Perimeter soil gas emissions criteria and associated management

Transcription

1 Ltd Perimeter soil gas emissions criteria and associated management

2 Using the ICOP 1. Background concentrations 2. Action limits 3. Compliance limits 4. Investigating gas migration 5. Mitigation options and verification strategies Brief quiz at the end of each section. For 3 lucky winners (drawn out of hat if more than 3 with all correct answers) there will be a 2GB memory stick prize 2

3 What migrating gas can do.. The Darvaza well known locally as Hells Door 3

4 Introduction - conceptual model Distances to residential areas, agricultural or urban sites Waterways Groundwater elevations Geological and hydrogeological conditions - permeability of potential gas migration pathways Risk of flooding, subsidence 4

5 Background concentrations Easy to set at sites with no background methane or carbon dioxide concentrations Difficult at sites with external sources of methane and carbon dioxide Difficult at or adjacent to old dilute and disperse sites the ICOP may not be suitable for these sites 5

6 Setting background can be tricky 6

7 Confounding factors No pre-disposal data? One part of site has different background to another? Active ground gas cycle? Intermittent mines gas from coal and shale seams? CO 2 natural respiration in topsoil, degassing from limestone and chalk Historically, well condition and data quality poor? 7

8 Changing background Lining a landfill changes the local background conditions because gas generated in the subsurface that has previously diffused to surface unimpeded now has to migrate around the impermeable lining 8

9 Key phase to collect background New cells must have perimeter boreholes before liner placed Four week period between the end of liner construction and waste deposit -intensive background monitoring is recommended Prior to the onset of methanogenic conditions within specific cells (should be cross referenced to on-site data) 9

10 Setting background (1) Background concentrations will be set on a monitoring well basis This has two advantages: 1. No discussion of zonal boundaries; 2. No complicated statistics are required to deal with multiple datasets that may not have equivalent data There is still merit in looking both at location specific and site/zone data 10

11 Setting Background (2) Sampling frequency is monthly Ideally 2 years background data prior to waste placement. 1 year minimum to understand seasonal variations. High monitoring frequency required after a liner installed (and before waste is placed) background data points a minimum before statistics employed 11

12 Setting Background (3) Background is the T max concentration stable datasets unstable datasets 12

13 13

14 Two dataset types Stable datasets with an approximately normal distribution - statistics can be used (later presentation) Unstable datasets - greater than 10% v/v fluctuations in soil gas concentrations may occur in short periods of time. Such data may exhibit a saw tooth pattern over time. 14

15 Stable dataset Background is the T max concentration T max CO 2 is 4.0%, T max CH 4 is 0.0% 15

16 Unstable dataset Mean: 6%. 95th percentile:19%. 95th percentile limit to maximum data value is >10% v/v 16

17 15% filtered unstable dataset Mean: 21%; 95%ile: 26% T max : 27% After filtering, statistics can be applied 17

18 Other unstable datasets Some datasets do not lend themselves to the use of statistics One common example is a dataset that has mainly zero values with an occasional peak that immediately disappears these will be outliers and removed so background will be zero. CCS scoring will need to account for rapid return to zero 18

19 Questions 1. When is it most important to get a good background dataset? 2. What is the minimum recommended number of datapoints before statistics are employed? 3. What type of site won t fit into the ICOP methodology? 4. What is the background concentration? 19

20 Action Limits 20

21 Action Limits (operator) First indication that conditions are outside the norm maybe that the landfill not operating properly, or natural variation Requires some action or attention from the operator Gas Management Plan Gives operator time to react before Compliance Limits breached 21

22 Action Limits (regulator) Action Limits not included in EPR permit Required as an integral part of a site s gas management plan If Action Limits exceeded, regulator not required to be involved, but if Compliance Limits are subsequently breached, then the actions taken will be scrutinised 22

23 Setting Action Limits Should allow for naturally occurring variation in methane concentrations from baseline conditions; and Should give sufficient time to take corrective or remedial action before regulatory Compliance Limits are breached 23

24 Action Limits - stable environments T max (background) methane concentration plus 0.5% 1% carbon dioxide above the T max if it is less than 5% 2% carbon dioxide above the T max if it is between 5 10% etc No Action Levels for T max carbon dioxide concentrations above 25% 24

25 Action Limits - unstable environments»unstable environments are suggested to occur when the range in concentrations values (between high and low data) is >8% For every well the action level for methane or carbon dioxide will be the T max concentration 25

26 Re-monitor First response Is the reading repeatable? Is the well and the sampler fit for purpose? Is the gas field balanced, with suction being applied to perimeter wells Has a low pressure front just come through? Has it been raining a lot? Are the engines performing - has total gas flow decreased recently? 26

