| The Purification of Biogas |
Of the many processes traditionally and presently employed, that have been used for large-scale desulphurization of technical gases, only the so-called "dry" process is suitable on a smaller scale for biogas plants. They are acceptable from the point of view of technical complexity and maintenance and the degree of purification is satisfactory.
The desulphurization of biogas is based on a chemical reaction of H2S with a suitable substance.
The lime process
The oldest process is the desulphurization of gases with quick lime, slaked lime in solid form or with slaked lime in liquid form. The process using quick or slaked lime has not been applied on a large scale for a long time. The large amounts of odourous residue that are produced cannot be satisfactorily disposed of. The handling of large amounts of dissolved or suspended slaked lime requires elaborate equipment.
Large concentrations of CO2 which are present in biogas make the satisfactory removal of H2S difficult. The CO2 also reacts with the quick and slaked lime and uses it up quickly. The Ca(HCO3)2 formed reacts with Ca(SH)2 which is formed by the reaction of H2S with Ca(OH)2 thus resulting in the reoccurance of H2S. However, a large scale biogas plant in Germany with the cogeneration of heat and power has recently been constructed using a lime purifier. The results of long term tests are not yet available.
In as far as enough lump, quick lime is available in the countries concerned, this process could be considered for desulphurization. The apparatus for utilizing quick lime corresponds in construction and function to that used for the desulphurization with iron-containing substances.
Ferrous materials in the form of natural soils or certain iron ores are often employed to remove H2S.
The ferrous material is placed in a closed, gas tight container (of steel, brickwork or concrete). The gas to be purified flows through the ferrous absorbing agent from the bottom and leaves the container at the top, freed from H2S.
The absorbing material must contain iron in the form of oxides, hydrated oxides or hydroxides. These react as follows:
2 Fe(OH)3 + 3 H2S Ã Fe2S3 + 6 H2O Fe(OH)2 + H2S Ã FeS + 2H2O
This process terminates, of course, after some time. The greater part of the iron is then present as a sulphide.
However, by treating the sulphidized absorbent with atmospheric oxygen, the iron can be returned to the active oxide form required for the purification of the gas:
2 Fe2S3 + 3 O2 + 6 H2O Ã 4 Fe(OH)3 +3 S2
2 FeS + O2 + 2 H2O Ã 2 Fe(OH)2 + S2
The used absorbent can, therefore, be "regenerated". This regeneration cannot be repeated indefinitely. After a certain time the absorbent becomes coated with elementary sulphur and its pores become clogged. Purifying absorbents in gasworks (coke plants) acquire a sulphur content of up to 25% of their original weight, i.e. 40% sulphur by dry weight.
There are three different, dry desulphurizing processes available.
The purification chamber consists of a box or drum. The absorbent is placed inside it on several, intermediate trays (sieve floors) to ensure that the depth of the absorbent is not more than 20-30 cm. Otherwise the absorbent would easily press together causing an increase in the resistance to the gas flow.
The biogas is fed in at the bottom of the box, flows through the absorbent and leaves the purification chamber at the top, freed from H2S. When the absorbent becomes loaded with iron sulphides, the gas leaving the chamber contains increasingly more H2S. The chamber is then opened at the top and the trays with the spent absorbent are removed. Then fresh absorbent is placed on the trays.
After the air in the purification chamber has again been displaced with biogas, the gas connection to the user is re-opened.
The spent absorbent is disposed of as described under the heading "Disposal of spent absorbent".
The spent, sulphide containing absorbent can also be regenerated by exposing it to oxygen. This can either be done by taking the used absorbent out of the chamber and exposing it to the air, or inside the purification chamber by simply sucking ambient air through it.
Since regeneration inside the chamber requires precautions against the formation of unwanted and dangerous air-gas mixtures and would require powerful fans, regeneration outside the chamber is usually preferred. The absorbent that is to be regenerated, is spread out on the ground in as thin a layer as possible. From time to time it is turned over with a shovel. After a few days it is ready for use again.
This regeneration process can be repeated up to ten times, after which the absorbent is finally spent.
Simultaneous regeneration and loading
Simultaneous regeneration and loading of the absorbent is a special case. Here a certain, small amount of air is added to the biogas. Then sulphide formation and regeneration occur at the same time and place. As such, the absorbent acts effectively as a catalyst.
Expensive gas-measuring and mixing equipment is required for this process, however, so that it is not suitable for small biogas plants.