Opening up the Gas Market (OGM) Project Oban - Laboratory and Field Testing Results

Transcription

1 Opening up the Gas Market (OGM) Project Oban - Laboratory and Field Testing Results SGN, Kiwa Gastec, D Lander & DNV-GL Mark Crowther Kiwa Gastec

2 Partial pre-mixed burner

3 What is Wobbe Index? Wobbe Index (WI) is Indicative of the energy input into an appliance and is expressed at the same metering and combustion conditions (in UK 15/15 C) as follows: WWWWWWWWWW IIIIIIIIII = GGGGGGGGGG CCCCCCCCCCCCCCCCCC VVVVVVVVVV RRRRRRRRRRRRRRRR DDDDDDDDDDDDDD

4 Effect of gas composition on Wobbe Index In ordinary natural gas (mostly methane), the WI of MJ/m3 is increased by the presence of ethane (C2), propane (C3), butane (C4) and higher hydrocarbon fractions (C5 and above). Conversely, the presence of carbon dioxide, nitrogen or other inert gases in natural gas decreases the WI. Gas % methane ethane propane nitrogen Wobbe Index (MJ/m 3 )

5 Appliance testing regimes Since the adoption of the Gas Appliance Directive 90/396/EEC in 1996 all new appliances gas produced in GB that use natural gas are tested with the following gases 1, which have Wobbe Index range exceeding the current GS(M)R limits: G20 (50.72 MJ/m 3 ) G21 (54.76 MJ/m 3 ) G23 (45.66 MJ/m 3 ) All appliances have met rigorous safety, efficiency and emissions criteria. In theory, GB could follow the proposal of EASEE gas and (for example) used in Spain and permit distribution of gas with a wider WI range, BUT it is known that a wide range of suboptimal conditions (such as burner corrosion/distortion ) can produce higher levels of CO than seen during laboratory test work on new appliances, and the GB gas industry is proud of the reduction in CO seen in recent years. Hence the laboratory and field trial test programmes. 1 gas composition and properties at 15 C, mbar as listed in BS EN 437: A1: 2009

6 Laboratory test programme A total of 18 appliance types (local space heaters, central heating/combination boilers, domestic cooking and commercial catering appliances) were tested in Kiwa laboratory with test gases G25, G23, G20 and G21 Example data is shown in following slides:

7 Laboratory data for partial pre-mixed burners - older boiler designs and hob burners Partial pre-mixed boiler Both burner types are partially pre-mixed and have fixed aeration. Cooker hob Gas leaving the injector draws air from the atmosphere into the venturi/mixing tube and thoroughly mixes with gas ensuring a constant gas/air ratio prior to combustion. The amount of combustion air supplied is dependent on the gas flow/pressure entering the venturi/mixing tube.

8 Laboratory data for gas fires and fully pre-mixed boilers Gas Fire For some gas fires CO emissions and CO/CO 2 ratio decreases with increasing WI. Fully premixed boiler For fully pre-mixed boilers the gas-air ratio is pre-set (for maximum thermal efficiency) and at extremes of gas WI (high or low) CO emissions and CO/CO 2 ratio increase creating the characteristic Bath-tub shaped emission profile as WI changes.

9 Interim conclusions from laboratory tests Domestic and small commercial appliances correctly installed, serviced and operated can safely burn gas with WI of up to MJ/m 3. BUT in real installations, appliance combustion performance is often sub-optimal, so allowances need to be made. This is termed Headroom Analysis of the laboratory test data indicates that the headroom should be 1.5 MJ/m 3 i.e. a maximum upper WI of MJ/m 3 would give adequate headroom to account for sub-optimal performance in the field.

10 Quantifying headroom This headroom allows for factors such as: Differences in the temperature of the combustion air temperature and gas temperature. Performance of Atmospheric Sensing Devices - Oxygen Depletion sensors fitted to open flued appliances The effect of gas WI on ASD activation point is shown below: Fig. X Effect of WI on the ASD activation point at 53.5 MJ/m 3 appliance meets CO limit Wobbe Index (MJ/m 3 ) CO limit Appliance age/condition the combustion performance of older appliances and appliances in poor condition are more sensitive to small gas WI changes. Adjustment of Gas/air ratio controls on full premixed boilers

11 Final conclusions from laboratory tests An upper WI limit of MJ/m 3 allows sufficient headroom for any deleterious unknowns in the field condition of the appliance. Increasing the WI to MJ/m 3 has negligible impact on the efficiency, performance and life of a domestic or small commercial appliances. The resultant WI is only modestly above the current Emergency Limit of MJ/m 3 and still lower than the German upper limit of MJ/m 3 The lower WI should be retained at MJ/m 3

12 Field testing 903 residential and commercial properties were visited and a total of 1280 appliances were supplied with test gases G20, G21 and G23 from a mobile test rig (below). Mobile test rig

13 Test data for partial and fully pre-mixed boilers 413 partial premix boilers 244 fully premix boilers (air/gas ratio)

14 Test data for flued space heaters and cooker hobs 212 Flued space heaters 661 Cooker hobs

15 Test data for cooker grills and flued tumble driers 82 Cooker grills 10 flued tumble driers

16 Burner safety sensitivity analysis The field trial data can be used to interpolate the likely increase in CO emissions when changing from a WI of MJ/m 3 to MJ/m 3 to indicate the sensitivity of particular appliances/burners to changes in gas WI as shown in the charts for flued and flueless appliances in the following slides. The charts are split into 3 zones, green, amber and red. An appliance that is positioned in the green shaded area is considered to be acceptable, could be considered at risk if positioned in the amber area and immediately dangerous if positioned in the red area.

