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Risk and mitigation of self-heating and spontaneous combustion in underground coal storage
Journal of Loss Prevention in the Process Industries 25 (2012) 617e622
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Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Risk and mitigation of self-heating and spontaneous combustion in underground coal storage Juha Sipilä a, d, *, Pertti Auerkari b, Anna-Mari Heikkilä b, Risto Tuominen b, Iris Vela c, Jyrki Itkonen a, Mikael Rinne d, Kalevi Aaltonen d a
Helsingin Energia, FI-00090 Helen, Finland VTT Technical Research Centre of Finland, FI-02044 VTT, Finland Bundesanstalt für Materialforschung und eprüfung, Berlin, Germany d Aalto University, FI-00076 Aalto, Finland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2011 Received in revised form 25 January 2012 Accepted 25 January 2012
While the self-heating and spontaneous combustion of coal is a known challenge at coal mines and storage sites, there are known methods for mitigating this challenge for typical open stockpile storage. However, closing the storage will reduce access for corrective action, and it is then important to manage the storage and its transport system with added attention without unduly adding cost or hindering availability. This paper aims to discuss the risk, prevention and extinguishing of fires in closed coal storage facilities, particularly in light of the experience with the Salmisaari underground rock storage facility in Finland. The observed autoignition events have indicated an array of contributing factors, some of which are unique to underground silo storage facilities. On the other hand, many features of the storage facilities can be compared with other extant closed storage systems. The factors affecting fire risk are described and the associated fault and event trees are outlined for autoignition at underground storage. Drawing upon the experiences with past events of self-heating and spontaneous combustion, recommendations are given on cost-effective preventive, corrective and other mitigating action for minimising fire risk and promoting storage availability. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Coal Underground storage Fire Risk Mitigation
1. Introduction The underground coal storage facility at the Salmisaari CHP power plant in Helsinki has been operating since 2004, with a total capacity of 250 000 tons of bituminous coal. The facility consists of four silos, B40 m 65 m each, with the silo bottom at a depth of 120 m, which are carved into granite rock that is strong enough not to need additional support structures. The advantages of a closed underground storage facility in comparison to the earlier above-ground open stockpile include automated lower cost operations, much reduced dust, noise and loss of heat content, less air ingress for a lower fire risk, and improved aesthetics. An important advantage was also freeing up urban real estate (about 100 000 m2) close to the city centre. The advantages are partly balanced by more limited access for any unplanned action, tighter requirements on safety systems and monitoring, and the cost of construction.
* Corresponding author. Helsingin Energia, FI-0090 Helen, Finland. Tel.: þ358 40 3346663. E-mail addresses: juha.sipila@iki.fi, juha.sipila@aalto.fi (J. Sipilä).
Self-heating and autoignition is not uncommon in the aboveground coal piles (Carpenter, Porter, Scott, & Walker, 2003; Nalbandian, 2010; Smith & Glasser, 2005), and the hazard must be accounted for also in a closed storage that, by definition, will provide reduced access for any mitigating action. In this paper, we summarize the experiences and views on the risk, prevention and mitigation of autoignition and spontaneous combustion incidents in closed coal storage facilities, with particular reference to and emphasis on the Salmisaari underground coal storage facility. With sufficient access to oxygen (air) and restricted heat removal, coal may self-ignite in storage as a result of internal heating by oxidation (Fig. 1). The process of self-heating and ignition tends to develop slowly and result in smouldering fires that can be challenging to extinguish without special precautions in the design and operation of the storage facility. Both external, or storage design, and conditions-related, and intrinsic (coal-related) factors will influence the likelihood of selfheating and ignition of stored coal. Known intrinsic factors that promote self-heating include, for example, low rank, high volatile content, high porosity, small particle size and high alkali content in ash (Beamish, Barakat, & St George, 2001; IMO, 2009; Nalbandian,
0950-4230/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2012.01.006
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Fig. 1. Evolution of a spontaneous coal fire: a) coal pile with varying particle sizes; b) initial self-heating by air ingress; c) heat loss by conduction, convection and radiation not exceeding self-heating; d) heating to a critical temperature to ignite a fire (adapted from DOE HDBK-1081, 1994).
2010; Sipilä & Auerkari, 2010; Sipilä, Auerkari, Heikkilä, Krause, 2010). Not all of the influential factors from the different batches of coal that have been delivered are known or routinely measured, and, therefore, it may not be self-evident how to rate the risk of self-heating. The other important issue has to do with preventing or extinguishing any initiated overheating or fire efficiently before significant losses or damage can occur. For this purpose, it is necessary to detect critical events, in other words, monitor the status of the storage facility regarding any signs of developing hot spots. Fortunately, this can usually be restricted to certain key locations to sufficiently characterise the condition and status of the whole storage facility and its transport systems. In addition, spontaneous combustion is also less likely if suitable scheduling and stacking is applied in filling and discharging the storage facility. Several current sites are using enclosed storage facilities to store coal at power plants. In 2008, elevated CO level from the silo no.4 of the Salmisaari facility indicated an initiated smouldering fire that resulted in damage to the hopper bellows and wall shotcrete of the silo containing Russian low sulphur coal, with an about 1 m layer of old Polish coal at the bottom. While the plant production was not limited during the incident, complete fire extinguishing took about four months by alternate nitrogen filling and coal discharging until the silo was empty. Thermal imaging from above showed a hot spot close to the bottom maintenance door. Discharged coal was often hot and to protect the conveyor belt, a layer of cool coal from another silo, and when necessary, belt watering was applied. Spalling of the shotcrete in the silo wall was observed on about one third of the silo area. Partly burnt coal, ash and slag indicated air ingress through the seals of the bottom maintenance door, where the measured outer surface temperature was about 350 C during the fire. City inhabitants living in the vicinity filed complaints about flue gas fan noise and sulphurous smell, and this was naturally considered harmful for the public image of the plant. Apart from possible particle size segregation to provide internal air passages, and relatively lengthy time of coal in storage (more than six months), a major contributing factor to the fire and its tedious extinguishing was the air leak through the maintenance door. Limited access for early detection (monitoring) and mitigation can be considered to be a specific features of the closed underground coal storage, influencing the size of the fire by the time of alarm. Furthermore, the coal grade may contribute to the likelihood of such an incident (see below). After introducing better bottom sealing against air ingress, no fires have occurred on a similar scale, in spite of leaving, for example, the sensor and alarm systems and the use of the storage or safety related practices and equipment unchanged. The safety system for protecting the personnel and equipment of the underground storage facility includes features that have been described in more detail by Sipilä et al. (2011). This paper is discussing aspects of risk, prevention and extinguishing of
