MSc Assignment 9

17 Dec

Mark Collinson

Advanced Biological Waste Management Technologies

Written Assignment 2

 

Abstract

Anaerobic Digestion (AD) and In-Vessel Composting (IVC) are processes utilised by the waste industry to manage organic wastes by controlling and even speeding up the breakdown of complex organic chemicals (fats, proteins and carbohydrates).  In doing so, many by-products are released, including acids, sugars and alcohols, and ultimately gases including methane, CO, CO2 and hydrogen.  One of these by-product gases is hydrogen sulphide – H2S, which is a major cause of corrosion within compost and digester vessels.  This piece of work will investigate the formation characteristics of H2S and study a number of processes which are employed in order to minimise its production and the effect the gas can have on the vessels themselves.

 

Anaerobic Digestion

AD is a natural process which takes various organic wastes such as crop waste, food waste, animal wastes (manure, slurry, etc) and allows micro-organisms to break them down, in the absence of oxygen, into their constituent elements (Defra 2012).  The process produces a number of by-products at each stage, ending with methane, carbon dioxide and water. It is a four stage process in which each stage relies upon the previous one.  These stages are called, progressively, hydrolysis; acidogenesis; acetogenesis and finally methanogenesis (ADBA, 2012). 

  1. Hydrolysis

This first stage breaks down the complex organic molecules – fats, proteins, carbohydrates – into the simpler molecules, fatty acids, amino acids and glucose (sugar), respectively. 

  1. Acidogenesis

The fatty acids, amino acids and sugars are then broken further into volatile fatty acids and alcohols, producing ammonia (NH3), hydrogen sulphide (H2S) and carbon dioxide (CO2) in the process

  1. Acetogenesis

The fatty acids and alcohols are reduced again to acetic acid, hydrogen (H) and carbon dioxide, the nitrogenous material gassing off as ammonia.

  1. Methanogenesis

The hydrogen and acetic acid is finally broken down by certain bacteria (methanogenic archaea) into methane and yet more carbon dioxide.

 

 

 

It is during the second phase, acidogenesis that hydrogen sulphide is produced, when sulphate (SO4) is reduced to sulphide (S2-) by a group of ‘sulphate-reducing bacteria’, thus:

SO42- + 8H+ + 8e  =>  S2- + 4H2O

For those processes which go on to utilise the AD produced biogas as a fuel product, the presence of hydrogen sulphide is a distinct problem as it is an very aggressive corrosive gas which can attack piping, equipment and instrumentation.  It is not a welcome addition to the mix of products to be added, for instance, to a £1million gas engine such as is often used in CHP plants, as internal combustion engines operate best when concentrations are below 100 ppm (although some designs of boilers can operate up to 1000ppm) (Bioenergy Consult, 2012a).  It is corrosive as it produces sulphur dioxide within the combuster, which can then go on to produce sulphuric acid, all of which can cause damage to the engine parts and processes.  H2S also leads (due to the sulphur element of hydrogen sulphide) to sulphur dioxide production, levels of which must be strictly controlled to comply with emission limits as set by the environmental permitting regime applied by the Environment Agency.  It is imperative therefore, that it is removed where possible.

Hydrogen sulphide removal.

The 2 most common methods for H2S removal are performed within the digester in the form of ‘oxygen dosing’ to the digester biogas and ‘iron chloride dosing’ to the digester slurry.  In the case of oxygen dosing, the Thiobacillus family of micro-organisms are utilised, as they are ‘sulphide oxidising’ organisms, which, when added to the biogas provide the simplest way to de-sulpherise the biogas.  When stoichiometric amounts of oxygen are added to the digester, Thiobacilli grow on the surface of the digestate, which provides the required nutrients and micro-aerophilic surface for them to flourish.  Sulphur is removed from the biogas and forms small yellow clumps on the substrate, which can then be removed.  A number of factors, including temperature, reaction time, the amount and position of oxygen added will affect the removal efficiencies, but H2S concentrations can be reduced by 95% to less than 50 ppm (Bioenergy Consult, 2012a).  Diaz, et al (2010) applied oxygen at a rate of 0.25NL per litre of sludge feed material and recorded removal efficiencies of over 98%. 

The other common method involves the addition of iron chloride into the slurry itself (or the feedstock, prior to digestion) as iron chloride reacts with the hydrogen sulphide to form particles of iron sulphide salts (Bioenergy Consult 2012b).  This method will only ever partially remove the H2S however, so a further upgrading process would still be required, especially if the biogas was intended as a high quality vehicle fuel.  Colin Risbridger, who runs Tuquoy Farm in the Scottish Western Isles spent over £10,000 on the installation of an iron chloride hydrogen sulphide removal system, a comparable cost to building a ‘gas-to-grid’ installation (Bywater 2010), which is considered by many to be too expensive.

