DESULPHURATION PLANTS FOR DIGESTER GAS-BIOGAS

Till now little attention was given to the removal of hydrogen sulphide. Higher environmental and safety demands require the removal of hydrogen sulphide out of the digester gas. The technically and economically best method is the classic removal of H2S by a cleaning compound based on iron oxide. This well-established process has essentially been improved. Meanwhile a small effort is needed to reach a high cleaning efficiency. Mode of operation The raw digester gas (or also another hydrogen sulphide containing gas) flows through a tower-type reactor. This reactor is filled with a cleaning compound based on a specific iron oxide in form of pellets. The hydrogen sulphide will be chemically bonded while passing the cleaning mass and so removed out of the gas. The cleaning compound will be naturally consumed by this process. Therefore a small amount of air is added to the raw gas in order to partially regenerate the cleaning mass and to minimise its consumption. The following chemical equations describe the process Cleaning 2 Fe(OH)3 + 3 H2S → Fe2S3 + 6 H2O ∆HR = - 63 kJ Regeneration Fe2S3 + 1,5 O2 + 3 H2O → 2 Fe(OH)3 + 3 S ∆HR = - 603 kJ By the addition of air the cleaning mass is restored to its prior condition. The cleaning efficiency will be reduced as soon as too much elementary sulphur has been accumulated on the surface of the cleaning compound pellets. This elementary sulphur can only be removed together with the cleaning mass. Only by this reason the catalytically running process cannot be perpetuated without consumption of the cleaning mass. Depending on the sulphur content in the raw gas, a high accumulation degree of sulphur can be achieved in the used cleaning mass, so that the absolute consumption of cleaning mass will be small.

DIGESTERS FOR PRODUCTION OF DIGESTER GAS-BIOGAS

DIGESTERS OUT OF REINFORCED STEEL Digester out of reinforced steel can be erected in high quality. The digesters can be economically produced in sizes up to 3000m3 by our own manufacturing. The complete equipment with mixing devices for sludge – as agitator or gas impressing – and the necessary installation of pipes from stainless steel can realised by us. The digesters are additionally protected with woven cased PE-HD-plates in the gas zone to reach a high protection in the high stressed areas.

GAS FLARES FOR DIGESTER GAS/BIOGAS

GAS FLARES FOR DIGESTER GAS/BIOGAS FUNCTION The gas flares have the task of burning digester gas if an utilisation by the combined heat and power unit or the heating is not available. The flares have to ensure that no unburned gas will be blown into the atmosphere. Thus hazards for humans and the environment are avoided. CONSTRUCTION The gas flare is composed of a flare burner and a flare substructure. It is designed according to the principle of an injection burner and consists of a nozzle, an injector with an air flow regulation, a flame protection tube, a fitting group and a flare control system. The gas flare is totally made out of stainless steel, whereas gas contacting parts are done in stainless steel AISI 316 Ti. The flare burner is made up of the flame protection tube, the injector and its supporting structure, the ignition and the flame monitoring. The flame protection tube and the injector are made out of stainless steel AISI 309. The flare substructure bears the flare burner and takes up the vertically installed fitting group. On demand it is able to provide it with a sheet-metal covering for protecting the fittings from the weather. Then a door enables a good accessibility. The fitting group consists of a magnetic or motor-driven valve (slow opening, fast closing, de-energised closed), a flame arrester and two pressure switches. The flare control system is fitted in a control cabinet out of stainless steel that is mounted on the flare substructure. The internal wiring to the fitting group, the ignition and the flame monitoring is completely done before delivery. MODE OF OPERATION Surplus gas is able to be burnt off by the flare once an adequate signal sets the flare into operation. Normally this signal is provided by the filling level indication of the gas holder. If the gas holder reaches its maximum filling level, the starting signal for the gas flare will be released. Then the flare control system initiates the ignition and opens the magnetic or motor-driven valve. The gas is now able to run through the valve and the flame arrester and is discharged with a high exit speed into the injector by the nozzle. Caused by the high speed of the gas the necessary combustion air is sucked into the injector from outside and a combustible gas/air mixture forms. Leaving the injector this mixture is ignited by an ignition electrode and burns off inside the flame protection tube. This tube provides a wind protection for a colourless burning flame. Once the gas has been ignited and a steady flame has been established the flare will burn until the filling level indication of the gas holder will send a shut-down signal. Then the magnetic or motor-driven valve will close and the burning process will be finished. An automatic restart of the gas flare will take place as soon as the starting condition, i.e. the maximum filling level of the gas holder, is fulfilled again. As an alternative to the filling level indication of the gas holder, the fluctuating gas pressure in the gas system can be used for switching the gas flare on and off. For this purpose there are two pressure switches available to adjust a maximum and minimum pressure as switch-on and shut-down value. The control cabinet contains the whole control system for the flame monitoring and the ignition of the flare. An ignition transformer feeds the ignition electrode that directly ignites the exhausting gas/air mixture at the injector. The combustion is monitored by an ionisation electrode that controls the ionisation current produced by the flame. If this ionisation current breaks down at a failure of the flame, it will be detected by the ionisation electrode and a re-ignition will be initiated by the automatic firing device. In case of a long running failure of the flame the gas supply will be interrupted and a malfunction message will be displayed.