fires in closed coal storage, particularly in light of the experience with the Salmisaari underground rock storage facility. 2. Recent experience on managing self-heating and fires in closed coal storage facilities Table 1 shows selected general issues of importance when considering potential self-heating and fires (and even dust explosions) for coal in enclosed storage facilities. Most of the listed issues are here external factors in character, suggesting that much can be done to design and manage the storage facility in such a way that self-heating can be avoided or minimised. Only the last issue listed in Table 1, coal grade and quality, is clearly associated with intrinsic or coal-related factors, and, as was noted above, even when it comes to the intrinsic factors, not all details of potential interest are routinely measured. However, an evaluation of the relative importance of different intrinsic factors suggests that only a few factors strongly correlate with the self-heating propensity of coal. Fortunately, some reasonable indication of such a propensity can be predicted from the quantities measured for the coal batches delivered to power plants. In particular, the content of volatile matter and intrinsic moisture appears to reflect the self-heating potential of coal of given particle size (Smith & Glasser, 2005). Volatiles roughly indicate the extent of lighter and more easily flammable combustibles in coal than carbon, and intrinsic moisture is proportional to the reactive surface area of coal. Unlike volatile content, intrinsic moisture is not widely specified and determined for coal deliveries, but the associated dependence on intrinsic moisture is much weaker than on volatiles (Smith & Glasser, 2005). However, when the (mean) intrinsic moisture is proportional to the routinely recorded total moisture, the SmitheGlasser reactivity index of self-heating propensity can instead be applied by using total moisture. Here we use the reactivity index r accordingly in the following modified form:
logr ¼ 0:89$m0:14 $v0:43 ;
(1)
where m is the (total) moisture (%) and v is the dry content of volatiles (%) in coal. A comparison of coal batches from two different sources is shown in Fig. 2, suggesting higher index values for Russian coal. It is notable that all fire incidents in the Salmisaari storage facility have involved Russian coal or its batch interface. The total self-heating risk is naturally not explained purely by intrinsic factors. Coal entering conveyors and storage facilities can already be heated, and heating may be further promoted by excessive ingress of air. In such cases, it is important to extract heated coal and divert it to combustion as soon as possible, and to seal off the potential air leaks of the storage facility, particularly from below. A closed storage facility can be easier to seal, but it may provide less cooling than open stockpiles if the air flow is only
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Table 1 Issues of importance for closed coal storage systems (silos, caverns, corridors and tunnels requiring monitoring of flammable, toxic and/or explosive gases or dust). Requirement
Action
Notes
Storage design
Features to discourage fires: - limited air and water ingress - efficient fire extinguishing - discharge of heated coal Declaration of coal properties before shipping and after control Storage space to be properly free of waste and impurities Dust should be washed away from free surfaces, belts must soon twist back on return Coal transfer points should include dust removal Use diesel vehicles, properly protected electrical equipment Humans present e ventilation; otherwise e nitrogen purging Every silo to be emptied once a year Self-heating coals to be used first or avoided if possible
For options see Sipilä et al., 2010; IMO, 2009; Beamish et al., 2001
Communication Cleaning
Safety against explosions Limits to contents of explosive gases from coal Utilization plan of storage facility Coal grade and quality
through leaks from drains or other channels of the coal bed. Increasing time in storage up to about six months also appears to increase the risk of self-heating and autoignition. The self-heating and autoignition incidents in the Salmisaari underground coal storage facility have been reviewed elsewhere (Sipilä & Auerkari, 2010; Sipilä et al., 2010). Table 2 briefly compares this experience with that from an underground storage facility in Sweden (Alsparr, 2000) and of an above-ground silo storage in Germany (Rosner & Röpell, 2011). Table 3 extends the comparison to the available fire retardant and fire fighting media. The observed cases of self-heating and autoignition in the Salmisaari storage facility generally agree with the trend shown in Fig. 2, that is to say, Russian coal shows a higher mean (and scatter) in its propensity to self-heat than Polish coal, although the individual coal batches responsible for the autoignition were not characterised later in detail. No significant change has been observed in the self-heating propensity of the acquired coals during the last ten years. As self-heating can limit the operational capacity and storage time, the Salmisaari facility is typically emptied once a year according to the utilization plan. To extinguish fires in the Salmisaari silos, the current system can apply nitrogen purging that is relatively costly, takes time
7.0 y = -0.3962x + 8.7612 R = 0.9307
6.5
log r
6.0
5.5 RUS PL 5.0
NZ Linear (NZ)
4.5 5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
Lower heating value (kWh/kg) Fig. 2. Self-heating propensity expressed as the modified reactivity index vs. the heating value for Polish (PL) and Russian (RUS) bituminous coals, with comparison to an independent range of New Zealand (NZ) coals (Beamish et al., 2001) and the linear fitted line for the NZ coals; each data point (PL and RUS) refers to mean properties of a shipload of delivered coal.
Operators need to know properties and possible deviations Also, removal of waste (e.g., wood and metals) from coal in the receiving end of the conveyor systems (before transport to silos)
See, for example, IMO, 2009 No human presence or access in case of exceeding limits To some extent, unpredictable for operational reasons To be considered in specifying, buying and handling of coal
(delivery, filling, holding and venting of N2) and prevents human presence in the storage facility for about two days during the extinguishing work cycle. Smoke venting fans are noisy and will disturb people within the vicinity of the plant. However, only minor smouldering fires have occurred more recently, and these have been successfully extinguished using water from fire hoses. For this purpose, water has the advantages of being immediately available and it does not require time-consuming venting or limit personnel access to the facility. However, water is only suitable for extinguishing near-surface fires, like those on conveyor belts or in the top layers of the silos. Fire-retardant gels have also been used successfully to prevent self-heating and fires in closed coal silos in Germany. A gel layer on top of the stored coal can largely seal it to prevent air draft through the coal bed, discouraging autoignition and the need for nitrogen purging (Rosner & Röpell, 2011).
