MSc Assignment 9

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 (S^2-) by a group of ‘sulphate-reducing bacteria’, thus:

SO₄^2- + 8H⁺ + 8e^–  =>  S^2- + 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),.

 

MSc Assignment 8

Mark Collinson

Advanced Biological Waste Management Technologies

Written Assignment 1

 

(a)      Calculate the potential calorific value of the OVERALL biodegradable fraction of the waste in 2020.

Of all the materials listed in tables 1 and 2, it can be ascertained from table 3 that only 5 of them have a level of biodegradable content (Paper/card; Putrescible; Textiles; Fines; Misc. combustible).  This assumes that the plastics element of the waste stream is 100% fossil-based plastics and not bio-based plastics, as otherwise the plastics element could also contain biodegradable material, an option which is overlooked by the report. 

If we take the five waste streams therefore, and apply the calorific value and % biodegradable factors listed in table 3 to the tonnage of waste for each stream in the year 2020, then we can calculate the potential calorific value for each waste stream, which when summed will total the potential calorific value for the overall biodegradable fraction.

N.B., although some composting/recycling should be expected, the question specifically asks for the potential calorific value of the OVERALL biodegradable fraction.  I have therefore assumed for the purposes of this exercise that no composting or recycling is carried out and instead the recycled, composted and residual material streams have been added together. 

 

Paper/card (100% biodegradable):

4,800 ktpa (recyclable) + 5,177 ktpa (residual) = 9,977 ktpa available material.

9,997 x 1000 = 9,997,000 tonnes available material per year.

9,997,000 x 1000 = 9,997,000,000 kg available material per year.

9,997,000,000 kg x 17.23 (CV in MJ/Kg) = 171,903,710,000 MJ

Therefore, 171 billion MJ available each year from the overall paper/card waste stream.

 

We can then follow the same process for each of the other waste streams:

 

Putrescible (100% biodegradable):

(11,496 + 8,873 x 1000) x 1000 = 20,369,000,000 kg material

20,369,000,000 x 6.55 (CV) = 133,416,950,000 MJ

Textiles (50% biodegradable):

(927 + 624 x 1000) x 1000 = 1,551,000,000 kg material

Apply the 50% factor as textiles are listed as only 50% biodegradable:

1,551,000,000 / 2 = 775,500,00 kg biodegradable x 16.12 (CV) = 12,501,060,000 MJ

 

Fines (50% biodegradable):

(0 + 1,802 x 1000) x 1000 = 1,802,000,000 kg material

(1,802,000,000 / 2) x 7.39 = 6,658,390,000 MJ

 

Misc. combustible (50% biodegradable)

(670 + 3,811 x 1000) x 1000 = 4,481,000,000 kg material

(4,481,000,000 / 2) x 9.25 = 20,724,625,000 MJ

 

Table 1: Calorific Value (MJ) of each waste stream

Paper/card	171,903,710,000
Putrescible	133,416,950,000
Textiles	12,501,060,000
Fines	6,658,390,000
Misc. combustible	20,724,625,000
Total	345,204,735,000

 

Therefore, the total calorific value of the biodegradable fraction of the combined waste streams in 2020 is 345,204,735,000 MJ.

 

 

 

(b)      Argue for or against the use of a NAMED biological treatment technology, versus a NAMED thermal treatment technology to manage the overall biodegradable fraction of the waste, in 2020.

 

For this section I will discuss anaerobic digestion (AD) following a mechanical biological treatment (MBT) process versus a traditional incineration process.  The reasons for this decision over other alternatives are listed below:

Open Windrow composting systems are cheap and relatively easy to manage, but to produce PAS100 quality material they cannot contain meat products and the environmental permit will not allow mixed putrescibles (just green waste).  Seeing as unsorted putrescibles are the second highest tonnage of material in the waste stream (20,000,000 tonnes pa), there’s a good chance that this feedstock will not comply with the permit and that the final product could not be classed as PAS100 compliant (WRAP, 2011).

In Vessel Composting (IVC) will happily utilise food waste as a feedstock, but the input of textiles can be a serious problem.  As textiles make up 1.6 million tonnes of the 2020 waste stream, this also makes it a difficult material to process within this treatment process.

AD will also use food waste as a feedstock, indeed, prefers it, but the technology is robust enough to manage other organics and putrescibles, so long as the inerts, plastics, metals and glass are first removed.  The mechanical element of the MBT process is capable of managing the combined fraction and should be capable of removing those contaminants, thereby cleaning the waste stream sufficiently to leave a high quality, high organic fraction suitable for AD.