27 Is well fit for purpose? Valve or end cap is damaged, seized, open or missing prior to sampling Well cap is loose or a perished rubber bung If the well cap is threaded, it is not tight and it doesn t have an o-ring seal If it is a combined well, Waterra tubing is protruding and there is no well cap No hydrated bentonite around the well Slotted screen section visible above ground Annulus has cracked or ground has dropped away around edge of well 27

28 Is sampler trained? Well Differential pressure before Carbon dioxide % Carbon dioxide trend Methane % Methane trend Oxygen % Differential pressure after Comment Stable 0 Absent Balance gas >85% Stable 0 Absent Balance gas >82% Peaked then Slight fall Slight increase 0 Absent Low permeability. Close to ambient air composition. 0 Absent Similar to ambient air composition Stable 0.6 Falling Similar to ambient air composition Slight increase 2.2 Falling Balance gas >80.5% Stable 0 Absent Balance gas > 88% Stable 0 Absent Balance gas >80.5% Stable 0 Absent Balance gas >82.5% 28

29 If Action Limit breached Refer to Gas Management Plan this should have pre-defined actions Re-monitor Check data quality (see above) Increase monitoring frequency Check easy wins gas abstraction system More actions discussed later 29

30 Gas Management Plan actions 30

31 Timeline Medium risk site Outcome Action Deadline Additional Monitoring Concentration above action level Re-monitor 48 hours Concentration above action level Check gas field 48 hours Concentration still above action level Verify conceptual model and plan for extended pathway assessment, if required 1 week Extended pathway assessment Concentration above action level Investigate sources and pathway 2 week fieldwork, 4 weeks report Concentration above action level Concentration still above action level In depth assessment of containment performance Verify conceptual model and review system performance. 3 week fieldwork, 4 weeks report 5 week report 31

32 Questions 1. What is the first thing to do if an Action Limit is breached? 2. For the purposes of setting action limits, what % difference between low and high concentrations indicates unstable conditions? 3. What is the most common problem with well infrastructure identified during monitoring? 4. What is the offset for methane above background (stable dataset)? 32

33 Compliance Limits 33

34 Compliance Limits These will be in the permit (can still use existing LFTGN03 limits) Breach taken as indication that a compliance failure of gas control has occurred Above a compliance limit presumption that elevated methane is from the landfill until proved otherwise 34

35 Setting Compliance Limits For every perimeter well under post-critical gas production conditions, the Compliance Limit will be 1% methane above the T max methane concentration No Compliance Limits where background methane is > 10% No Compliance Limits for carbon dioxide 35

36 Unstable datasets ICOP methodology not appropriate 36

37 Gas Management Plan actions 37

38 Safety first If potential receptors are close to the landfill and if there is any uncertainty then take the safety first option, which would be to instigate actions in the next box down 38

39 Timeline High risk site Outcome Action Deadline Additional Monitoring Concentration above compliance level Re-monitor 6 hours Concentration above compliance level Check gas field 24 hour Concentration above compliance level Verify conceptual model and plan 48 hour for extended pathway assessment, if required Extended pathway assessment Concentration above compliance level Off-site receptor analysis and risk 1 week action plan report Concentration above compliance level Investigate sources and pathway 1 week fieldwork, 3 weeks report Concentration above compliance level In depth assessment of containment performance 2 week fieldwork, 3 weeks report 4 week report Concentration still above compliance level Verify conceptual model and review system performance. Concentration still above compliance level Additional contingency actions 4 week report 39

40 Timeline for event LANDFILL OPERATOR ACTIONS ACTION LEVEL COMPLIANCE LIMIT Tmax +0.5 % Methane Tmax + x% carbon dioxide Or Tmax carbon dioxide ACTION e.g., resampling Tmax +1% Methane INCREASED ACTION* ENVIRONMENT AGENCY ACTIONS ACTION LEVEL COMPLIANCE LIMIT Potential CCS SCORE Tmax +0.5% Methane Tmax + x% carbon dioxide Or Tmax carbon dioxide Tmax +1% Methane NO ACTION ACTION** 40

41 Dilute and disperse sites ICOP methods may be appropriate but should accurately reflect the site conceptual model Investigation and mitigation methods may be different Proportionate risk based response may be appropriate (next slide) 41

42 Hazard assessment methodology British Standard 8485:2007. This uses the concept of hazardous gas flow rate Q hg Q hg = C hg /100 * q 42

43 Questions 1. What is the methane off-set above background for Compliance Limits? 2. Above what methane concentration are limits not appropriate? 3. Above a Compliance Limit, where is the gas presumed to have come from? 4. Are LFTGN03 methods still valid? 43