3. Discussion 3.1. Fire risk in storing solid fuels The fault tree of Fig. 3 (left side) aims to roughly describe the main preconditions to consider when assessing the frequency of autoignition events for the Salmisaari underground storage facility. The corresponding event tree is shown in Fig. 3 (right side) describing the impacts of the mitigating actions on the consequences of autoignition events. The likelihood of a coal batch selfheating while in storage depends on the contents of the volatiles and the moisture in the batch, and on the amount of time that the coal has been in the storage facility. It should be noted that the event tree may in reality include multiple sequential actions for the removal of heated/burning coal and extinguishing it with nitrogen or water. However, the expected end effects materialising as damage in the conveyors, the rock walls of the silos and/or the hopper bellows would remain as shown in Fig. 3. Fires in a closed storage facility may introduce a new (or “emerging”) risk for the operator, but, as noted above, there is also already some accumulated experience suggesting means that can help to reduce the frequency and impact of self-heating and autoignition incidents (see Table 4). Nevertheless, some future threats are also possible or even likely. First, the expected low frequency of autoignition events may gradually reduce the alertness and ability of organizations to respond to the initiating events, which may be small and not clearly symptomatic at first. Second,
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Table 2 Comparison of experiences with self-heating in closed coal storage facilities (Alsparr, 2000; Rosner & Röpell, 2011; Sipilä & Auerkari, 2010; Sipilä et al., 2010). Issue or feature
Salmisaari (FI, underground)
Tiefstack (DE, above-ground)
Värtan (SE, underground)
Operating since Capacity Storage principle Crushed coal grain size
2004a 4 60 000 t silos Last in first outb 30 mm (crushing to size before transport to silos) Introduction of coal to silos in thin even layers, closed storage, tightly closed doors, drains, etc. Maintenance door, hopper and bellows below silo, drain pipes, loose coal at silo wall Anecdotal evidence only, along rock walls/wall drains CO, CH4, O2 detectors þ odour (human nose) Mainly CO, odour N2, water (foam)c Reported cases of self-heating and fires
1996a 2 50 000 m3 silos Last in first outb Not reported
1990 100 000 t cavern Last in first out After storage, to pasta of 0e6 mm coal þ dolomite, 25% total moisture As free-standing piles in storage caverns
Principles of excluding or reducing air access Observed or suspected air ingress from Thermal draft in silo Self-heating & fire detection from Alarm indication by Extinction/retardant media Observed incidents a b c
Introduction of coal to silos in even 0.2 m layers, closed storage, tightly closed doors, drains, etc., gel on top Inspection doors of silo bottom, leaking drain pipes and fittings
See above
Air leakage amplified by draft due to heated coal and low outside temperature CO, CH4, O2 detectors þ thermal imager
See above CO detectors
Mainly CO Gel, water (N2, foam)c Only self-heating cases reported
Mainly CO Not reported Self-heating occurred
Same design origin. Oldest coal last out. (In italics when not applied/needed).
Table 3 Comparison of media used against self-heating and fires in closed coal storage. Medium
Advantages
Limitations
Notes
Water (spray)
Cheap, widely available, with cooling effect
Fire fighting foam
Relatively cheap, easy to apply, with cooling effect
Can form flammable water gas (CO þ H2) if not used in abundance Can make substrate surfaces greasy or sticky
Nitrogen
Fully inert cooling medium, replaces oxygen, can be introduced from below Reduces air flow through coal bed, reduces loss of heating value, low tendency to reduce coal flow, cheaper than N2, cooling by slow evaporationa
To be applied in large quantity, will reduce the heating value of coal, can reduce coal flow Reduces the heating value of coal, can reduce coal flow Expensive, diluted if not contained in a gasetight system Requires proper and even spreading to be effective
Fire-retardant gel
a
a Prevents quick water evaporation and formation of CO þ H2 on hot coal surfaces
Rosner & Röpell (2011).
even if coal was to be gradually phased out as a proper fuel, at least in plants without carbon capture and storage (CCS), this would not exclude solid renewable fuels like biomass as an alternative. As with low-rank coals, biomass tends to have high volatile and
Top event
Likelihood of coal batch to self-heat
moisture contents, and it ignites easily when dry. Also, in order to replace a significant percentage of coal, the low heating value and density of biomass will greatly increase the amount of required transport and storage space.
Ignition Alarm Coal removal Extinguishing Damage Consequence Removal of burning coal Safe state Operator alerted
Self-heating coal in silo
Extinguished Safe state Shutdown & repair Conveyor
Autoignition in silo
Burning coal not removed
Failed to put out
Rock wall Bellows
Annual no. of batches entering silo
Extinguished
Air ingress to silo/coal
Repair Repair
Safe state Shutdown & repair Conveyor
Late alarm & major fire
Failed to put out
Rock wall
Repair Repair
Bellows
Fig. 3. Simplified fault tree (left) and event tree (right) presentation for assessing the risk of self-heating and autoignition incidents in the underground coal storage silos of the Salmisaari power plant.
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Table 4 Risk factors related to the autoignition of coal in closed storage facilities. Unwanted feature
Nature of loss
Mitigating factors
Notes
CO2, other emissions from stored coal Plant shutdown
Tax on emissions, global warming
Low temp in storage, limited air ingress
Underground rock silos help (cool and closed)
Economical loss, security of supply (district heat) Economical loss from derating and added maintenance Economical loss (including early failures and added disturbances) Economical loss, can be extensive, from fire or dust explosion Loss of customers Unauthorized entry during fire/extinguishing Dust explosion Unplanned heavy maintenance work, added personnel risk
Fire prevention
Disturbed fuel flow Extra wear Structural damage Public image loss Safety hazard to personnel
Monitoring, training for prevention
For example, due to accepting of heated coal
Monitoring, training for prevention
For example, hot coal hardens belts and bellows
Monitoring, cleaning, training for prevention
For example, due to dust explosion, uncontrolled fire or lost silo lining For example, emissions, smoke, dust, odour, noise Challenge to train for rare events
Proper communication Preventive action Access control, monitoring, training, use of protective & alarm equipment Cleaning, monitoring, training Overtime control, correct work methods & tools, training, monitoring
Table 5 Expected advantages and possible limitations of the recommended actions. Action
Advantages
Limitations
Notes
Add fire-retardant gel system
Reduced fire incidents, reduced need for N2 purging Prevents water drip channels in coal and corrosion in structures To avoid hopper destruction & dust explosion in local fire Systematic goal setting to avoid incidents, follow-up for permanence
Applied when stopping discharge, adds some water to coal May hinder visual inspection of the rock ceiling afterwards Complicated to build into an existing system
Stops air ingress through coal bed
Add silo sealing top membrane Hopper (N2) inertising New goals, KPI’s and continuity
3.2. Recommendations For the future operations of underground coal storage facilities, we can make a few recommendations. First, we propose selected improvements to the storage facility and its operation and precautions that can be taken to minimise self-heating and autoignition events and, ideally, to completely avoid smouldering fires. The first three actions listed in Table 5 aim to serve this purpose. The first of these actions is to apply fire-retardant gel on the top layers of coal to stop air flow through the coal bed, following the positive experience with this approach at the Tiefstack power plant (Rosner & Röpell, 2011). Secondly, we recommend to seal the silo ceiling with a water- and fireproof membrane in order to protect the coal bed surfaces from air channel formation and the internal steel structures from corrosion by drip water. Our third recommended action is to facilitate nitrogen purging at the hoppers in order to protect the structure and personnel from the impact of potential local fires or dust explosions. In contrast, our fourth recommendation in Table 5 aims to assess and quantify the condition and improvements in the performance of the systems related to autoignition; it is important to monitor and control the outcomes for the future of the facilities. This includes defining quantified quality objectives and adopting the principle of continuous improvement in the management of the storage facility. This may not be formally a very extensive effort, if the plant operation is already covered by ISO 9001/ISO 14001 systems. Nevertheless, in order to quantify and possibly refine the objectives, appropriate (key) performance indicators (KPI) should be defined, and these are likely to be specific to the storage facility. Suggested performance indicators are shown in Table 6 for both immediate fire risk (alarms) and for longer term or post-assessment indicators of the system performance and follow-up assessments.