If we return to the Oakdene Hollins report and assume that the 2020 waste streams are now separated into compost/recyclate and residual, and deal here with just the residual, then we can concentrate on the residual material as the rest will have been dealt with.  If the compost/recyclate is added to the mix, then the process to be followed described below will still stand.  The residual element will be the black bag/black bin waste from households and the residues which remain following the separation of other recyclates at a material recovery facility (MRF).  This material is the material which will provide the feedstock for the MBT/AD process:

Mechanical Biological Treatment

The idea behind an MBT process is that contaminant material is removed from the residues, leaving a cleaner ‘organic’ fraction for the biological treatment process.  The mechanical process is very similar to that employed in many MRFs across the country, utilising shredders to size minimise the material, trommels to screen the material, optical sensing equipment (NIR scanners or X-ray fluorescence scanners) and density separators such as wind sifters or floatation tanks. 

The process is thus:

 

 

1. Size reduction

A shredder is used on the bagged waste to split the bags, remove the material and reduce where possible the larger fragments.

1. Screening

The material is then passed through a trommel (or similar screening device), usually set to 40mm, so that all material smaller than 40mm falls through the screen and is classed (regardless of its biodegradable nature or otherwise) as ‘organic’ material.  This is the material which will be processed by the biological process (see AD).  The material larger than 40mm passes through the trommel and continues onto the mechanical process. 

1. Optical sensing equipment

Although not all MBT employ this technology, it is becoming more commonplace, and all new MBT plant have them installed as a matter of course.  They scan the material as it passes beneath them and identify the material according to its chemical make-up.  They are especially proficient at identifying certain plastics (HDPE, PP, PE, PVC), and with a suitable removal device are capable of removing 99% of plastic material from a waste stream

1. Metals

Metals are very easily removed from a mixed waste stream, through the use of over-band magnets to attract and remove ferrous materials and eddy current separators to remove aluminium and copper. 

1. Density separation

The remaining material is now made up of plastic film, inerts, textiles and composite or contaminated materials.  This final stage will pass that material through a series of wind sifters or floatation tanks to remove the remaining aggregates, inerts and any remaining heavy materials, leaving the film and textiles to go on as a final (probably fuel) product.

Anaerobic Digestion

AD is a process which takes various organic wastes such as crop waste, food waste, animal wastes (manure, slurry, etc) and breaks them down into their constituent elements to produce methane, carbon dioxide and water (ADBA, 2012).  It is a four stage process in which each stage relies upon the previous one.  These stages are called hydrolysis; acidogenesis; acetogenesis and finally methanogenesis. 

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, hydrogen sulphide and carbon dioxide in the process

 

 

1. Acetogenesis

The fatty acids and alcohols are reduced again to acetic acid, hydrogen and carbon dioxide.

1. Methanogenesis

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

 

Figure 1. The four stages of anaerobic digestion (after ADBA, 2012)

 

 

There remains an inevitable solid material following the breakdown process, known as anaerobic digestate, and a liquid fraction, black liquor.

Incineration

Biodegradable waste is not the ideal fuel for either moving grate or fluidised bed incinerators, mostly due to the high moisture content inherently found in biodegradable waste.  Combustion conditions are difficult to maintain, ash is difficult to manage (Anthony, 1995) and emissions are difficult to control, especially NO (Hui-Chao, et al, 2007).  Overall, a dry, homogeneous material feedstock is ideally required, and this is not generally the case with putrescible, biodegradable materials, which can contain up to 80% water content.  Specifically, the additional problems encountered by fluidised bed combustors (FBCs) when attempting to utilise putrescible biodegradable waste have been outlined by Van Caneghem, et al, 2012. These additional issues are caused by the wet, ‘cloggyness’ that can destroy the generally sensitive nature of FBCs and the dry and loose nature of the material that is used to create the bed.  In the absence of this loose, dry bed material, the combustor tends to clog up and prevents both mixing and aeration of the fuel material which is required to ensure full combustion.

