Hofstetter BV

Performance of the low-emission flare


Courtesy of Courtesy of Hofstetter BV

Job report / Elbisgraben (Bale / Switzerland)

The summer of 1992 had finally arrived. The 5MW low emission furnace in the district heating station Liestal was ignited, supplied with landfill gas from Elbisgraben landfill.

Since then the plant has been running continuously and the comprehensive measurements taken, verify that all targets aimed at have been achieved: plant efficiency 90%, low NOX, almost pollution free flue gas, no dioxin and furane.

Something to the previous history
Since 1983, the Elbisgraben landfill has been taking non-hazardous commercial and industrial waste from the county of Bale in Switzerland, some 900'000 tons of waste have already been deposited todate.

The gas production of the first phase was burnt off with Hofstetter mobile flares.

After extensive preliminary investigations and studies, the landfill gas utilization project Elbisgraben/ Liestal passed through the county's political authorities and the appropriate municipal councils. In 1991 the constructional work was started; beginning 1992 the assembly of the Hofstetter equipment could begin.

The gas utilization project Elbisgraben

Special project requirements
Location: The Elbisgraben site is situated on the outskirts of a very built-up area, in the middle of the Liestal region recreation area.
Structure of the landfill: Household waste, sewage sludge and non-hazardous commercial / industrial waste, deposited fairly well separated. 120'000 t yearly, up todate 900'000 t. The gas collection system is laid horizontally in layers.
Clean air plan of the County of Bale: This, compared to the Swiss Clean Air requirements, contains more stringent values. As the landfill gas also contains halogenated organic compounds, the authorities requirements are particularly strict.

  • high temperature combustion
  • retention time of the flue gas
  • very low limit values for harmful substances in flue gas
  • low NOX
  • maximum utilization rate

Public interest: Due to the special circumstances surrounding this project, the media and politicians took a very lively interest from the start.

In particular the critical location, the stringent clean air specifications and the lively, public, political interest, gave the chance to exhaust the technical possibilities of the project in respect of:

  • complete utilization
  • flue gas quality and
  • safety

in almost unique dimensions.

The gas utilization Elbisgraben can therefore be looked upon as an example of a project with the unmistakable priority 'environmental protection/utilization/safety.'

Location of the utilization plant

For reasons of area planning and technical utilization as well as other factors, the district heating station Liestal as utilization location was practically a must.

This gave the project considerable planning data.

The process station Elbisgraben
Location: At a higher level than the landfill, which makes the dewatering of the gas easier.
Distinction between good and poor gas: Due to the structure of the landfill with larger/smaller gas production ranges, the possibility of separated collection and degassing is given.
Gas dewatering: This is done by 2 cyclone separators with continuous removal of the condensate.
Gas pumping: 2 rotary blowers (A + B) with a total pressure difference of p 600 mbar, r.p.m. controlled by frequency transformer, maximum output 1800 Nm3/h.
Flare station: 2 low emission flares EGH-06, dimensioned differently for optimization of the regulating range A: 500 Nm3/h; B: 1000 Nm3/h
Measuring and analysis: The considerable distance between landfill and utilization point, required the set-up of two autonomic measuring and analysing stations.

Transport pipeline Elbisgraben - heating station Liestal
A transport pipeline 2'500 meters long had to be laid out for the transport of the landfill gas from the process station to the heating station Liestal. Up to 1'800 Nm3/h of gas has to be conveyed through the HDPE plastic pipe with a diameter of DN 300 to the heating station.

In view of the fact that the compressed gas cools and water vapour condenses during the conveyance through the underground pipeline, condensate tanks with automatic discharge of the condensate into the drains, are mounted at the deepest points of the transport pipeline. Both plants are built in an accessible shaft construction and are maintained weekly by the landfill operating personnel. Moreover, in the vicinity of the ground water current called 'Ergolz', the transport pipeline passes through an additional protection pipe.

Landfill gas utilization plant in heating station Liestal

Landfill gas delivery station
The district heating station Liestal lies near the centre of the town. On this account, special safety precautions were necessary.

The function of the underground constructed delivery station outside of the district heating station, is to control and process the landfill gas for utilization.

For control, the gas is analysed in respect of the main components and condition, i.e. CH4 and O2 content, temperature, pressure, volume. Inadmissible situations are thereby percepted; faults can be immediately acted upon: leak checks, shut down of the gas supply, etc.

For processing, the landfill gas is heated up to around 30°C, in order to prevent the precipitation of remaining condensate in the sensitive regulating and safety fittings of the gas burner train.

The Hofstetter low emission combustion chamber

Installation in existing heating station operation
For the landfill gas utilization, a former steam boiler had to be dismantled and replaced by the new 5MW low emission combustion chamber incl. waste heat steam boiler, flue gas recycling, economizer, control, etc. Of course the complete conversion had to take place without interrupting operation.

Dual-fuel combustion
The project demanded that the burner plant is capable of burning landfill and natural gas at the same time, in order to uphold the required heat production even at fluctuating landfill gas supply. The supply, control, regulating and mixing of the fuels is executed by a sophisticated control system. The combustion takes place at a regulated temperature of 1200 and a retention time of minimum 0,3s.

Flue gas recirculation
The feedback of flue gas serves to hold the allowed combustion temperature at every gas flow and at the same time ensure maximum efficiency, complete combustion and lowest NOX value.

In view of the emphatic request for an optimum utilization of the energy (= high thermal degree of efficiency) it was important to reduce the loss of energy by further reducing the flue gas temperature to 140 C. This was achieved by installing after the steam boiler, an economizer to preheat the feed water.

Control and regulation
The optimum combination of all the gas utilization components and their complete process, demanded an extremely efficient control/regulation. On the basis of the well-tried Hofstetter PLC, this requirement could be solved with excellent results. The chosen PLC conception has therefore proven its extraordinary flexibility and expansibility.

Measuring results / official acceptance
The plant operator entrusted the measuring laboratory of Ciba-Geigy AG with the execution of this comprehensive exacting measuring.
The tests were carried out in two phases:

Phase 1:

  • continuous analysis of CO, NOX, SO2 and C org. (unburnt hydrocarbons)
  • measuring by upgrading of dust, ammonia, halogenide

Phase 2:

  • measuring of the dioxin and furane content

With results that were way under the required limit values throughout, this plant from Hofstetter had once again succeeded in passing the official acceptance with honours.

The measuring results:

Average composition of the landfill gases

Flue gas analysis of district heating station Liestal

Measuring values are related to normal conditions, dry and 3.0 Vol. % oxygen

Sum equivalence TEQ related to 11 Vol. % residual oxygen (according 17.BImSch V)

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