Requires dedication, rare events may limit motivation
Available in fire-retardant materials Could be introduced by using N2 bottles Possibly to be integrated into a wider system
It should be noted that, although self-heating and fires in stored coal are far from unknown (Beamish et al., 2001; IMO, 2009; Nalbandian, 2010; Sipilä & Auerkari, 2010; Smith & Glasser, 2005; Porter & Ovitz, 1912), published experience on underground coal storage fires is uncommon and such fires can be seen to represent an emerging risk issue (IRGC, 2010). As shown above, this risk can nevertheless be systematically managed for future mitigation.
Table 6 Suggested immediate and post-assessment performance indicators of the emerging risk issues related to self-heating and ignition/fires of stored coal. Issue
Suggested performance indicators
Notes
Self-heating and ignition
Coal temperature 40 C for safe state Monitored CO 5 ppm for safe state
Fires Fires
Number of recorded fires/10 years Number and trend of heating incidents (true alarms)/1, 5 & 10 years Number and depth of public reactions (newspaper/TV, commentaries, blog responses, etc.), number and volume of lost customers Added losses (V) from derating (lost availability, max yearly production loss,added cost of O&M, disturbance, damage or failures) Challenge to personnel safety: number of injuries due to unplanned heavy maintenance work, fires or dust explosions Time to intervene and extinguish a heating incident (from alarm)
Immediate; limits possibly by location, measurement, ventilation, etc. Post-assessment Post-assessment
Fires
Fires
Fires
Fires
Post-assessment
Post-assessment
Post-assessment
Post-assessment
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4. Summary The underground coal storage facility of the Salmisaari power plant in Helsinki was built to free up valuable real estate from the previous open storage facility, and to remove a source of noise and dust. Self-heating and autoignition is not uncommon in aboveground stockpiles, and must be accounted for also in a closed storage facilities with reduced access for fire extinguishing services or other mitigating actions. In this paper, we have summarized recent experiences with and recommendations for managing the risk of spontaneous combustion in closed coal storage facilities. Although no similar underground storage site is known to exist, the experience from smouldering fires in the Salmisaari storage facility together with the available information from other sites of closed storage have provided useful insight into the array of variables that contribute to self-heating, ignition and extinguishing of such fires. Both storage system related and coal-specific factors influence the risk of spontaneous combustion. Of the coal-specific factors, the content of volatiles appears to have a relatively strong impact on the propensity of coal to self-heat and ignite, as we have shown for the bituminous coal batches delivered to the Salmisaari storage silos. However, storage-related factors like air ingress and heat retention in the coal bed are also very important in promoting autoignition. The experiences from the Salmisaari plant and elsewhere suggest that proper sealing against water and air flows through the stored coal can go a long way towards preventing excessive self-heating. Nitrogen purging, water spraying, fire fighting foams and fire-retardant gels have been used to extinguish fires. These methods can also be combined, but they differ significantly in terms of cost and useful range of application. New challenges may emerge from the increasing use of biomass fuels that e similarly to low-rank coals e have relatively high volatile and moisture contents, and ignite easily when dry. They also have low heating value and density so that high volumes are needed to replace equivalent amounts of coal. However, for the Salmisaari underground coal storage facility, we recommend the following: - adding fire-retardant gel to the top layer of coal in the silos to prevent air flow through the coal bed - sealing the silo ceiling with a water- and fireproof membrane to protect the coal bed surfaces and internal engineering structures from coal channel formation and corrosion by drip water
- facilitating inertising by nitrogen purging at the hopper to protect the structure and personnel from the impact of local fire or dust explosion - complementing the quality system for continuous improvement and defined performance indicators.
Acknowledgement Technical support by the partnership of the European project iNTeg-Risk, and financing by the 7th Framework Programme (FP7/ 2007-2013, under Grant Agreement no. 213345) of the European Union, Helsingin Energia and VTT are gratefully acknowledged.