The addition of iron oxide covered wood chips are a popular option, especially in the US, as they are light in weight yet have a large reactive surface which allows a collection ratio of 1:5 (i.e., 1kg of wood chip can collect 200g of sulphur).  This can be achieved as iron oxide reacts with H2S to produce iron sulphides which are then ‘regenerated’ to produce elemental sulphur and iron oxide once more. It is a low cost option, but operational costs can rise, as monitoring requirements increase due to a tendency for the materials to overheat when regenerating.  Alternatively, iron oxide pellets can be used rather than wood chips, and having a much higher density, they can have a collection ratio of 1:2 (i.e., 1kg of iron oxide pellets can collect 500g of sulphur) (IEA Bioenergy, 2000).  Meeyoo et al (1997) investigated the use of various porous materials as adsorbants to collect H2S and found that microporous carbons had the highest removal efficiency, due to their capacity to oxidise the hydrogen sulphide whereas the other sorbents could remove the gas by adsorption only.  Cosoli et al (2008) went on to show that common zeolites (micro porous aluminium silicate materials) are also an effective method of H2S removal when biogas is passed through, as it allows the adsorption of the gas on to the zeolite’s structure.  This is a slow process though, and an expensive one when compared to oxygen dosing or iron chloride dosing, mostly due to the expense of the zeolite structures.  There are over 40 patented zeolite structures which have been designed, but they remain primarily manufactured for high value pharmaceutical and chemical industries, and unless the cost falls significantly then the use of zeolites as a cost effective adsorber of H2S looks unlikely.  Cookney et al (2012) have investigated the use of silicone based membrane materials (specifically poly-di-methyl-siloxane or PDMS) as a gas removal technology, and although the work was primarily aimed at measuring rates of methane removal, it was noted that the system has significant potential in removing H2S from biogas.  Filho et al (2010) studied packing materials to investigate their potential as bifiltration materials and found that recovery rates as high as 99.3% could be obtained.  Testing filters packed with polyurethane foam, sugarcane bagasse and coconut fibre, H2S was passed through the filtration media at rates varying between 184 and 644 ppmv and was found to be significantly captured.  It was concluded that the packing materials present a viable long-term biofiltration medium. Wet scrubber systems are also employed in some cases, although again costs can be high and their effectiveness can be questioned when removal efficiencies are compared to those of dosing or filtration methods.  Also, wet scrubbing systems carry the additional burden of producing high volumes of water (sulphur contaminated) requiring disposal (IEA Bioenergy, 2000).

Discussion

It is evident, then, that many processes are capable of removing the hydrogen sulphide from the biogas, and that many studies have been carried out, looking at ingenious methods for removal utilising new materials.  Unfortunately, it would seem that the cost of some of the technologies would prove prohibitive for a low value biogas at the moment.  It would require an increase in the value of the gas (by government intervention via incentive payments and grants or by market forces) or a reduction in the cost of the technologies.  Even the simple idea of removing the sulphur from the feedstock prior to digestion is flawed, as sulphur is a vital component of the process (the optimal ratio of carbon, nitrogen, phosphor and sulphur is 600:15:5:1) (Al Seadi et al, 2008).  The most common removal technology is that of oxygen dosing, and with good reason as it is cheaper than most other systems and removal efficiencies are high.  It is a simple process which requires little management and little monitoring.  Unless the value of the gas increases dramatically or new technologies are discovered or cost-reduced, then it will remain so for some time, especially in the commercial world.

References

 

ADBA, 2012, 2012-last update, What is AD ? [Homepage of ADBA], [Online]. Available: http://www.adbiogas.co.uk/about-ad/ [October 2012, 2012].

AL SEADI, T., RUTZ, D., PRASSL, H., KOTTNER, M., FINSTERWALDER, T., VOLK, S. and JANSSEN, R., 2008. The Biogas Handbook. Esbjerg: University of Denmark.

BIOENERGYCONSULT, 2012a, 2012-last update, Biological removal of hydrogen sulphide in biogas [Homepage of Bioenergy Consult], [Online]. Available: http://www.bioenergyconsult.com/biological-desulphurization-of-biogas/ [December 4th 2012, 2012].

BIOENERGYCONSULT, 2012b, 2012-last update, Common methods for hydrogen sulphide removal [Homepage of Bioenergy Consult], [Online]. Available: http://www.bioenergyconsult.com/hydrogen-sulphide-removal-from-biogas/ [December 4, 2012].

BYWATER, A., 2011. A review of anaerobic digestion plants on UK farms – barriers, benefits and case studies. 1. Warwickshire: Royal Agricultural Society of England.

COSOLI, P., FERRONE, M., PRICL, S. and FERMEGLIA, M., Hydrogen sulphide removal from biogas by zeolite adsorption Part I. GCMC molecular simulations. Chemical Engineering Journal; Chem.Eng.J., 145(1), pp. 86-92.

DEFRA, December 3 2012, 2012-last update, Anaerobic Digestion Strategy and Action Plan [Homepage of Defra], [Online]. Available: http://www.defra.gov.uk/publications/files/anaerobic-digestion-strat-action-plan.pdf [December 3, 2012].

DÍAZ, I., PÉREZ, S.I., FERRERO, E.M. and FDZ-POLANCO, M., 2010. Effect of oxygen dosing point and mixing on the micro-aerobic removal of hydrogen sulphide in sludge digesters. Bioresource technology, 102(4), pp. 3768-3775.

FILHO, J.L.R.P., SADER, L.T., DAMIANOVIC, M.H.R.Z., FORESTI, E. and SILVA, E.L., Performance evaluation of packing materials in the removal of hydrogen sulphide in gas-phase biofilters: Polyurethane foam, sugarcane bagasse, and coconut fibre. Chemical Engineering Journal, 158(3), pp. 441-450.

IEA BIOENERGY, 2000. Biogas Upgrading and Utilisation. 1. Stockholm: IEA Bioenergy.

MANNUCCI, A., MUNZ, G., MORI, G. and LUBELLO, C., Biomass accumulation modelling in a highly loaded bio-trickling filter for hydrogen sulphide removal. Chemosphere, 88(6), pp. 712-717.

MEEYOO, V., TRIMM, D. and CANT, N., 1997. Adsorption-reaction processes for the removal of hydrogen sulphide from gas streams. Journal of Chemical Technology and Biotechnology, 68(4), pp. 411-416.

MOLINO, A., NANNA, F., DING, Y., BIKSON, B. and BRACCIO, G., Biomethane production by anaerobic digestion of organic waste. Fuel, (0),.

 

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