DESULPHURISATION UNITS FOR BIOGAS

The removal of hydrogen sulfide (H₂S) from biogas has so far received little attention. The increased requirement for environmental protection and the improvement of the reliability of the operated biogas fuels involves the removal of hydrogen sulfide from the biogas. Adsorbers with activated carbon or iron hydroxide can be the optimal solution.

THE PROCESS

For the removal of hydrogen sulfide from the biogas activated carbon and iron hydroxide are very suitable as absorbents. Both activated carbon and iron hydroxide have a high capacity for hydrogen sulfide and can guarantee a high final purity of the gas.

The adsorber is designed as a continuous system. The activated carbon is in a cylindrical container. The activated carbon pellets or pelleted iron hydroxide are flowable. The gas is introduced into the lower part of the filter and flows from the bottom upward through the cylinder. The activated carbon is fed into the cylinder from the top by a butterfly valve. An appropriate subset of activated carbon can then be easily removed on the flap at the bottom of the container. This has several advantages:

• The activated carbon inside the adsorber can be used more efficiently, i.e. the consumption of activated carbon decreases. The material flows slowly through the tank. The activated carbon (or the iron hydroxide) situated at the bottom of the cylinder is responsible for the primary purification of the gas. The carbon freshly inserted from above is responsible for the precision purification. In this way, the quality of the purification can be controlled and the carbon is steadily filled into the cylinder.
• The replacement of the carbon does not need to be affected completely. Depending on the gas quality just, a little subset is discharged and replaced by fresh activated carbon. The work expended is thus low and is not a strain.

REGENERATION AIR

By admixing a low quantity of oxygen (air) the activated carbon can be loaded much higher as the carbon is impregnated or doped with a special addition agent. The parallel processed regeneration is reached by an addition of air of approximately 1 – 3 vol. % (depending on the H2S content₂S-content of the gas) to the crude gas. Elemental sulfur is developed by the reaction of hydrogen sulfide with oxygen and remains in the activated carbon.

While the binding of the hydrogen sulfide is carried out by adsorption in the activated carbon and regeneration directly according to the equation

H₂S + ½ O₂ → S + H₂O

occurs, run the cleaning step and the regeneration of the iron hydroxide by the following chemical reactions:

Cleaning

2 Fe(OH)₃  +  3 H₂S    →    Fe₂S₃    +  6  H₂O   ∆  H_R =    –  63 kJ (1)

Regeneration

Fe₂S₃   +   1,5  O₂  +  3 H₂O  →   2 Fe(OH)₃  + 3 S      ∆  H_R   =  ‑ 603 kJ (2)

In practice, however, no difference in the operation is found.

The parallel proceeded regeneration is obtained by the addition of approximately 1 –3 % by volume air (depending on the H₂S content of the gas) to the raw gas. A signal is taken from the gas volume-measuring device to adjust the admixture quantity. A dosing unit adds the regeneration air according to the actual flowing gas quantity.

The dosing installation for regeneration air includes:

• Control unit for the air admixture with non-return valve and magnetic valve
• Temperature monitoring of the pure gas
• Measuring the charged air quantity inflow
• Display for volume flow, analogue
• Potential-free output for notice of the malfunction

FUNCTION OF THE AIR ADMIXTURE DEVICE

The gas quantity flowing through the desulphurisation unit is measured by the gas quantity measuring device. In a proportional way, a certain air quality of approximately 1 – 3 vol. % is admixed via a speed-controlled side channel blower. This air quantity is measured as well. The signal as well as the gas quantity is sent to the control device. This control device compares the gas quantity and the air quantity and keeps a constant gas/ air quantity ratio. The air admixture process is stopped as soon as an exceedance of a predetermined limit value occurs.

A shutdown of the air admixture process is also affected if the gas temperature at the outlet of the desulphurisation unit is higher than 60 °C.

DESULPHURISATION UNITS FOR BIOGAS

Example Unit

Activated Carbon Adsorber For Biogas Plant

General Information

Hydrogen sulphide content

Crude gas, max.	:	1000 ppm
Cleaned gas	:	< 10 ppm
Maximum allowable operating pressure	:	100 mbar
Permissible temperature range	:	5 – 35 oC
Permissible relative gas humidity	:	< 70 %
Pressure loss	:	app. 10 mbar

Adsorption tank

Material	:	1.4571
Feed hopper for activated carbon	:	DN 200
Butterfly Valve	:	DN 200
Butterfly valve for removal	:	DN 150
Option
Heat insulation	:	100 mm mineral wool
Panelling	:	Aluminium sheet

Regeneration air admixture

Purpose	:	provision of regeneration air for optimal utilisation of the activated charcoal
Air Compressor
– Design type	:	side channel blower
– Volume flow air	:	max. 10 m³/h
– Pressure	:	max. 120 mbar
Air Flow Meter	:	variable area flow meter
– Measuring range	:	0.9 – 9m³/h
Flow and temperature monitors	:	1
Mechanical non return valve	:	½”
Solenoid valve	:	½”
Ball valve	:	½”
Control unit	:	400 x 400 x 210 mm

Size Chart

ADSORBER FOR ACTIVATED CHARCOAL AND IRON HYDROXIDE

Size		m³/h	120	200	280	420	500
Height unit	Hges	mm	1.820	1.870	1.920	1.970	1.990
Diameter unit	ØZyl	mm	960	1.200	1.500	1.750	1.900
Weight unit – empty	mL	kg	350	460	570	690	770
Gas nozzle inlet	DN	DN	80	100	125	150	150
Gas nozzle outlet	DN	DN	80	100	125	150	150
Feeding nozzle for pellets	DN	DN	150	150	150	150	150
Removal nozzle for pellets	DN	DN	150	150	150	150	150

ADSORPTION AGENTS FOR ADSORBER

Activated carbon – doped

Raw material base	:	Charcoal
Properties		Doped activated carbon with alkali and alkaline earth Fully open pore system
Spent activated carbon	:	Use as a fertilizer additive possible
Diameter pellets	:	4 mm
Water content	:	< 10 %
Bulk density	:	455 ± 20 kg/m³
BET surface	:	> 850 m²/g
Micropore volume	:	> 0.35 ml/g

ALTERNATIVE

Raw material base	:	Iron hydroxide
Properties		Pellets
Diameter pellets	:	2 – 8 mm
Length	:	2 – max. 20 mm
Active substance	:	Iron(III)-oxidhydrate
Iron content (Fe3+)	:	ca. 20 % based on the dry matter
Dry matter content	:	min. 80 %
Bulk density	:	570 ± 80 kg/m³ (loose bed), app. 85 % TS 610 ± 100 kg/m³ (compacted bed), app. 85 % TS

Initial Filling Chart for Celsigas Adsorber

REQUIRED INITIAL FILLING

SIZE	m³/h	120	200	280	420	500
Activated charcoal	kg	340	530	820	1,110	1,310
Iron hydroxide	kg	420	650	1,000	1,380	1,620