References Alsparr, J. (2000). PFBC of Birka Energi: ten years of operating experience (in German). VGB Kraftwerkstechnik, 80, 63e67. Beamish, B. B., Barakat, M. A., & St George, J. D. (2001). Spontaneous-combustion propensity of New Zealand coals under adiabatic conditions. International Journal of Coal Geology, 45, 217e224. Carpenter, A. M., Porter, D., Scott, D. H., & Walker, S. (2003). Transport, storage and handling of coal. IEA Clean Coal Centre. DOE HDBK-1081e94. (1994). Primer on spontaneous heating and pyrophoricity. FSC6910. Washington DC: U.S.: Department of Energy. IMO. (2009). International Maritime Solid Bulk Cargoes Code. London: IMO. IRGC. (2010). The emergence of risks: Contributing factors. Geneva: International Risk Governance Council. ISO 14001. (2004). Environmental management systems-Requirements with guidance for use. Geneve: ISO. ISO 9001. (2008). Quality management systems e Requirements. Geneve: ISO. Nalbandian, H. (2010). Propensity of coal to self-heat. Report CCC/172. IEA Clean Coal Centre. Porter, H. C., & Ovitz, F. K. (1912). Deterioration and spontaneous heating of coal in storage. The Journal of Industrial and Engineering Chemistry, 4, 5e8. Rosner, C., & Röpell, H. (2011). Experiences with fires in silos for coal storage in the Tiefstack CHP plant (in German). VGB Powertech, 91, 84e87. Sipilä, J., Auerkari, P. Fire incidents in underground coal storage. In International Conference on Maintenance of Power Plants (Baltica VIII), Helsinki-Stockholm, May 2010. Sipilä, J., Auerkari, P., Heikkilä, A.- M., Krause, U. Emerging risk of autoignition and fire in underground coal storage. In Integ-Risk Conference on Emerging Risk, Stuttgart, June 2010. Sipilä, J., Auerkari, P., Malmén, Y., Heikkilä, A.- M., Vela, I., Krause, U. Experience and the unexpected: Risk and mitigation issues for operating underground storage silos for coal fired power plant. In Integ-Risk Conference on Emerging Risk, Stuttgart, June 2011. Smith, M. A., & Glasser, D. (2005). Spontaneous combustion of carbonaceous stockpiles. Part II. Factors affecting the rate of low-temperature oxidation reaction. Fuel, 84, 1161e1170.
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Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Risk and mitigation of self-heating and spontaneous combustion in underground coal storage Juha Sipilä a, d, *, Pertti Auerkari b, Anna-Mari Heikkilä b, Risto Tuominen b, Iris Vela c, Jyrki Itkonen a, Mikael Rinne d, Kalevi Aaltonen d a
Helsingin Energia, FI-00090 Helen, Finland VTT Technical Research Centre of Finland, FI-02044 VTT, Finland Bundesanstalt für Materialforschung und eprüfung, Berlin, Germany d Aalto University, FI-00076 Aalto, Finland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2011 Received in revised form 25 January 2012 Accepted 25 January 2012
While the self-heating and spontaneous combustion of coal is a known challenge at coal mines and storage sites, there are known methods for mitigating this challenge for typical open stockpile storage. However, closing the storage will reduce access for corrective action, and it is then important to manage the storage and its transport system with added attention without unduly adding cost or hindering availability. This paper aims to discuss the risk, prevention and extinguishing of fires in closed coal storage facilities, particularly in light of the experience with the Salmisaari underground rock storage facility in Finland. The observed autoignition events have indicated an array of contributing factors, some of which are unique to underground silo storage facilities. On the other hand, many features of the storage facilities can be compared with other extant closed storage systems. The factors affecting fire risk are described and the associated fault and event trees are outlined for autoignition at underground storage. Drawing upon the experiences with past events of self-heating and spontaneous combustion, recommendations are given on cost-effective preventive, corrective and other mitigating action for minimising fire risk and promoting storage availability. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Coal Underground storage Fire Risk Mitigation
1. Introduction The underground coal storage facility at the Salmisaari CHP power plant in Helsinki has been operating since 2004, with a total capacity of 250 000 tons of bituminous coal. The facility consists of four silos, B40 m 65 m each, with the silo bottom at a depth of 120 m, which are carved into granite rock that is strong enough not to need additional support structures. The advantages of a closed underground storage facility in comparison to the earlier above-ground open stockpile include automated lower cost operations, much reduced dust, noise and loss of heat content, less air ingress for a lower fire risk, and improved aesthetics. An important advantage was also freeing up urban real estate (about 100 000 m2) close to the city centre. The advantages are partly balanced by more limited access for any unplanned action, tighter requirements on safety systems and monitoring, and the cost of construction.
* Corresponding author. Helsingin Energia, FI-0090 Helen, Finland. Tel.: þ358 40 3346663. E-mail addresses: juha.sipila@iki.fi, juha.sipila@aalto.fi (J. Sipilä).
Self-heating and autoignition is not uncommon in the aboveground coal piles (Carpenter, Porter, Scott, & Walker, 2003; Nalbandian, 2010; Smith & Glasser, 2005), and the hazard must be accounted for also in a closed storage that, by definition, will provide reduced access for any mitigating action. In this paper, we summarize the experiences and views on the risk, prevention and mitigation of autoignition and spontaneous combustion incidents in closed coal storage facilities, with particular reference to and emphasis on the Salmisaari underground coal storage facility. With sufficient access to oxygen (air) and restricted heat removal, coal may self-ignite in storage as a result of internal heating by oxidation (Fig. 1). The process of self-heating and ignition tends to develop slowly and result in smouldering fires that can be challenging to extinguish without special precautions in the design and operation of the storage facility. Both external, or storage design, and conditions-related, and intrinsic (coal-related) factors will influence the likelihood of selfheating and ignition of stored coal. Known intrinsic factors that promote self-heating include, for example, low rank, high volatile content, high porosity, small particle size and high alkali content in ash (Beamish, Barakat, & St George, 2001; IMO, 2009; Nalbandian,
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Fig. 1. Evolution of a spontaneous coal fire: a) coal pile with varying particle sizes; b) initial self-heating by air ingress; c) heat loss by conduction, convection and radiation not exceeding self-heating; d) heating to a critical temperature to ignite a fire (adapted from DOE HDBK-1081, 1994).
