Improving Anaerobic Digestion operations. Matthew Smyth & Nigel Horan Aqua Enviro

Transcription

1 Improving Anaerobic Digestion operations Matthew Smyth & Nigel Horan Aqua Enviro

2 Front end processing of feedstocks Matthew Smyth Operations Director, Aqua Enviro

3 Feedstocks = Sources of Energy The nature and composition of the feedstock is of critical importance, it influences the overall economics of digestion, the way the plant is operated and the quality of the whole digestate. The operator should therefore understand the risks and rewards associated with different feedstocks. Organic material that can be converted into methane by AD is either composed of: Carbohydrates Proteins Lipids (Lignin) Organic waste feedstocks, which are composed of carbohydrates, proteins and lipids (fats) can be broken down by the process of anaerobic digestion to produce biogas (principally methane and carbon dioxide). The rate of conversion to methane and the amount of energy recovered (biogas) is directly related to the feedstock composition. If the operator understands the feedstock it becomes possible to set operating ranges for critical metrics to meet plant specific targets (e.g. maximise throughput, achieve PAS110, maximise methane yield). It is rarely possible, however to configure a process to meet every target, a balance must be struck. Wrap, 2010

4 COD & Energy Potential Compound Classification Composition Theoretical COD (g COD/g material) Energy potential from 1kg of compound (kwh) Glucose Carbohydrate C 6 H 12 O Alanine Protein C 3 H 8 O Glycerine Trioleate Lipid C 57 H 104 O If the chemical formula of the feedstock is known it is possible to calculate the COD (chemical oxygen demand) of the material.

5 COD & Energy Potential 1 kg of COD has the potential to deliver 3.56 kwh of energy. On an equivalent dry weight of different materials, long chain lipids have the potential to deliver up to three times the energy than the same dry mass of a carbohydrate. Waste glycerol (a by product of producing bio-diesel) is thus an attractive feedstock from an energy perspective, but may pose operating problems if the waste characteristics are not fully understood. For example waste glycerol can be rich in contaminants (e.g. methanol), of high ph and being very high strength has the potential to organically overload the digester. Operators accepting third party waste materials should have a robust quality management system in place to minimise the risk associated with trade waste imports.

6 Energy Content of methane 9.97 kwh/nm 3 methane Therefore with a concentration of methane in biogas of typically 65% - we have about 6.48kWh/Nm 3 biogas of energy.

7 Feed Carbon to nitrogen ratio Total carbon: total nitrogen ratio If N is too high (e.g. FYM) then there is a danger of ammonia inhibition If C is too high (e.g. food waste) then too much VFA production, inadequate alkalinity, and the ph will drop

8 Digester operation is the biggest influence on C:N

9 The management of ammonium-nitrogen levels in the digester is of critical importance and a challenge for operators. Nitrogen is required as a nutrient by the bacteria, but if present in too high a concentration becomes toxic. Ideally the feedstock carbon: nitrogen (C:N) ratio should be balanced to provide an optimum. Where a digestion plant recycles return liquors from dewatering (e.g. belt press or centrifuge) or digestate to the head of the process and uses it for dilution water (as opposed to e.g. fresh water) the potential for ammonia inhibition to occur is greatly increased. One of the biggest challenges facing operators is how to dilute down the feedstock which is generally in the range 20-25% dry solids to a level where contaminants can be removed and the feedstock pumped, which generally means <14% DS. This can be done with: Water, the disadvantage of this is that it increases the volume out of the digester and hence transportation costs; Recirculating digestate back to the front of the process. Whilst this minimises transportation costs digestate is rich in ammonium and salts, both of which are inhibitory to the process, will reduce the biogas yield and increase VFA levels and the RBP of the digestate.

10 Achieving PAS 110 Compliant Sanitisation and Digestate Nigel Horan Director Aqua Enviro

11 PAS110 The purpose of this PAS is twofold: to ensure that digested materials are made using suitable inputs and effectively processed by anaerobic digestion (AD) for sufficient time; and to ensure that the process has been well managed and monitored so as to produce digested material that meets market needs and protects the environment. (PAS110:2010)

12 Key elements of PAS110 It specifies:- Controls on input materials and management systems for AD process Covers AD system that accept source segregated biowastes/biodegradable non-wastes that are depackaged Minimum quality of whole digestate, separated fibre and liquor (pathogens, physical contaminants, PTEs, stability) Information required to be supplied to digestate recipients

13 Anaerobic Digestion Quality Protocol PAS 110 Waste Product Feedstock Digestion Digestate End-user Input restrictions HACCP Product quality Industry (Hazard Analysis & specification Critical Control Points) Suitable QMS and auditing procedures

