Biogas

The use of activated carbon in biogas upgrading

There is a global consensus that reliance on fossil fuels in energy production is no longer viable, with great interest in renewable sources as the future of energy production. Of such interest is the use of waste materials to generate energy, this simultaneously provides power and deals with the issue of removing waste. The production of biogas is one favourable option as it utilises existing infrastructure in transporting gas (can use the same pipes that transport natural gas/injection into the gas grid). Biogas is mixture of gases, mainly compromised of methane (CH₄). It is produced under anaerobic conditions, i.e. the absence of oxygen, by anaerobic bacteria. The feed material must be biodegradable (wood is exempt). Anaerobic conditions are vital for the anaerobic bacteria to survive, digest and ferment this waste into useful CH₄, this is called anaerobic digestion (AD). The table below shows the percentage composition of biogas:

Table 1: Composition of biogas species and their effects

From the table above it is clear that some of these species need to be removed to prevent problems when using biogas. Jacobi Carbons’ has a range of products that are tailored to address the specific issues that these impurities present. The level of treatment provided allows classification of the biogas into 3 categories:

1. Low-grade fuel; fuel for boilers, furnaces, and micro-turbines. Desirable to include some degree of hydrogen sulfide removal to prevent premature wear on collection and utilisation equipment.
2. Medium-grade fuel; broader range of fuel applications because of the reduction in corrosive constituents. Used in industrial boilers, reciprocating engines, gas turbines and combined cycle systems.
3. High-grade fuel; upgrading biogas to pipeline gas quality, fuel cells and fuel for vehicles.

Treatment steps involve:

• Preconditioning/ pretreatment; removal of particles, droplets, siloxanes, other trace components
• Biogas desulfurisation
• Compression
• Biogas upgrading; separation of CO₂ and H₂O
• Final conditioning; dewpoint control, adjustment of heating value, offgas treatment

Activated carbon (AC) is a highly porous, carbonaceous material that is activated either chemically or physically, to give a series of micro -, meso – or macropores and a large internal surface area. These properties make AC a highly useful adsorbent in a variety of applications. The high surface area also allows impregnation whilst maintaining capacity for adsorption. Impregnated ACs can be designed to target specific types of species by utilising the appropriate chemistry.

A number of the trace compounds above must be removed in order to purify the biogas. As the table shows some of these compounds have adverse effects when using biogas in application. Odour is also a concern, both NH₃ and H₂S are malodourous compounds that are also harmful to health at high concentrations. NH₃ is described as having a pungent odour whilst H₂S is known to have the smell of rotten eggs. Impregnated ACs can be used to treat NH₃ and H₂S in gas streams. NH₃ is treated with acid impregnations such as phosphoric acid (H₃PO₄) trapping NH₃ in salt form, as diammonium phosphate. H₂S is removed by catalytically cracking it into sulfur (S₈), which is physically adsorbed.

Hydrogen Sulfide

Figure 1: Effect of dissolution of H₂S and O₂ in moisture on activated carbon surface.

This reaction needs the presence of H₂O and O₂ to proceed.

It is possible to use unimpregnated carbon in this application. Physical adsorption of H₂S is not generally the preferred method of removal as adsorption is reversible which is why oxidation (see above) is used. Ammonia as a catalyst for uptake does have the added benefit of NH₃ being lighter than air so it does not act as an impregnant and thus when combined with activated carbon, capacity is not reduced. In this process, a small amount of ammonia is needed to facilitate the oxidation of H₂S in the following stoichiometric reaction:

3H₂S + 1.5O₂ ⇒ 3S +3H₂O (NH₃ catalyst)

Or via the following polymeric reaction:
xH₂S + 2NH₃ +[(x-1)/2]O2 ⇒ (NH₄)₂S_x + (x-1)H₂O

It is not known which mechanism is used but both are accepted.

One study done by Jacobi Carbons shows the effects of varying parameters on the adsorption of H₂S, including pH, temperature and humidity. Their outcomes were noted and the results summarised in the paragraph below.

The adsorbing of S₈ is dependent on the activity level of the base carbon material. The higher the activity and the greater the pore volume means greater adsorption of S₈. The most catalytic breakdown of H₂S was seen in the temperature range of 50-80 °C and a relative humidity between 80-95%. The concentration of the impregnation has no effect on the adsorption of H₂S, however too much VOC content could prevent H₂S adsorption by blocking the active sites. The effects of temperature, humidity and contact time are shown in the figures below.

Figure 2: H₂S loading of the KI impregnated activated carbon with variation in gas temperature (2000 ppm; 2 sec; O₂: H₂S ratio 2:1).

It is clear that at 50 °C and 100% RH, the greatest loading of H₂S per 100g carbon is seen.

Figure 3: Translation: H₂S loading of the KI impregnated activated carbon with variation in residence time (2000 ppm H₂S, O₂: H₂S ratio of 2:1; 0 and 100% relative humidity, 23 °C.

This figure shows the best H2S loading per 100g of carbon is achieved at 100% relative humidity with the highest at 8s contact time.