2010; Sipilä & Auerkari, 2010; Sipilä, Auerkari, Heikkilä, Krause, 2010). Not all of the influential factors from the different batches of coal that have been delivered are known or routinely measured, and, therefore, it may not be self-evident how to rate the risk of self-heating. The other important issue has to do with preventing or extinguishing any initiated overheating or fire efficiently before significant losses or damage can occur. For this purpose, it is necessary to detect critical events, in other words, monitor the status of the storage facility regarding any signs of developing hot spots. Fortunately, this can usually be restricted to certain key locations to sufficiently characterise the condition and status of the whole storage facility and its transport systems. In addition, spontaneous combustion is also less likely if suitable scheduling and stacking is applied in filling and discharging the storage facility. Several current sites are using enclosed storage facilities to store coal at power plants. In 2008, elevated CO level from the silo no.4 of the Salmisaari facility indicated an initiated smouldering fire that resulted in damage to the hopper bellows and wall shotcrete of the silo containing Russian low sulphur coal, with an about 1 m layer of old Polish coal at the bottom. While the plant production was not limited during the incident, complete fire extinguishing took about four months by alternate nitrogen filling and coal discharging until the silo was empty. Thermal imaging from above showed a hot spot close to the bottom maintenance door. Discharged coal was often hot and to protect the conveyor belt, a layer of cool coal from another silo, and when necessary, belt watering was applied. Spalling of the shotcrete in the silo wall was observed on about one third of the silo area. Partly burnt coal, ash and slag indicated air ingress through the seals of the bottom maintenance door, where the measured outer surface temperature was about 350 C during the fire. City inhabitants living in the vicinity filed complaints about flue gas fan noise and sulphurous smell, and this was naturally considered harmful for the public image of the plant. Apart from possible particle size segregation to provide internal air passages, and relatively lengthy time of coal in storage (more than six months), a major contributing factor to the fire and its tedious extinguishing was the air leak through the maintenance door. Limited access for early detection (monitoring) and mitigation can be considered to be a specific features of the closed underground coal storage, influencing the size of the fire by the time of alarm. Furthermore, the coal grade may contribute to the likelihood of such an incident (see below). After introducing better bottom sealing against air ingress, no fires have occurred on a similar scale, in spite of leaving, for example, the sensor and alarm systems and the use of the storage or safety related practices and equipment unchanged. The safety system for protecting the personnel and equipment of the underground storage facility includes features that have been described in more detail by Sipilä et al. (2011). This paper is discussing aspects of risk, prevention and extinguishing of
fires in closed coal storage, particularly in light of the experience with the Salmisaari underground rock storage facility. 2. Recent experience on managing self-heating and fires in closed coal storage facilities Table 1 shows selected general issues of importance when considering potential self-heating and fires (and even dust explosions) for coal in enclosed storage facilities. Most of the listed issues are here external factors in character, suggesting that much can be done to design and manage the storage facility in such a way that self-heating can be avoided or minimised. Only the last issue listed in Table 1, coal grade and quality, is clearly associated with intrinsic or coal-related factors, and, as was noted above, even when it comes to the intrinsic factors, not all details of potential interest are routinely measured. However, an evaluation of the relative importance of different intrinsic factors suggests that only a few factors strongly correlate with the self-heating propensity of coal. Fortunately, some reasonable indication of such a propensity can be predicted from the quantities measured for the coal batches delivered to power plants. In particular, the content of volatile matter and intrinsic moisture appears to reflect the self-heating potential of coal of given particle size (Smith & Glasser, 2005). Volatiles roughly indicate the extent of lighter and more easily flammable combustibles in coal than carbon, and intrinsic moisture is proportional to the reactive surface area of coal. Unlike volatile content, intrinsic moisture is not widely specified and determined for coal deliveries, but the associated dependence on intrinsic moisture is much weaker than on volatiles (Smith & Glasser, 2005). However, when the (mean) intrinsic moisture is proportional to the routinely recorded total moisture, the SmitheGlasser reactivity index of self-heating propensity can instead be applied by using total moisture. Here we use the reactivity index r accordingly in the following modified form:
logr ¼ 0:89$m0:14 $v0:43 ;
(1)
where m is the (total) moisture (%) and v is the dry content of volatiles (%) in coal. A comparison of coal batches from two different sources is shown in Fig. 2, suggesting higher index values for Russian coal. It is notable that all fire incidents in the Salmisaari storage facility have involved Russian coal or its batch interface. The total self-heating risk is naturally not explained purely by intrinsic factors. Coal entering conveyors and storage facilities can already be heated, and heating may be further promoted by excessive ingress of air. In such cases, it is important to extract heated coal and divert it to combustion as soon as possible, and to seal off the potential air leaks of the storage facility, particularly from below. A closed storage facility can be easier to seal, but it may provide less cooling than open stockpiles if the air flow is only
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Table 1 Issues of importance for closed coal storage systems (silos, caverns, corridors and tunnels requiring monitoring of flammable, toxic and/or explosive gases or dust). Requirement
Action
Notes
Storage design
Features to discourage fires: - limited air and water ingress - efficient fire extinguishing - discharge of heated coal Declaration of coal properties before shipping and after control Storage space to be properly free of waste and impurities Dust should be washed away from free surfaces, belts must soon twist back on return Coal transfer points should include dust removal Use diesel vehicles, properly protected electrical equipment Humans present e ventilation; otherwise e nitrogen purging Every silo to be emptied once a year Self-heating coals to be used first or avoided if possible
For options see Sipilä et al., 2010; IMO, 2009; Beamish et al., 2001
Communication Cleaning
Safety against explosions Limits to contents of explosive gases from coal Utilization plan of storage facility Coal grade and quality
through leaks from drains or other channels of the coal bed. Increasing time in storage up to about six months also appears to increase the risk of self-heating and autoignition. The self-heating and autoignition incidents in the Salmisaari underground coal storage facility have been reviewed elsewhere (Sipilä & Auerkari, 2010; Sipilä et al., 2010). Table 2 briefly compares this experience with that from an underground storage facility in Sweden (Alsparr, 2000) and of an above-ground silo storage in Germany (Rosner & Röpell, 2011). Table 3 extends the comparison to the available fire retardant and fire fighting media. The observed cases of self-heating and autoignition in the Salmisaari storage facility generally agree with the trend shown in Fig. 2, that is to say, Russian coal shows a higher mean (and scatter) in its propensity to self-heat than Polish coal, although the individual coal batches responsible for the autoignition were not characterised later in detail. No significant change has been observed in the self-heating propensity of the acquired coals during the last ten years. As self-heating can limit the operational capacity and storage time, the Salmisaari facility is typically emptied once a year according to the utilization plan. To extinguish fires in the Salmisaari silos, the current system can apply nitrogen purging that is relatively costly, takes time
7.0 y = -0.3962x + 8.7612 R = 0.9307
6.5
log r
6.0
5.5 RUS PL 5.0
NZ Linear (NZ)
4.5 5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
Lower heating value (kWh/kg) Fig. 2. Self-heating propensity expressed as the modified reactivity index vs. the heating value for Polish (PL) and Russian (RUS) bituminous coals, with comparison to an independent range of New Zealand (NZ) coals (Beamish et al., 2001) and the linear fitted line for the NZ coals; each data point (PL and RUS) refers to mean properties of a shipload of delivered coal.