Discussion

It would seem reasonable, therefore, to believe that incineration of biodegradable waste is more problematic and less beneficial than AD, especially if the AD feedstock material is carefully source segregated, uncontaminated food waste (Bouallagui, et al, 2005).  Even the difficulties often encountered utilising AD if co-digesting a number of materials at the same time, (Agdag and Sponza, 2007) can be overcome by careful monitoring and mixing of input materials.  This is the basis then for the additional step introduced by the MBT process – loss of contaminants and purification of the biodegradable fraction, leading to an easier to manage feedstock material and greater yields of biogas.  Xiao, et al (2012) report a “high biogas production rate” of 4.25 m³ (m³ d)^-1 in trials of municipal biodegradable biomass (mixed kitchen and fruit/vegetable) waste when managed through a co-digested AD plant.  Running an AD plant though is more difficult than running an incinerator, CAPEX and OPEX are high, as are the technical complexities (Shuguang et al, 2007), but the benefits can be high when the difficulties are overcome.  The biogas produced by the AD plant can be used to produce electricity or heat, some of which can be used to fund the parasitic load required to keep the digester at the optimum temperature.  This helps minimise fuel bills and reduces dependence upon fossil fuels.  The electricity can be sold to the National Grid, and in addition the producer can claim the government’s renewable incentive payments to offset the CAPEX/OPEX investment encountered, and at the same time deliver towards the UK’s requirement to deliver 15% of its energy from renewable sources by 2020.  The solid digestate can be used as a soil conditioner in agriculture and the black liquor is a worthwhile fertiliser.  Both these applications themselves also reduce our dependence on fossil fuels as fertiliser is often a product of mineral oil distillation.  In addition, the utilisation of food waste as a fuel product can minimise waste management costs, and the non-organic outputs from the MBT plant are numerous.  Anecdotal evidence suggests that ferrous metals from MBT can currently be worth around £40 per tonne, non-ferrous £80 per tonne, mixed solid plastics (HDPE, PP, PE, PVC) £10 per tonne and mixed glass £10 per tonne, with 2 separate fuel products produced in addition, one at 10-12 MJ/Kg (RDF) and one at 20 MJ/Kg (SRF).  These materials are used as a fuel replacement and can themselves claim an element of government incentive payment if used, for example, in advance conversion technologies such as high quality gasification.

Compare the use and the marketability of these outputs with the limited benefits of incineration and it is no surprise that Viridor, operators of the largest waste management contract in Europe, when faced with developing an integrated waste management policy in Greater Manchester, chose to follow exactly this route to manage the residual black bin waste (MBT to cleanse the residual, AD treatment of the organics).  Other than some hiccups at the commissioning phase (a broken valve leading to a digester emptying and a ‘minor’ gas leak), they have reported great success in working towards minimising landfill levels of biodegradable household waste arisings, helping meet the targets of the 1999 EU Landfill Directive.  At the same time, New Earth Solutions are working with the West of England Partnership to manage the 200,000 tpa of biodegradable residual black back waste from counties around Bath, Bristol, Wiltshire and Somerset.  Again employing a large scale MBT to cleanse the organic fraction, in this case the <40mm fraction is placed into an IVC where it produces a ‘compost like output’ which is used as a soil substitute on old landfill sites.  As we work toward a zero waste economy, toward meeting EU targets on biodegradable landfill minimisation and toward EU requirements for greater renewable energy generation, these are excellent examples for the waste industry to follow going forward.

 

 

References

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

AGDAG, O.N. and SPONZA, D.T., 2007. Co-digestion of mixed industrial sludge with municipal solid wastes in anaerobic simulated landfilling bioreactors. Journal of hazardous materials, 140(1–2), pp. 75-85.

ANTHONY, E.J., 1995. Fluidized bed combustion of alternative solid fuels; status, successes and problems of the technology. Progress in Energy and Combustion Science, 21(3), pp. 239-268.

ANTHONY, E.J., 1995. Fluidized bed combustion of alternative solid fuels; status, successes and problems of the technology. Progress in Energy and Combustion Science, 21(3), pp. 239-268.

BOUALLAGUI, H., TOUHAMI, Y., BEN CHEIKH, R. and HAMDI, M., 2005. Bioreactor performance in anaerobic digestion of fruit and vegetable wastes. Process Biochemistry, 40(3–4), pp. 989-995.

HUI-CHAO, C., CHANG-SUI, Z., YONG-WANG, L. and DUAN-FENG, L., 2007.  NO emission from incineration of organic liquid waste in a circulating fluidized bed. Korean Journal of Chemical Engineering, 24, pp. 906-910.

LIU, X., GAO, X., WANG, W., ZHENG, L., ZHOU, Y. and SUN, Y., 2012. Pilot-scale anaerobic co-digestion of municipal biomass waste: Focusing on biogas production and GHG reduction. Renewable Energy, 44(0), pp. 463-468.

LU, S., IMAI, T., UKITA, M. and SEKINE, M., 2007. Start-up performances of dry anaerobic mesophilic and thermophilic digestions of organic solid wastes. Journal of Environmental Sciences, 19(4), pp. 416-420.

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

VAN CANEGHEM, J., BREMS, A., LIEVENS, P., BLOCK, C., BILLEN, P., VERMEULEN, I., DEWIL, R., BAEYENS, J. and VANDECASTEELE, C., 2012. Fluidized bed waste incinerators: Design, operational and environmental issues. Progress in Energy and Combustion Science, 38(4), pp. 551-582.

WANG, Q., KUNINOBU, M., OGAWA, H.I. and KATO, Y., 1999. Degradation of volatile fatty acids in highly efficient anaerobic digestion. Biomass and Bioenergy, 16(6), pp. 407-416.

WRAP, 2011, 2012-last update, PAS100:2011, Specification for composted materials [Homepage of WRAP], [Online]. Available: http://www.wrap.org.uk/system/files/private/PAS-100-2011.pdf [October 2012, 2012].