Where stoichiometric oxygen is present, AddSorbTM VA12 can be used. It is a KI impregnated AC that catalytically cracks H2S as explained above. In the absence of O2, AddSorbTM VA4 (coal based) or AddSorbTM VA15 (coconut shell based) are recommended. These have a CuO impregnation and function in the absence of oxygen. However their capacity is significantly lower than KI impregnated carbons, therefore working best at low concentrations of H2S.

Care has to be taken when concentrations of H2S exceed 100 ppm, as the heat of adsorption and the mass flow of H2S would be too great, to expect a reasonable life expectancy. The enthalpy of reaction for H2S reaction with impregnated carbon can be calculated using established enthalpy of formation values. When using caustic materials the following can be expected; -38.72 kJ mol-1 (KOH) and -39.06 kJ mol-1 (NaOH). It has been documented that these impregnations will form a carbonate salt on the surface of the activated carbon, and therefore the enthalpy of reaction becomes -914.05 kJ mol-1 for KOH!

If we assume, a concentration of 1000 ppm H2S then we would expect to see approximately 5 days life expectancy before the carbon needs changing, when using AddSorbTM VA12, based on the information given in figure 2:

1000 ppm × (34/22.4)=1517.86 mg/m₃

1517.86 mg/m₃ × 1000 Nm/h =1,517,860 mg/h=1.518 kg/h

1000 kg of carbon × 17% breakthrough capacity (VA12) = 170 kg

170 kg / 1.518 kg⁄h = 112 hours = 4.67 days (based on 24/7 continuous operation)

At 100 ppm, we can expect to see a 100-fold increase in the life expectancy:

10 ppm × (34/22.4)=15.1786 mg/m₃

15.1786 mg/m₃ × 1000 Nm/h = 15,179 mg/h=0.0152 kg/h

1000 kg of carbon × 17% breakthrough capacity (VA12) = 170kg

170 kg / 0.0152 kg/h = 11184 hours = 466 days (based on 24/7 continuous operation)

Impregnated carbons cannot be regenerated in situ or thermally. Jacobi Carbons solves this issue by being able to offer the grade AddSorb™ Sulfox, a non-impregnated activated carbon, with capacity for H₂S adsorption. Complementary techniques such as amine treatment and wet chemical scrubbing are used to help further remove H₂S from the biogas stream. Amine treatment removes H₂S via the following:

RNH₂ + H₂S ⇒ RNH₃+ + SH

Activated carbon is then used in the recirculating leg of this process to regenerate the amine. Wet scrubbing removes H2S when in contact with caustic materials (this neutralises H2S).

Siloxanes

Siloxanes are found in many household consumer products so will find their way into municipal waste. Siloxanes in biogas can volatise during the anaerobic digestion process. If the siloxanes are then allowed to remain in the biogas which is then burned, silicon dioxide (silica) is produced, the major constituent of sand/quartz, causing the problems as mentioned in Table 1.

Siloxanes are a group of organosilicon compounds, with the Si-O linkage and organic chemistry functionality (they possess methyl, ethyl groups etc). This class of compound can either be linear or cyclic. The nature of the structure of siloxane compounds can affect the life of the carbon. The simplest linear siloxane is polydimethylsiloxane (PDMS) and the simplest, stable cyclic siloxane is octamethylcyclotetrasiloxane.

For linear siloxanes, the greater the value of n (see figure 4), the longer the chain length will be. This increases the molecular weight of the siloxane. Heavier siloxanes will decrease the life of the carbon as the mesopores of the activated carbon are more easily blocked and thus reduces capacity. This also applies for cyclic siloxanes. Low molecule weight cyclic siloxanes occupy a large physical space in comparison with a linear molecule of the same size. This too has an impact on the life of the carbon and both cases should be considered when applying activated carbon to biogas applications.

Non-impregnated AC is used to physically adsorb siloxanes (due to size and the non-polar nature of both the AC and siloxane). The AC is thermally regenerated but it is very difficult to desorb siloxanes, so regular replacement of the bed is needed. Bed life is improved by moisture removal ahead of carbon bed. For dealing with physically adsorbed species, like siloxanes, Jacobi Carbons recommends the use of EcoSorb^TM GXB.

Table 2: Comparison of siloxane capacity on ACs from different raw materials

As biogas can be produced from a variety of source materials then it is possible that the biogas produced will have a different composition with different species that will need removal. The table below shows the Jacobi Carbons recommendation for treatment of the biogas source material

Figure 4: Repeat unit of siloxane

Figure 5: Top; linear siloxane- octamethyltrisiloxane. Bottom; cyclic siloxane- octamethylcyclotetrasiloxane

Table 3: Biogas source material, likely pollutant and recommended grade for treatment

Activated carbon is a tried and tested material for the purification of biogas. It is not without its caveats, which have been addressed here at Jacobi Carbons. It is clear that Jacobi Carbons has the solutions to treat impurities in biogas effectively from a variety of source materials fermented in the production of biogas, by offering a variety of grades to meet the requirements of the customer.

Below is a summary of the properties of grades mentioned above that can be used as an effective treatment for either hydrogen sulfide or siloxane removal. It is intended that this is used as a guide when deciding on the most appropriate treatment of biogas.

Table 4: Summary of grades suitable for this application