44 Investigating gas migration 44

45 Gas migration Gas migration in soil is primarily controlled by pressure gradients - along paths of least resistance, from areas of high to low pressure The rate of gas migration depends on the gas pressure in the source area, and the effective gas-phase permeability of the subsurface Diffusive transport is typically much lower than pressure-driven flow in a permeable porous medium 45

46 Controlling characteristics Length of flow path Formation permeability (equivalent porous media or fracture controlled) Permeability variations (i.e., sand and clays); Presence of a near-surface capping layer Water table depth and fluctuation The presence of barriers to lateral flow Preferential pathways 46

47 Conceptual model At operational and closed landfills the application of a negative pressure should mitigate against gas migration The potential for gas migration out of the landfill only exists in pockets of waste that are not under the influence of the gas abstraction system 47

48 Physical conceptual model Atmospheric pressure Recent weather Any activities in the area that may affect readings Pressure within the well Reading variability Any odours Groundwater levels Flow rate Oxygen Balance gas - any input of landfill gas comprising ~ 60% methane and 40% carbon dioxide would displace and reduce the 80% of nitrogen in the pore space Graphs should be used to identify trends 48

49 Detailed investigation Trace component testing CH 4 : CO 2 ratio Borehole purging Helium Isotopes carbon and tritium (best if enough CH 4 ) 14 C Activity (Percent Modern) δ 13 C (%o) CH 4 CO 2 CH 4 CO 2 GA ± ± Well ± ± H of the CH 4 Bq/litre H 2 O (TU) 1730 ± ± ± ±

50 Source term management Total abstraction m 3 - gas losses or volume increasing? Site vacuum, operation of carrier mains, is the gas mix at the Plant richer than normal etc. Assess status of gas extraction wells (open, percentage open, closed) and condition is satisfactory If wells open and well-maintained but gas production has increased, check for interference suction on wells Condensate build up in gas carrier pipe work If operational model has changed, full performance assessment may be required 50

51 Quick reference Process Likely Gas Migration Unlikely Gas Migration Differential High pressure in the Pressure greater landfill, low pressure in than 30mB the Borehole in the Borehole Presence of Helium Presence of Chlorinated hydrocarbons in off-site wells Carbon isotopes If no Helium is found, this does not mean that the gas has migrated Chlorinated hydrocarbons are most likely to have come from the landfill 14 C is found to be modern and the gas is biogenic Low pressure in the landfill, High pressure Helium present in the borehole. Low 14 C level, not biogenic Tritium isotope High Tritium level. Background Tritium Level Carbon Dioxide 60:40 CH 4 : CO 2 ratio High flow with either ratio. with high flow CH 4 or CO 2 but not both Comments Diffusion can drive a gas against a pressure gradient. Not all non-landfill gas sources contain Helium. If no chlorinated hydrocarbons are found, they may not be present in the landfill, or may be adsorbed en route. Data is rarely conclusive for CO 2 because this does not discriminate mixed sources or influences Need enough methane to get adequate tritium sample If the gas is flowing quickly over a short distance the amount of adsorption or methane oxidation will be limited. 51

52 Questions 1. What is the primary controlling factor for gas migration? 2. What other factor controls the rate of gas migration? 3. What gas ratio can be worth investigating? 4. What is the most useful isotope and what is the limitation of this technique? 52

53 Mitigation strategies 53

54 Cost should be proportionate to risk Risk Gas into building Gas into confined space Gas off-site Adverse CCS score Cost Maximise gas field efficiency Additional monitoring Additional on-site gas abstraction Off-site gas abstraction Risk Ranking Very high High Medium High Cost Ranking Low Medium High Very high 54

55 CIRIA guidance Risk to housing 55

56 Risk to confined spaces The majority of the incidents associated with landfill gas migration involve the accumulation of gas in confined spaces, such as within buildings, culverts, and manholes 56

57 Risk of gas off-site CCS scoring (see later presentation) Corporate image 57

58 Off-site gas management If on-site gas management is as good as it can be and there is still a loss of containment, then an off-site gas management plan will be required Two types Passive systems are not normally used to manage gas and are now typically viewed as a poor solution 58

59 Active systems Well or trench vents extract gas and form a negative pressure gradient, or air is injected to form a positive pressure gradient Use biofilter to oxidise abstracted methane Biowindow to release pressure in area of poor gas control 59

60 Summary Background concentrations being set on a well by well (or zonal) basis Statistical techniques are proposed to define background limits for stable and unstable data No Compliance (formerly trigger) Limits need be set for carbon dioxide Carbon dioxide data should continue to be collected and assessed against a lower Action (formerly control) Limit 60

61 Questions 1. What is the off-site location where the majority of LFG gas incidents occur in? 2. What is required when all gas management options have been explored and there is still migration? 3. What kind of system is now viewed as a poor solution? 61