MSc Assignment 7

A short critical review for a Local Authority on the applicability of ‘advanced thermal technologies’ for treating MSW-derived plastic waste streams.

 

 

This review is intended to act as an overview for Local Authorities (LAs) wishing to explore the potential for ‘Advanced Thermal Technologies’ (ATT) to deal with the plastic fraction of a municipal solid waste (MSW) stream.  It will briefly outline the technologies available; the environmental impact of ATT when utilising plastic as a fuel; the regulatory and emission controls issues to be dealt with; the regulatory and legislative issues to bear in mind and the cost of each option.

 

Advanced Thermal Technologies.

Whereas incineration is an exothermic chemical reaction based on full oxidation of the fuel materials, the suite of treatment options known as ATT are endothermic reactions – i.e., they require heat from an external source (Arena 2011), and they rely on heavily controlled oxygen conditions in order to produce various by-products.  The three most common technologies are: gasification; pyrolysis and plasma gasification, although there is increasing interest in the production of fuel and fuel oils through the thermal depolymerisation of plastics.  Each of these is explained below:

 

Gasification:

The gasification of waste material occurs at temperatures between 500 and 1400^oC (Morrin, et al, 2010) and pressures up to 40 bars.  The supply of oxygen is managed, in so far as it is controlled to allow a partial combustion of fuel, but not full combustion.  If one considers the fire triangle (Fig 1), which stipulates that each of the three elements of the triangle must be present to produce a fire, then a reduction of any of the elements will stifle it. 

 

 

Figure 1, The Fire Triangle.

 

Following this rule, the oxygen allowed into the combustion area is managed so that the waste cannot create a fire, despite its flammability and despite it being subjected to such high temperatures.  Instead, the waste decomposes due to the heat within the reaction chamber and releases a synthetic gas which is produced from the constituent elements of the waste material (syngas).  This syngas, consisting mostly of CO, CO₂, H₂ and N₂ (Ricaud 2011) has a calorific value in the region of 5 to 10 MJ/Nm³ (Defra 2007) and can then be collected and subsequently combusted in the presence of oxygen, thus providing heat energy.  The other major product of this process is an ash, which generally retains any heavy metals present in the fuel and persistent organic compounds.  First exhibited in the UK by Energos on the Isle of Wight through Defra’s New Technology Demonstrator Programme (Defra 2003), there are numerous demonstrator project in the UK, and now 2 major modular gasification plants reaching the commissioning stage in Plymouth (O-Gen) and Avonmouth (New Earth Solutions).

 

 

Figure 2. 1MW gasification reactor (ITI, Sheffield).

 

Pyrolysis:

Pyrolysis works in a similar manner to gasification, but operates in a total absence of oxygen, rather than a managed amount, and can operate at much lower pressures and temperatures of 1.5 bar and 300-800^oC.  It will produce a syngas but of a higher density, and consequently it’s CV is generally double that of a gasified syngas.  In addition, it produces a carbon rich ‘char’ (residue) and a tar-like oil. 

 

 

Figure 3.  Typical pyrolysis feedstocks and outputs

 

The respective levels of these 3 outputs vary according to the speed of the process and good management of the pyrolyser can lead to either high gas, high char or high oil.  It therefore depends upon the desired output as to how the pyrolyser is run, as an oil to be refined into a fuel product is produced from the same waste input material, but in a very different manner to gas for usage in driving a steam cycle.  Harper Adams University College in Shropshire has, in conjunction with Aston University, recently developed a 1MW pyrolyser whose early tests show great promise at utilising a number of plastic polymer fuels, specifically polypropylene (PP) and polyethylene (PE).

 

 

Figure 4. Harper Adams’ pyrolysis reactor.

 

Plasma Gasification:

Plasma is a stream of superheated ions (similar to a lightning bolt) which can be used to gasify waste in temperatures of over 15000^oC.  At these temperatures, a high amount of syngas is released, along with a metal product containing the melted metallic atoms and a ‘slag’ product containing any other remnant materials.  The advantage of this process is that the gaseous product is extremely clean due to the high temperatures involved in the gasification, which is high enough to destroy hazardous elements such as dioxins, pesticides and PCBs (European IPPC Bureau, 2006).  Unfortunately, this comes at a high price, as the energy overhead required for the production of the plasma stream can mean a high parasitic load (i.e., it uses much of the energy it creates).

 

Environmental Impact.