GAS HOLDER FOR DIGESTER GAS/BIOGAS

Why do we store digester gas? The energy stored in the biogas cannot be used directly and transformed into heat and electric current at all times. For example the energy demand at night is usually lower as the one in the daytime. But, of course, the digester gas will be also generated at night and might be used to supply the higher energy demand during the day. But for that purpose storage of the gas is necessary. Also for the operation of the digester a gas holder might be necessary. The scum is often removed out of the digester by opening the scum door: Because of taking out a larger amount of liquid in this case, gas has to flow back from the gas holder into the digester to avoid damages caused by a vacuum. Limits of the digester gas storage Digester gas is an energy form with a very low energy density. For comparision: 1 m³ digester gas ≈ 0,6 l fuel oil Therefore the storage of gas produced excessively in the summer to supply the higher energy demand in the winter is not feasible for practical and economic reasons. Reasonably only the production of one day will be buffered. So the energy not used at night or during the weekend is able to be stored and utilized during the day. Especially for sewage plants which use the gas in a combined heat and power unit this is recommended. The produced electric energy can be used in the daytime in far better economic way.

LOW PRESSURE GAS HOLDER FOR DIGESTER GAS-BIOGAS

Function Low pressure gas holders were often inserted in municipal or industrial waste water treatment plants. This dryly working gas holder consists of a cylindrical steel tank that is built gas tight in its lower half. The ballast bearing plate consists of steel. It is conducted by a (telescope) guide tube. Ballast plate and steel tank are connected by a membrane. The smaller diameter of the membrane is attached to the ballast plate and the bigger one to the cylindrical steel wall at the middle altitude of the tank. When the gas holder will be filled, the inflowing gas raises the ballast plate. The membrane forms the ealing between ballast plate and wall. Mode of operation The operating pressure in the gas distribution system of a waste water treatment plant is given by the weight of the ballast plate of the low pressure gas holder. The sewage gas is generated without any pressure in the digester as long as it could freely exhaust. When the gas flows into a low pressure gas holder, it has to raise the ballast plate. At first the space in which the gas could flow in is consequently limited. The gas compresses until its pressure has compensated the counterforce of the ballast plate. From this point on the generated gas raises the ballast plate and the gas holder fills. During the discharging of gas out of the gas holder, the weight of the ballast plate keeps up the pressure of gas. The amount of ballast on the plate consequently determines the pressure of the discharged gas. Since the gas distribution system is a communing system, the gas pressure in the digestion and in front of the consumers is the same –discounting the inevitable pressure losses of the gas lines. Design Low pressure gas holders could be delivered in two versions: • welded steel tanks with corrosion inhibiting primer • enamelled steel tanks