ Tcomb. 1'000°C
- sufficient retention time tret.at the defined retention time temperature, usually tret. is ≥ 0,3 s
- almoast complete combustion, resp. low emission combustion
The following construction and thermodynamic requirements must be met in order to satisfy the above specifications:
- almost adiabatical combustion: for this purpose the combustion chamber is lined with generous internal insulation
- combustion in optimal range of excess air
- optimal mixing of landfill gas and combustion air
- homogeneous temperature distribution in the combustion chamber, prevention of cold zones in the combustion process, minimum drop in temperature towards the outside wall
- internal combustion: the end of the flame must lie under the upper edge of the combustion chamber at full load, so that a sufficient burn out zone remains
- quick acting automatic regulation of the optimal gas/air mixtures in order to keep a constant ratio at volume flow and heating value fluctuations.
Under these conditions, the following illustrated combustion characteristic can be achieved.
Combustion characteristic in relation to excess air and combustion temperature
The High Temperature Combustion oxidises hydrocarbons almost entirely and destroys the high molecular weight compounds - including the polycyclic aromatics and other complex HC compounds and enables the combustion with low emission.
The following diagram depicts the characteristic relationship between excess air coefficient (Lambda ratio), combustion temperature, typical combustion products and trend of pollutants from the high temperature combustion of Landfill Gas:
Lambda ratio λ:
Lambda ratio is a number indicating the ratio of a mixture of fuel and air in the combustion process. (Lambda) is the ratio of the theoretical minimal combustion air needed by a fuel (stoichiometric combustion) to the effective amount of air put into the combustion process. From this number conclusion can be made about the combustion process, the temperatures, the formation of pollutant substances and the efficiency of the process.
From the above diagram it is clear that the minimum emission of pollutants is given in the combustion range O2-res. of 8 ..10% by vol. (Lambda = 1,6 .. 1,9).
Profile of the high temperature combustion chamber
The combustion chamber is lined with a generously dimensioned high temperature resistant insulation with an extremely low heat transition coefficiency.
Owing to the design of the burner, the flame is spaciously distributed. Accordingly, the temperature is evenly distributed over the profile section of the combustion chamber and very quickly achieved.
The following diagramme shows the temperature and retention time profile at peak and low load operation:
Retention time
The retention time is the period that the gas is held in the specified combustion temperature during combustion process, or above.
By ensuring that combustion gases are held at the specified combustion temperature for a minimum duration, it is possible to ensure that near-complete burnout will occur.
The minimum recommended retention time is 0,3 seconds at a minimum temperature of 1,000°C (sometimes 900°C). This is an indicative standard that is likely to achieve the required emission standard.
Emissions - exhaust gas measurements
Authorised Measuring Institutes in Switzerland, Germany, England, Austria and France carried out series of measurements.
The average value of the exhaust gas related to 3 Vol. % O2-residual, is shown in the following table.
The concentrations of dust, sulphur dioxide compounds, of chlorine and fluorine as well as heavy metals in exhaust gas depend on the substances in the raw gas.
By analysing the raw gas, these can be determined in the planning phase. If they are known, a prognosis of the expected emission value is recommended. Hofstetter Umwelttechnik AG have the appropriate calculation programme in order to determine if pre-treatment of the crude gas is necessary.
These exhaust gas monitoring results show that high temperature combustion, as defined above, ensures the complete combustion of landfill gas and low exhaust gas emission levels. The stringent limit values imposed by the clean air specifications can be observed, in certain cases they are even considerably lower than the permitted level.
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