 

The environmental impact of diverting waste from landfill comes primarily from the minimisation of carbon release due to decomposition of waste in anaerobic landfill conditions.  However, if that carbon is then released through the emission stack, it could  be argued that there’s little, if any, carbon benefit from using the waste as fuel.  Indeed, a WRAP (2008) study suggested that from 3 scenarios studied, only an (unachievable) 100% efficiency CHP pyrolysis plant, displacing a coal fired alternative would have any environmental benefit.  Indeed, considering that plastic waste streams are derived from oil based, and therefore fossil carbon based sources, it is easily argued that the plastic waste stream is environmentally better diverted back into the ground (i.e., landfilled) rather than being released into the atmosphere.  This way, the fossil carbon is retained within the earth’s crust rather than being atmospherically released.  It is argued that with plastic waste streams, recycling is always the better environmental option then fuel usage.  Some plastic waste streams (LDPE film, for example) are notoriously difficult to recycle, due to contamination and lack of source segregation, making recycling a hard to realise proposition.

 

Emissions and Emission Control.

 

Whenever waste materials are used as a fuel product, the issue of emissions is inevitably raised, not least by the anti-incineration lobby.  The heavy metals and persistent organic compounds which are said to be produced and released are a great risk to public health and are allowed to be released in an uncontrolled manner.  In fact, nothing could be further from the truth, as any process which utilised waste materials as fuel must comply with the Waste Incineration Directive (2000).  These stringent rules are designed to prevent or limit any negative effects on the environment (WRAP, 2012). In particular, it looks to achieve significant levels of environmental and health protection through the administration of emission limit values which far exceed those imposed on traditional coal fired power stations.  Although there are exceptions to WID applicability (e.g., vegetable waste, ‘untreated’ wood waste, cork waste), the imposition of the limit values when utilising waste materials should ensure a safe level of emissions from any ATT plant operating within the Directive. 

 

Regulatory and Legislative issues.

 

In addition to the WID, there are other regulatory issues to bear in mind when attempting to set up and run an ATT plant, not least of which is the requirement for an Environmental Permit (EPR, 2010).  Environmental permits are the overarching mechanism for regulating energy from waste facilities and fall into 2 categories: Environment Agency (EA) permits and Local Authority (LA) permits.  Any facility proposing to use more than one tonne per hour of fuel will require an EA permit, those using less can instead work with the (in theory simpler) LA permit.  The permit will have conditions attached which must be followed to prevent any processing activity from damaging the environment or human health.  The permit itself will dictate what fuel feedstocks are allowed, the calorific value of fuel, how the material is to be stored, the chemical composition of the fuel, emissions to the atmosphere (and how they comply with the WID) and certain process controls.  Strict monitoring systems are set out, and any breach of the allowances will see the permit revoked and a subsequent shut down of the plant.

 

 

Cost.

 

Costs of ATT plant are always difficult to ascertain, partly due to commercial sensitivities and partly due to a constantly evolving market place.  Primary research by the author has highlighted 4 gasification plant systems which claim to be able to fully utilise the MSW plastic waste stream, ranging in capital expenditure (capex) from £1m (40,000 tpa) to £17m (80,000 tpa).  Taking only the £1m example, if an LA with a plastic waste stream of 100,000 tpa was hoping to utilise this technology to manage its plastics entirely, it would need to invest £2.5m on the plant alone.  Added to this would be land cost, building cost, planning, due diligence and consultancy costs, before ever considering collection, transport and operating expenditure.  With an expected life time of 10 years, this equates to a capex cost of £250,000 per year, and possibly a similar amount in opex.  With a landfill cost at the current rate of £72 landfill tax, £20 gate fee and maybe £10 transport (around £100 per tonne), 100,000 tpa would cost £1m per year in disposal to landfill.  This would represent a reduction to 50% of previous costs for management of the plastic waste stream alone.  Any such business plan would require, of course, a much more detailed breakdown than offered here.

 

Summary

 

The technologies generally described as Advanced Thermal Treatments have had a difficult evolution over the past few decades, with many examples failing to live up to their early promise.  Many European states have attempted to introduce them as part of the national waste strategies, but have walked away following disappointing results.  Japan, on the other hand, has embraced the idea of gasification, especially, and now has 40 plant up and running.  Recent technological developments however, have helped to overcome some of the perceived barriers associated with ATT and the UK is now seeing a number of new plant in development, due in part to the government’s incentive programme offering greater incentives for renewable power generation.  With an apparent reduction in cost for LAs to utilise this waste stream as opposed to disposing of it, it can be argued that ATT to manage plastic waste is economically viable.  However, environmentally it could still be argued that plastic waste is better off in landfill.  Maybe a reduction in landfill tax for unrecyclable plastics would avoid the fossil carbon based within it becoming inevitably released into the atmosphere, and ATT could then focus their efforts more on renewable biomass materials instead.