Operators need to know properties and possible deviations Also, removal of waste (e.g., wood and metals) from coal in the receiving end of the conveyor systems (before transport to silos)
See, for example, IMO, 2009 No human presence or access in case of exceeding limits To some extent, unpredictable for operational reasons To be considered in specifying, buying and handling of coal
(delivery, filling, holding and venting of N2) and prevents human presence in the storage facility for about two days during the extinguishing work cycle. Smoke venting fans are noisy and will disturb people within the vicinity of the plant. However, only minor smouldering fires have occurred more recently, and these have been successfully extinguished using water from fire hoses. For this purpose, water has the advantages of being immediately available and it does not require time-consuming venting or limit personnel access to the facility. However, water is only suitable for extinguishing near-surface fires, like those on conveyor belts or in the top layers of the silos. Fire-retardant gels have also been used successfully to prevent self-heating and fires in closed coal silos in Germany. A gel layer on top of the stored coal can largely seal it to prevent air draft through the coal bed, discouraging autoignition and the need for nitrogen purging (Rosner & Röpell, 2011).
3. Discussion 3.1. Fire risk in storing solid fuels The fault tree of Fig. 3 (left side) aims to roughly describe the main preconditions to consider when assessing the frequency of autoignition events for the Salmisaari underground storage facility. The corresponding event tree is shown in Fig. 3 (right side) describing the impacts of the mitigating actions on the consequences of autoignition events. The likelihood of a coal batch selfheating while in storage depends on the contents of the volatiles and the moisture in the batch, and on the amount of time that the coal has been in the storage facility. It should be noted that the event tree may in reality include multiple sequential actions for the removal of heated/burning coal and extinguishing it with nitrogen or water. However, the expected end effects materialising as damage in the conveyors, the rock walls of the silos and/or the hopper bellows would remain as shown in Fig. 3. Fires in a closed storage facility may introduce a new (or “emerging”) risk for the operator, but, as noted above, there is also already some accumulated experience suggesting means that can help to reduce the frequency and impact of self-heating and autoignition incidents (see Table 4). Nevertheless, some future threats are also possible or even likely. First, the expected low frequency of autoignition events may gradually reduce the alertness and ability of organizations to respond to the initiating events, which may be small and not clearly symptomatic at first. Second,
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Table 2 Comparison of experiences with self-heating in closed coal storage facilities (Alsparr, 2000; Rosner & Röpell, 2011; Sipilä & Auerkari, 2010; Sipilä et al., 2010). Issue or feature
Salmisaari (FI, underground)
Tiefstack (DE, above-ground)
Värtan (SE, underground)
Operating since Capacity Storage principle Crushed coal grain size
2004a 4 60 000 t silos Last in first outb 30 mm (crushing to size before transport to silos) Introduction of coal to silos in thin even layers, closed storage, tightly closed doors, drains, etc. Maintenance door, hopper and bellows below silo, drain pipes, loose coal at silo wall Anecdotal evidence only, along rock walls/wall drains CO, CH4, O2 detectors þ odour (human nose) Mainly CO, odour N2, water (foam)c Reported cases of self-heating and fires
1996a 2 50 000 m3 silos Last in first outb Not reported
1990 100 000 t cavern Last in first out After storage, to pasta of 0e6 mm coal þ dolomite, 25% total moisture As free-standing piles in storage caverns
Principles of excluding or reducing air access Observed or suspected air ingress from Thermal draft in silo Self-heating & fire detection from Alarm indication by Extinction/retardant media Observed incidents a b c
Introduction of coal to silos in even 0.2 m layers, closed storage, tightly closed doors, drains, etc., gel on top Inspection doors of silo bottom, leaking drain pipes and fittings
See above
Air leakage amplified by draft due to heated coal and low outside temperature CO, CH4, O2 detectors þ thermal imager
See above CO detectors
Mainly CO Gel, water (N2, foam)c Only self-heating cases reported
Mainly CO Not reported Self-heating occurred
Same design origin. Oldest coal last out. (In italics when not applied/needed).
Table 3 Comparison of media used against self-heating and fires in closed coal storage. Medium
Advantages
Limitations
Notes
Water (spray)
Cheap, widely available, with cooling effect
Fire fighting foam
Relatively cheap, easy to apply, with cooling effect
Can form flammable water gas (CO þ H2) if not used in abundance Can make substrate surfaces greasy or sticky
Nitrogen
Fully inert cooling medium, replaces oxygen, can be introduced from below Reduces air flow through coal bed, reduces loss of heating value, low tendency to reduce coal flow, cheaper than N2, cooling by slow evaporationa
To be applied in large quantity, will reduce the heating value of coal, can reduce coal flow Reduces the heating value of coal, can reduce coal flow Expensive, diluted if not contained in a gasetight system Requires proper and even spreading to be effective
Fire-retardant gel
a
a Prevents quick water evaporation and formation of CO þ H2 on hot coal surfaces
Rosner & Röpell (2011).
even if coal was to be gradually phased out as a proper fuel, at least in plants without carbon capture and storage (CCS), this would not exclude solid renewable fuels like biomass as an alternative. As with low-rank coals, biomass tends to have high volatile and
Top event
Likelihood of coal batch to self-heat
moisture contents, and it ignites easily when dry. Also, in order to replace a significant percentage of coal, the low heating value and density of biomass will greatly increase the amount of required transport and storage space.
Ignition Alarm Coal removal Extinguishing Damage Consequence Removal of burning coal Safe state Operator alerted
Self-heating coal in silo
Extinguished Safe state Shutdown & repair Conveyor
Autoignition in silo
Burning coal not removed
Failed to put out
Rock wall Bellows
Annual no. of batches entering silo
Extinguished
Air ingress to silo/coal
Repair Repair
Safe state Shutdown & repair Conveyor
Late alarm & major fire
Failed to put out
Rock wall
Repair Repair
Bellows
Fig. 3. Simplified fault tree (left) and event tree (right) presentation for assessing the risk of self-heating and autoignition incidents in the underground coal storage silos of the Salmisaari power plant.