17 Burner safety sensitivity charts for flueless and flued appliances

18 Burner safety sensitivity charts for flued appliances categorised by flue type

19 What is carbon monoxide? Carbon monoxide (CO) is a colourless, odourless gas created by the incomplete combustion of natural gas. It combines with the blood to form carboxyhaemoglobin which stops oxygen transfer and causes ill health and eventually death. Whilst a very toxic and dangerous gas at high concentrations, street levels can be up to 7ppm 1 and the Health and Safety Executive (HSE) sets a long term (8 hour) exposure limit of 30ppm and short term (15 minute) exposure limit of 200ppm 2. Interestingly, smokers exhaled breath contains high level of CO 3 : Smokers 17ppm Passive smokers 5.2ppm Non-smokers 3.6ppm 1. DEFRA 2. EH40/2005 Workplace exposure limits 3.

20 CO fatalities from appliances using mains gas in GB The real question is: Does widening the permitted WI range materially affect the risk of CO incidents? Assumed that numbers of CO incidents are proportional to the fraction of appliances with CO levels above an arbitrary level, but incidents have continued to fall since records began, despite little change in WI of distributed gas (in fact WI has risen over the period since 1990). Issue is complex, but suggests that high proportion of incidents are associated with factors other than gas quality

21 Why flue gas CO is not room CO In practice: Flueless appliances have small gas input rates, and only produce a small volume of combustion products, typically with low CO emissions. Room sealed appliances CO in the combustion products have no direct path to room. Open flued evacuate combustion products via a chimney and have atmospheric safety devices in the event of blockage/partial blockage of the flue.

22 Conclusion flue CO vs room CO Although the burner sensitivity graphs show appliances with high CO emissions that exceed the HSE long term (8 hour) exposure of 30ppm, it must be remembered that: Flued appliances are designed and installed to vent their products of combustion outside of the property. Room sealed appliances are sealed relative to the room in which they are installed and there is no direct path for the CO to enter the living space. All flueless appliances are either of very low gas input and/or for intermittent use.

23 Potential increased risk from flueless appliances The combustion products of flueless appliances are dispersed directly into the room where the appliance is installed. BUT to understand whether the flueless appliances would still lead to dangerous situation it is necessary to take into consideration a number of other key factors. such as: CO exposure model developed in order to aid assessment of the risk posed by an appliance emitting flue gases containing a given level of CO. Model used to establish the theoretical room concentration using the emissions data of a grill with a net heat input of 2kW producing CO of 2500ppm, operating for 24 minutes in a room of 17m 3 with a 0.25 room air changes per hour.

24 CO exposure model from a faulty grill The model predicts that when the grill is operated for 24 minutes the maximum peak concentration will be 100ppm, which is less than the HSE short term (15 minute) exposure limit of 200ppm Exposure concentration of CO C(t), mg/m time, h In practice kitchen doors and windows will be open and additional air changes will occur.

25 Examples of appliances with pre-existing Faults Of the 903 properties visited, there were very few appliances that were considered to be emitting combustion products (CO/CO 2 ) into the living space and potentially harmful to the user. Two of the worst examples are shown below. gas fire with smashed glass A corroded boiler - reported to be setting off the CO detector which the householder was ignoring. Unfortunately the appliance was deemed too hazardous to test.

26 Conclusions The field trial confirmed the results of the laboratory testing and demonstrated that: Domestic and small commercial appliances correctly installed, serviced and operated can safely burn gas with WI of up to MJ/m 3. An upper WI limit of MJ/m 3 allows sufficient headroom for any deleterious unknowns in the field condition of the appliance. Maintenance of the lower WI at MJ/m 3 (or more realistically MJ/m 3 because of the thermal regulations) allows safe adjustment of fully pre-mixed burners with any gas supplied in this range.

27 Conclusions Using Oban as a statistical representation of GB it is estimated that: 4% of the GB appliance population would be classified as at risk against the Unsafe Situations Procedure. 2% of the GB appliance population would be classified as immediately dangerous against the Unsafe Situations Procedure There are an extremely small number of already unsafe appliances (pre 1996) producing high levels of CO. Laboratory tests performed on unsafe appliances removed from Oban appliance population found that whilst CO emissions increased as a result of WI increases, this was not considered to materially increase the risk to the occupant. It is the view of Kiwa Gastec that renewed efforts should be made to identify and replace these appliances.

28 Thank you Questions?