 

 

 

 

 

 

 

 

 

 

 

References

 

Advanced Plasma Power (2011) Advanced Plasma Power – The energy from waste solution: Technology Overview, available from www.advancedplsmapower.com/technology.aspx

 

Arena, U. (2011). Gasification: An alternative solution for waste treatment with energy recovery. Waste Management, 31(3), 405-406 

 

DEFRA (2003) New Technologies Demonstrator Programme – Summary and Key Findings, available from http://archive.defra.gov.uk/environment/waste/residual/newtech/demo/documents/TAC-Summary.pdf

 

DEFRA (2007) Advanced Thermal Treatment of Municipal Solid Waste

 

Environment Agency (2002) Solid Residues from Municipal Waste Incinerators in England and Wales, available from http://www.environment-agency.gov.uk/business/sectors/133342.aspx

 

European IPPC Bureau (2006) Reference Document on the Best Available Techniques for Waste Incineration. European Comission.

 

Mininni, G., De Stafanis, P., Barni, E, Chirone, R, and Urciuolo, M (2008) New Technologies for MSW Thermal Treatment: The state of the art, available from http://www.acs.enea.it/documentazione/editoria/11.pdf

 

Morrin, S., Lettieri, P, Mazzei, L and Chapman, C (2010) Assessment of Fluid Bed + Plasma gasification for energy conversion from solid waste.  In: CISA, Environmental Sanitary Engineering Centre.  Proceedings Venice 2010, Third International Symposium on Energy from Biomass and Waste.  Venice, Italy.

 

Ricaud, Anne-Lise (2011) Practical and Economic Viability of Small-scale Energy from Waste.  Imperial College London, London.

 

Williams, P.T., (1994) Pollutants from Incineration: An Overview IN Hester R.E. and Harrison R.M, Issues in Environmental Science and Technology – Waste Incineration and the Environment.  The Royal Society of Chemistry, Letchworth.

 

WRAP (2008) LCA of CHP incineration for disposal of mixed waste.  Available from www.wrap.org.uk/mixed_plastics

 

WRAP (2012) Energy from Waste Development Guidance.  WRAP.  Available from www.wrap.org.uk/efw

 

MSc Assignment 6

During an interview following protest from environmentalists after Hull City Council and East Riding of Yorkshire Council signed a 25 year contract with Waste Recycling Group to burn 165,000 tonnes of municipal solid waste annually, Mr Clive Carr, Managing Director of WRG stated that:

 

“The level of emissions that would be produced [by the proposed incinerator] are equivalent to the level of dioxins in urban soils” and “the dioxins released on bonfire night are equivalent to that released from all the UK’s incinerators in one year”.

 

 

This often quoted phrase that the level of dioxins released on bonfire night is equivalent to that produced by the UK’s waste incinerators in a year is primarily based on a line released by the Environment Agency, in which it was claimed that “during the Millennial celebrations in London the emissions from one 15 minute, 35 tonne firework display equalled 120 years of dioxin emissions from the SELCHP waste incinerator”.  In addition, the Director of INCPEN (the Industry Council for Packaging and the Environment) is quoted as saying “more dioxins were emitted in one hour by the Millennium fireworks than will be produced by all the UK’s incinerators in 120 years.  Dioxins are no longer a problem in modern incinerators, more dioxins are given off by the domestic gas cooker” (UKWIN).  How true are these statements, and are bonfires/fireworks really as bad as this for dioxin formation ?  In order to evaluate this statement, we first need to understand what dioxins are, how they are formed, and how the production of them from EfW and fireworks relate to each other. 

 

Dioxin properties.

 

The term dioxin (polychlorinated dibenzo-para-dioxin) is applied to a family of organic compounds which share similar properties.  Mostly (but not always) toxic, they are generally the first of issues raised by the anti-incineration lobby (due to health concerns) when attempting a facility opposition.  Chemically based on 2 benzene rings (Environment Australia, 1999), with 2 oxygen atoms conjoining the rings, they are shown in their basic form in Figure 1. 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1:  Basic dioxin structure.

 

As the ring is made up of carbon atoms with a strong desire to bond (due to the lack of a full electron shell), there exists the possibility to conjoin with other available atoms, producing variants.  Chlorine atoms (prevalent in some waste stream feedstocks) can occupy some or all of the eight vacant positions to produce any one of 75 variants.    

 

The structure of the most toxic dioxin (tetra chloride dibenzo-dioxin – TCDD) is shown in Figure 2.  Only the dioxins with four or more chlorine atoms are considered toxic (4=tetra, 5=penta, 6=hexa, 7=septa, 8=octa) and their number is shown in Table 1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2:  Tetra chloride dibenzo-dioxin

 

 

Number of Chlorine atoms	Number of PCDD isomers
1	2
2	10
3	14
4	22
5	14
6	10
7 8	2 1
Total	75

 

Table 1:  Numbers of dioxin variations

 

Similarly toxic and similar in make-up are the sister compounds known as furans, also produced by waste incinerators, but with a single oxygen atom rather than 2 and a carbon bond replacing the oxygen (Figure 3).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3:  Basic structure of polychlorinated dibenzo furan.