14 PAS110 and pasteurisation: Why and what? Intended to minimise risk from microbiological hazards Human, animal and plant Applies to all AD processes within PAS110 Except where inputs arise, are digested, and used on the same premises or holding Site-specific criteria Non-ABP* operators can opt for one of the three standard approaches in the UK ABPRs** No deference to thermo or mesophilic *ABP-Animal By-Products **ABPR- Animal By-Products Regulations

15 ABPR Definitions of Main Caegories Many feedstocks must be treated to achieve compliance with the Animal by-product Regulations which requires a heat stage. This heat stage is based on a time temperature relationship that is dependent on the risks posed by the particular feedstock source. ABPR clearly outlines the type of wastes that fall within each of the three categories. Category Definitions Animals suspected of infection with BSE, specific risk materials, catering wastes from international transport. Manures, digestive tract content, milk not fit for human consumption, killed or fallen animals, solid particles >6mm from the wastewater stream from slaughterhouses. Default status of any animal by-product not defined as either category 1 or category 3 material. Animals fit for slaughter but not intended for human consumption. Animal by-products from food production and catering waste.

16 Digester feedstocks and their impact on the digestion process Matthew Smyth Operations Director Aqua Enviro

17 The 4 Stages of Anaerobic Digestion 1. Hydrolysis 2. Acidogenesis 3. Acetogenesis What happens Bonds in molecules are broken through the addition of water Fermentation of dissolved, complex organics into volatile fatty acids (ph ) Volatile fatty acids broken down into Acetic acid 4. Methaneogenesis Acetic acid converted into methane and carbon dioxide (ph )

18 Digester Operation VFAs (Volatile Fatty Acids) VFAs Formic Acetic Propionic and Lactic Butyric Iso-Butyric Valeric Iso-Valeric The final stage of anaerobic digestion is methanogenesis. The bacteria which carry out this reaction are sensitive to changes in the digester and are thus often the first to be inhibited or upset if the process is non-optimal. When this occurs the methanogens are unable to complete the final step of converting volatile fatty acids into methane, therefore the first thing that will be observed in the data is a change in the quantity and type of volatile fatty acids in the digestate. We want our digester to be undertaking methanogenesis if it is not the above tell us what it is doing and that tells us why!

19 Alkalinity Whether or not VFAs shift the ph is dependent upon another factor, the alkalinity. The alkalinity refers to the buffering capacity of the digester, its ability to withstand fluctuations in VFAs. The ratio of VFAs: alkalinity is an important critical metric and indicator of plant stability. Base HCO 3 - CO 3 2- OH - NH 4 + PO 4 3- Name Bicarbonate Carbonate Hydroxyl Ammonium Phosphate

20 Effect of ph on Methanogen Growth The resulting ph is critical to the performance of the digester and should ideally be within the range Should the ph fall below 6.0 or register over 8.5 there is risk of catastrophic process failure.

21 VFA (mg l -1 ) Co, Se (mg l -1 ) VFA (mg l -1 ) Co and Se dilute-out curves - VFA profile Food waste has been shown to be deficient in trace nutrients, for example selenium is a component of shampoo, but would not be expected to form part of the feedstock to a digester. It is now standard practice to add in a solution of trace nutrients for food waste digestion plants. It is important to monitor levels in the digestate over time as too many micro-nutrients can also be bad for the digester Co - measured Se - measured Co - calculated Se - calculated VFA measured Control OLR=2 Control OLR=3 Se, Mo Se, Mo, Co, W Se, Mo, Co, W, Fe, Ni Se, Mo, Co, W, Fe, Ni, Zn, Cu, Mn, Al, B Banks, 2011 Time (days) Time (days)

22 (k) Temperature % reducion in methanogenesis Temp ( C) 31% reduction in methanogenesis 7 C reduction 5 o C reduction Digestion occurs optimally between 32 and 42 o C. Should plant operation fall outside of this range the ability of the methanogenic bacteria to convert VFAs is measurably reduced. It is common practice to therefore reduce the load to the digester in this or any other scenario where the process is identified as being stressed. Failure to reduce the loading rate can exacerbate the problem and lead to an extended recovery time and unstable whole digestate, fiber or digestate.

