Anaerobic digestion = biogas process

Transcription

1 Anaerobic digestion = biogas process Input Manure Organic household waste Industrial waste Sludge Products Energy: in the form of biogas (methane) Substitution of existing energy production Fertilizers: Substitution of fertilizer production 1 Copyright Anders Damgaard & Morton A. Barlaz, NC State University 1

2 Introduction Focus on OFMSW (Organic fraction of MSW) 1970s: tried to digest mixed MSW and it is not a good idea All feedstock issues from aerobic processes apply purity participation Methane recovery is major advantage Copyright Anders Damgaard & Morton A. Barlaz, NC State University 2

3 Anaerobic Processes Solids Content Number of stages Temperature Operation strategy Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 3

4 Solids Content Wet digestion <20% solids Add biosolids, recycled process water or some other liquid Dry Digestion > 20% solids Still may be adding some liquid Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 4

5 Number of Stages Recall anaerobic biodegradation process Hydrolysis Fermentation/acidogenesis Acetogenesis Methanogenesis Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 5

6 Anaerobic conversion to biogas Initial hydrolysis of particulate matter and larger molecules Fermentation (acidogenesis) (formation of acids) generating primarily acetate but also other Volatile Fatty Acids (VFA) Acetogenesis (formation of acetate), Hydrogen is used as an electron acceptor Methanogenesis Acetate CO 2 + CH 4 (major pathway app. 70%) 4H 2 + CO 2 CH 4 + 2H 2 O 6 Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 6

7 Number of Stages In a two phase system, hydrolysis and acidogenesis occur in one stage and acetogenesis/methanogenesis occur in stage 2 First stage less sensitive to ph Optimize mixing, retention times, loading rates (gm VSS/m 3 ) Better control of methanogenesis which is more sensitive Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 7

8 Psychrophilic 5 25 o C (typically ambient temperature) used for low technology treatment without heating Temperature range Mesophilic o C (typically o C) used in many process applications for household waste, industrial waste, and animal slurry with a high content of ammonia Thermophilic o C used for high yielding processes, often with more advanced process technology and where sanitation is required Faster Uses more energy (from the biogas) Pathogen reduction Different temperatures for each stage TPAD = temperature-phased anaerobic digester 8 Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 8

9 Batch vs. continuous Operating Strategy Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 9

10 Commercial Systems Primarily Europe DRANCO and Kompogas Continuous, high solids, thermophilic, single stage BIOCEL Batch, high solids, mesophilic BTA Low solids, upflow sludge blanket, continuous Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 10

11 Post Digestion The solids will require additional stabilization Aerobic windrow curing (5-25% methane potential left) Contaminant removal Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 11

12 Post Digestion Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 12

13 BTA Process: Single Stage Copyright: Anders Damgaard & Morton A. Barlaz, NC State University 13

14 BTA Process Mechanical wet pretreatment Feedstock and recycled process water mixed into a pulp Heavy fraction trap Grit removable and then pump slurry to single- or multi-stage reactor In multi-stage system the pulp is separated into a solid mass and liquid phase Solid is treated in a hydrolysis reactor for 4 days, dewatered and the liquid goes to a methane phase reactor with a 2-day retention time Copyright Anders Damgaard & Morton A. Barlaz, NC State University 14

15 East Bay Municipal Utility District (EBMUD in Oakland) Excess digestion capacity as food processors closed Food waste from San Francisco where food waste separation is now required CA air permit limits on turbines They are flaring some gas while engines sit idle!! Copyright Anders Damgaard & Morton A. Barlaz, NC State University 15

16 Collects SSO Expanding Copyright Anders Damgaard & Morton A. Barlaz, NC State University 16

17 Apeldoorn, The Netherlands There is landfill ban on anything that can recycled, burned or treated biologically Landfill tax is 90 Euros ( 1 = $1.30) per metric ton for MSW Pay as you throw is in place for waste compostables and recyclables are collected for free Biowaste = kitchen waste + green waste They have excess WTE capacity and the operators are looking for tonnage Copyright Anders Damgaard & Morton A. Barlaz, NC State University 17

18 The Waste Receive 230,000 metric tons of biowaste Recently contracted with McDonalds More food waste in winter, more garden waste in summer I was there on first nice week in April and lots of green waste Food waste has more methane potential than yard waste Copyright Anders Damgaard & Morton A. Barlaz, NC State University 18

19 The Waste They estimate about 50% participation Feedstock quality is good, some plastic is present and they try to remove it mechanically In larger cities with more people living in apartments, product quality and participation are both lower Copyright Anders Damgaard & Morton A. Barlaz, NC State University 19

20 The Process The 230,000 tons/year are screened Material passing a 70 mm screen goes to AD (70,000 tons/yr) Other material is shredded and composted directly (160,000 tons/yr) The overs contain a lot of woody material which has low methane potential so they do not want it in the AD facility Copyright Anders Damgaard & Morton A. Barlaz, NC State University 20

21 The Process The material for AD is digested in large static tanks for 20 days It is turned by a shaft with arms extending out off the shaft Next it is dewatered in a screw press and then sent to composting piles for curing They pull air in through the compost piles All air exposed to waste is captured and sent to biofilters By pulling, they can use an outdoor operation Copyright Anders Damgaard & Morton A. Barlaz, NC State University 21

22 The Process They estimate 20% sand by mass it accumulates and reduces effective volume in tank The process is operated at about 30% moisture (semi-dry); they add water to the process This water comes from the screw press and is recycled Copyright Anders Damgaard & Morton A. Barlaz, NC State University 22

23 Markets They sell product in bulk as potting soil Market is excellent (remember that Holland is a major flower growing country) The product offsets peat mining Copyright Anders Damgaard & Morton A. Barlaz, NC State University 23

24 Final Comment There was a lot of process engineering Moving parts to move solids Adding feed while maintaining anaerobic conditions in tanks Why Anaerobic Digestion? Landfill ban on degradable organics When the company bids on contracts, the cities typically require a statement of the carbon footprint Note that they recover both methane for energy conversion and a soil amendment Copyright Anders Damgaard & Morton A. Barlaz, NC State University 24

25 Greenhouse Gas Considerations Avoided methane from landfill disposal How this factors in is a function of as collection and utilization Food waste decomposes rapidly so presumably little methane collection at a landfill Compost consumes energy Some fertilizer offset Copyright Anders Damgaard & Morton A. Barlaz, NC State University 25

26 Copyright Anders Damgaard & Morton A. Barlaz, NC State University 26

27 Copyright Anders Damgaard & Morton A. Barlaz, NC State University 27

28 Global Warming Potential associated with each subprocess used to compost 1 ton food waste lb yard waste Copyright Anders Damgaard & Morton A. Barlaz, NC State University 28

29 Avoided Emissions, GWP and Energy Use Associated with the Production of Fertilizer from One Ton of Food Waste CO (lbs) SO 2 (lbs) NO x (lbs) Total PM (lbs) Energy Use (kbtu) GWP (lbs CO 2 ) Copyright Anders Damgaard & Morton A. Barlaz, NC State University 29

30 Global Warming Potential (lb CO 2 ) and energy use in kbtu from managing 1 ton food waste lbs of yard waste Copyright Anders Damgaard & Morton A. Barlaz, NC State University 30

31 Anaerobic Digestion: Limiting Factors Economics of the process Economics of collection Stimulating factors Carbon tax or cap and trade policy that put a cost on methane emissions Value of energy Copyright Anders Damgaard & Morton A. Barlaz, NC State University 31