 

Table 2 lists the number of possible variants of the furan structure

 

Number of Chlorine atoms	Number of PCDF isomers
1	4
2	16
3	28
4	38
5	28
6	16
7 8	4 1
Total	135

 

Table 2:  Number of furan variations.

 

In order to produce these chemical variants then, a number of constituent elements are required, namely carbon, oxygen and chlorine.  They are a natural by-product of the combustion process (Arc21, 2012), as is shown in a French study of 2003 (Robin des Bois) which suggests that a typical 2 hour barbeque can release 12-22 nanograms of dioxins into the atmosphere.  This is quoted as equivalent of 220,000 cigarettes, and is way above the limits set for waste incinerators. 

 

Dioxin Control

 

Waste Incinerators, as can be guessed from the title of the legislation, are governed by the Waste Incineration Directive (Directive 2000/76/EC of the European Parliament and of the Council of 4 December 2000 on the incineration of waste [OJ L332, P91 – 111]), which sets limit values for the emission of many substances, including dioxins.  Annex II (cement kilns) and Annex V (waste incinerators) set the limit for dioxin/furan emissions at 0.1 ng/m³ over a sample period between 6 and 8 hours (Waste Incineration Directive, 2000).  In order to limit the emission of dioxins/furans, it is important for plant to understand the processes of their formation and the opportunities to control them.  They are produced by incineration processes (when the constituent elements are present) at between 250 – 350^oC (Santoleri, et al, 2000), so the obvious control measures are to either remove the required chemical constituents, or avoid the temperature range within which they can form.  As the majority of dioxins require chlorine as a constituent, it follows that the control and removal of chlorine in the feedstocks will deliver a reduction in the dioxin formation levels.  A 2010 study investigated the role that PVC (polyvinyl chlorine) removal from feedstocks can play in the reduction of chlorinated dioxin formation, but maybe somewhat surprisingly concluded that pre-combustion PVC removal “offers no discernible emission reduction benefit” (Vanezia, et al, 2010).  It may be assumed that removal of PVC would facilitate in the reduction of dioxin formation, but it is noted that chlorine sources include food waste (in the form of salt – Sodium Chloride [NaCl]) and paper waste (chlorine replaced sodium hypochlorite in the 1930’s as the main bleaching agent in the paper industry) (Themelis, 2010).  As organic food waste and unrecyclable paper are unlikely to be ever removed from the waste stream, removal technologies are therefore the most feasible solutions. 

 

Removal Technologies

 

As previously stated, dioxins are produced in a temperature range of 250-350^oC, therefore, if this temperature range can be avoided, then it should stand that dioxin production is impossible.  There are 2 possible points in the incineration process whereby these temperatures can be reached – the incineration process itself (where temperatures will exceed this range) and the gas cooling process that follows (where the gases will reduce in temperature until cooled).  Therefore control of both these processes is required. 

To control the incineration process of waste, WID requires that the feedstock is held at a minimum temperature of 850^oC for a minimum of 2 seconds.  This is to ensure complete oxidation of the material and in part to prohibit the formation of dioxins.  In addition, many process ‘quench’ the exhaust gases to reduce the temperature quickly, thereby removing the ability for dioxins to form (Carroll, 2003).

 

Despite these efforts, dioxins can still be found in the exhaust gases, so further treatment processes are put in place, including scrubber systems and electrostatic precipitators (Environment Australia, 1999).  These systems work by physically ‘grabbing’ the dioxins, either through a filtration mechanism, or by utilising the capacity of the molecules to bond with other atoms.  One such system was investigated by Hajidzadeh and Williams (2011).  They studied, in addition to the suite of standardly utilised pollution abatement techniques, the application of activated carbon from the pyrolysis of waste car tyres to adsorb dioxins from the flue gas stream, based upon the capacity of the carbon atoms to bond with the vacant electron positions as shown in Figure 1.  The results showed an overall reduction of 84% of dioxins in bench scale tests, highlighting the potential for this material as a control measure in waste incinerator flue gases.  Additionally, Ruegg and Stigg (1992) and Everet, Baeyens, et al (2003) noted the excellent properties of activated carbon in attracting and bonding with waste incinerator flue gas dioxins. 

 

Fireworks emissions

 

All well and good, studying incinerator flue gases, but how do these compare to bonfire/firework emissions, as mentioned within the title of the question ?  The Health Protection Agency (2009) say that the vast majority of human exposure to dioxins comes from dietary intake (>90%), mostly from meat, fish and eggs.  Incinerator emissions, they say, contribute less than 1% of UK emissions.  Fleischer, et al (1999), studies the release of polychlorinated dibenzo-p-dioxins from fireworks and found that modern day pyrotechnics are produced using a large variety of products, including chlorine based oxidisers, flame-colouring copper based salts and pulverised PVC.  These positions in the production of dioxins is well known, but prior to the study, it was assumed that high temperatures within the conflagration process would prohibit dioxin production.  Fleischer pointed out, however, that dioxin formation could potential occur on the edge of the process, where temperatures are lower, and where temperatures are allowed to reduce naturally (i.e., not ‘quenched’, as they are in a waste incinerator).  Dyke and Coleman (1994) reported a 4-fold increase in ambient air dioxin concentrations during a fireworks/bonfire celebration, backing up the assertion that fireworks/bonfires do indeed produce dioxins.