23 Optimising digester performance and operation Nigel Horan

24 Digester Design 1. The Shell Organic loading rate Hydraulic retention time Feed solids concentration Shape 2. The peripherals Mixing Heating Feeding Gas collection Digestate removal

25 Organic Loading Rate (OLR) Takes into account the feed strength OLR = kg VS/(m 3.d) or kg TS/(m 3.d) Increase the feed strength - OLR increases, HRT stays constant Increase the feed flow rate OLR increases, HRT reduces

26 OLR (kg TS/[(m3.d)] % TS 7% TS 10% TS HRT (d) OLR is a dependent variable and a function of HRT and TS concentration. HRT is independent of the TS concentration Sludge feed is usually 4 5% (thickener limitations), thus OLR typically 3.5 to 4.5 kg VS/(m 3.d) Waste feeds up to 10% and OLR kg VS/(m 3.d)

27 Organic Loading Rate The amount of food we give the bacteria each day Too much and they don t convert it all to methane (low gross yield) Too little and it all goes to methane (high net yield) but the specific yield is low

28 Methane/kg VS added Yield and RBP (Residual biogas Potential) HRT (d) Methane production for the vast majority of wastes decays exponentially which means that the majority of methane potential in the waste feedstock can be recovered in a relatively short time. It is an operator decision as to how much of the methane is recovered, reduce the HRT and the methane potential of the remaining digested material exiting the digester increases. Increase the retention time to recover more energy from the feedstock, but note that the amount recovered decreases with time.

29 Methane/kg VS added 0.35 Yield and RBP HRT (d) A site must therefore determine its optimum HRT and OLR for its own business case.

30 VFA: alkalinity Start-up Acclimatisation To asses if the OLR is too high VFA and alkalinity levels are measured, the higher the ratio the more likely plant operation will become less stable.

31 Impact of Mixing Mixing often a major cause of problems Thick feeds (>10%) difficult to mix Mixing power often under specified (needs >4W/m 3 drawn power) Solids and plastic accumulate in digester Foaming attributed to poor mixing

32 Common Operational problems and their solutions Matthew Smyth Operations Director Aqua Enviro

33 Comparison of Mesophilic & Thermophilic Digesters Feature Mesophilic digester Thermophilic digester Loading rates Lower Higher Destruction of pathogens Lower Higher Sensitivity to toxicants Lower Higher Operational costs Lower Higher Temperature control Less difficult More difficult Source: Gerardi, 2003

34 Maintaining temperature

35 VFAs Volatile fatty acid analysis is an important parameter to measure to evaluate plant stability and performance. Increasingly there is a trend not only to measure total VFAs but also species present. Analysis of the different species helps the informed operator to understand where the process is struggling and can be optimised. The relative toxicity of VFAs varies according to the species present and the digester ph. 50% inhibition concentration ph Acetate Propionate mg/l COD

36 Operational Conditions Associated with Foam Production Condition Alkalinity increase Contributing factor Lysis of nocardiaforms or microthrix CO 2 increase VFA increase Mixing Polymers Surfactants Temperature fluctuations Scum Change in fermentation reactions Excess grease, and fats Gas vs mechanical Excess from thickening units Present within feedstock Intermittent \ slug feeding Rapid breakdown leading to increased VFA s Source: Adapted from Gerardi, 2003

37 Team working for successful plant operation Nigel Horan

38 Who does what? Smaller digesters can be package, or off-the-shelf plants and involve a vendor, eg: Dragon Anaerobic Digester But usually a bespoke design is needed involving: Consultant (designs the facility) Legal team (draws up the contract) Contractor (builds it) Operator (operates and maintains it) And sometimes venture capital is involved

39 Delivery routes The main parties are generally integrated: Turnkey delivery (probably the commonest) BOO (Build Own Operate), BOT (Build, Own, Transfer), BOOT (Build Own Operate Transfer)- (Municipalities) Service vehicles (private finance projects and a variation of the above)

40 Taking Control of the Digester Takeover is the key point for a Client, for this point onwards the digester is your responsibility So the contractually binding take-over tests must be designed to show it meets all contractual requirements (but not so arduous and expensive they can t be carried out!). Describe the tests clearly and unambiguously, keep them simple and ensure they are realistic Ensure the appropriate teamwork to make them happen (they are really important)

41 It is important to define the point at which the digester is completed at which point its operation is solely your responsibility and any outstanding retention payments are made. It is crucial by this stage to ensure it is fit for purpose and will do what you expect of it. The role of takeover tests is to demonstrate this. There is a tendency by Clients to provide an arduous and complicated round of takeover tests under the impression it provides them better protection. Often at the point of takeover, they cannot be undertaken, because (for instance) the volume of feedstock is not available, or the cost of testing is simply to expensive. Keep them simple and cheap and they will be more effective.

42 Preparing for the Worst The contract protects both the Client and Contractor But often favours the Contractors (lack of data to meet contractual onus) So check any contractual protections Many routes for contractual disputes Mitigation Mediation Arbitration If things go badly wrong, the courts are the last resort