 

Summary

 

It has been shown that dioxins are produced by a variety of natural processes, all of which require an element of incomplete combustion of fuel materials.  The duel benzene/oxygen/chlorine formation of most is a highly toxic combination, very harmful to human health.  The management and control of these products therefore is of huge importance to the waste industry when utilising waste materials as fuel (which include chlorinated plastics such as PVC, ‘bleached’ paper and food waste containing salts).  The Waste Incineration Directive delivers stringent and measureable limits for emission releases. In addition, abatement technologies such as the use of pyrolysed tyre waste can control dioxins, post-production.  In contrast, the unregulated, unmonitored and positively promoted usage of fireworks on a regular basis is subject to none of these controls.  It is eminently believable, therefore, that dioxin release from fireworks is exhibited at a higher level than that from waste incinerators.

 

 

 

 

 

References

AEA Technology (2008), Investigation of waste incinerator dioxins during start-up and shutdown operating phases. AEA Energy and Environment, The Gemini Building, Harwell, Didcot.

 

Alcock, R. E., Behnisch, P. A., Jones, K. C., Hagenmaier, H (1998).  Dioxin-like PCBs in the environment – human exposure and the significance of sources Chemosphere, Volume 37, Issue 8, 1457 – 1472

 

Carroll, W., (2003) Incinerator Design and Operation:  The Robust Approach to PCDD/F Minimisation, Chlorine Chemistry Division of the American Chemistry Council. 

 

Bell, S and McGillivray, D (2006) Environmental Law. Oxford University Press, 2006.  Oxford.

 

Environment Agency (2002) Solid Residues from Municipal Waste Incinerators in England and Wales, available from http://www.environment-agency.gov.uk/business/sectors/133342.aspx

 

EPA, (2002), Polychlorinated Biphenyls (PCBs), available from http://www.epa.gov/opptintr/pcb/

 

Environment Australia (1999), Incineration and Dioxins: Review of Formation Processes, consultancy report prepared by Environment and Safety Services for Environment Australia, Commonwealth Department of the Environment and Heritage, Canberra

 

Everaert, K., Baeyens, J., Creemers, C., (2003)  Adsorption of dioxins and furans from flue gases in an entrained flow or fixed/moving bed reactor. Journal of Chemical Technology and Biotechnology, 78, 213-219]

 

Fleischer O, Wichmann H, Lorenz W, (1999) Release of Polychlorinated Dibenzo-P-Dioxins and Dibenzofurans by Setting Off Fireworks.  Institute of Ecological Chemistry and Waste Analysis, Technical University of Braunschweig, Germany.

 

Hajizedeh, Y., and Williams, P. T., (2011) Tyre Pyrolysis activated carbon for Dioxin Control in Waste Incinerators. Energy and Resources Research Institute, University of Leeds. 

 

Hawkins, RGP and HS Shaw (2004) The Practical Guide to Waste Management Law.  Thomas Telford Publishing, London.  ISBN 07277 32757.

 

Health Protection Agency (2009), The Impact on Health of Emissions to Air from Municipal Waste Incinerators.  Available from http://www.hpa.org.uk/webc/HPAwebFile/HPAweb_C/1251473372218

 

Reducetheuse (2012), Energy from Waste summary document. Available from www.reducetheuse.co.uk/?p=420

 

Ruegg, H., and Sigg, A. (1992) Dioxin Removal in a wet Scrubber and Dry Particulate Remover Chemosphere, 25, 143-148

 

Themelis, N. J., (2010) Chlorine sources, Sinks, and impacts in WTE Power Plants”, presented at Annual North American Waste to Energy conference, 2010.

UKWIN (2011)  Unpunished Breaches (2012).  Available from:  http://ukwin.org.uk/resources/health/unpunished-breaches

Venezia D, Van Brunt M, Joshi S, Szurgot A (2010) Energy from Waste and Dioxin emission control:  Is there a role for PVC separation ?  NAWTEC, Orlanda, Florida.

 

Williams, P.T., (1994) Pollutants from Incineration: An Overview IN Hester R.E. and Harrison R.M, Issues in Environmental Science and Technology – Waste Incineration and the Environment.  The Royal Society of Chemistry, Letchworth.

 

World Health Organisation (2010) Dioxins and their effects on Human Health. Available from www.who.int/mediacentre/factsheets/fs225/en

 

MSc Assignment 5

To come.