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Table 4 Risk factors related to the autoignition of coal in closed storage facilities. Unwanted feature
Nature of loss
Mitigating factors
Notes
CO2, other emissions from stored coal Plant shutdown
Tax on emissions, global warming
Low temp in storage, limited air ingress
Underground rock silos help (cool and closed)
Economical loss, security of supply (district heat) Economical loss from derating and added maintenance Economical loss (including early failures and added disturbances) Economical loss, can be extensive, from fire or dust explosion Loss of customers Unauthorized entry during fire/extinguishing Dust explosion Unplanned heavy maintenance work, added personnel risk
Fire prevention
Disturbed fuel flow Extra wear Structural damage Public image loss Safety hazard to personnel
Monitoring, training for prevention
For example, due to accepting of heated coal
Monitoring, training for prevention
For example, hot coal hardens belts and bellows
Monitoring, cleaning, training for prevention
For example, due to dust explosion, uncontrolled fire or lost silo lining For example, emissions, smoke, dust, odour, noise Challenge to train for rare events
Proper communication Preventive action Access control, monitoring, training, use of protective & alarm equipment Cleaning, monitoring, training Overtime control, correct work methods & tools, training, monitoring
Table 5 Expected advantages and possible limitations of the recommended actions. Action
Advantages
Limitations
Notes
Add fire-retardant gel system
Reduced fire incidents, reduced need for N2 purging Prevents water drip channels in coal and corrosion in structures To avoid hopper destruction & dust explosion in local fire Systematic goal setting to avoid incidents, follow-up for permanence
Applied when stopping discharge, adds some water to coal May hinder visual inspection of the rock ceiling afterwards Complicated to build into an existing system
Stops air ingress through coal bed
Add silo sealing top membrane Hopper (N2) inertising New goals, KPI’s and continuity
3.2. Recommendations For the future operations of underground coal storage facilities, we can make a few recommendations. First, we propose selected improvements to the storage facility and its operation and precautions that can be taken to minimise self-heating and autoignition events and, ideally, to completely avoid smouldering fires. The first three actions listed in Table 5 aim to serve this purpose. The first of these actions is to apply fire-retardant gel on the top layers of coal to stop air flow through the coal bed, following the positive experience with this approach at the Tiefstack power plant (Rosner & Röpell, 2011). Secondly, we recommend to seal the silo ceiling with a water- and fireproof membrane in order to protect the coal bed surfaces from air channel formation and the internal steel structures from corrosion by drip water. Our third recommended action is to facilitate nitrogen purging at the hoppers in order to protect the structure and personnel from the impact of potential local fires or dust explosions. In contrast, our fourth recommendation in Table 5 aims to assess and quantify the condition and improvements in the performance of the systems related to autoignition; it is important to monitor and control the outcomes for the future of the facilities. This includes defining quantified quality objectives and adopting the principle of continuous improvement in the management of the storage facility. This may not be formally a very extensive effort, if the plant operation is already covered by ISO 9001/ISO 14001 systems. Nevertheless, in order to quantify and possibly refine the objectives, appropriate (key) performance indicators (KPI) should be defined, and these are likely to be specific to the storage facility. Suggested performance indicators are shown in Table 6 for both immediate fire risk (alarms) and for longer term or post-assessment indicators of the system performance and follow-up assessments.
Requires dedication, rare events may limit motivation
Available in fire-retardant materials Could be introduced by using N2 bottles Possibly to be integrated into a wider system
It should be noted that, although self-heating and fires in stored coal are far from unknown (Beamish et al., 2001; IMO, 2009; Nalbandian, 2010; Sipilä & Auerkari, 2010; Smith & Glasser, 2005; Porter & Ovitz, 1912), published experience on underground coal storage fires is uncommon and such fires can be seen to represent an emerging risk issue (IRGC, 2010). As shown above, this risk can nevertheless be systematically managed for future mitigation.
Table 6 Suggested immediate and post-assessment performance indicators of the emerging risk issues related to self-heating and ignition/fires of stored coal. Issue
Suggested performance indicators
Notes
Self-heating and ignition
Coal temperature 40 C for safe state Monitored CO 5 ppm for safe state
Fires Fires
Number of recorded fires/10 years Number and trend of heating incidents (true alarms)/1, 5 & 10 years Number and depth of public reactions (newspaper/TV, commentaries, blog responses, etc.), number and volume of lost customers Added losses (V) from derating (lost availability, max yearly production loss,added cost of O&M, disturbance, damage or failures) Challenge to personnel safety: number of injuries due to unplanned heavy maintenance work, fires or dust explosions Time to intervene and extinguish a heating incident (from alarm)
Immediate; limits possibly by location, measurement, ventilation, etc. Post-assessment Post-assessment
Fires
Fires
Fires
Fires
Post-assessment
Post-assessment
Post-assessment
Post-assessment
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4. Summary The underground coal storage facility of the Salmisaari power plant in Helsinki was built to free up valuable real estate from the previous open storage facility, and to remove a source of noise and dust. Self-heating and autoignition is not uncommon in aboveground stockpiles, and must be accounted for also in a closed storage facilities with reduced access for fire extinguishing services or other mitigating actions. In this paper, we have summarized recent experiences with and recommendations for managing the risk of spontaneous combustion in closed coal storage facilities. Although no similar underground storage site is known to exist, the experience from smouldering fires in the Salmisaari storage facility together with the available information from other sites of closed storage have provided useful insight into the array of variables that contribute to self-heating, ignition and extinguishing of such fires. Both storage system related and coal-specific factors influence the risk of spontaneous combustion. Of the coal-specific factors, the content of volatiles appears to have a relatively strong impact on the propensity of coal to self-heat and ignite, as we have shown for the bituminous coal batches delivered to the Salmisaari storage silos. However, storage-related factors like air ingress and heat retention in the coal bed are also very important in promoting autoignition. The experiences from the Salmisaari plant and elsewhere suggest that proper sealing against water and air flows through the stored coal can go a long way towards preventing excessive self-heating. Nitrogen purging, water spraying, fire fighting foams and fire-retardant gels have been used to extinguish fires. These methods can also be combined, but they differ significantly in terms of cost and useful range of application. New challenges may emerge from the increasing use of biomass fuels that e similarly to low-rank coals e have relatively high volatile and moisture contents, and ignite easily when dry. They also have low heating value and density so that high volumes are needed to replace equivalent amounts of coal. However, for the Salmisaari underground coal storage facility, we recommend the following: - adding fire-retardant gel to the top layer of coal in the silos to prevent air flow through the coal bed - sealing the silo ceiling with a water- and fireproof membrane to protect the coal bed surfaces and internal engineering structures from coal channel formation and corrosion by drip water
- facilitating inertising by nitrogen purging at the hopper to protect the structure and personnel from the impact of local fire or dust explosion - complementing the quality system for continuous improvement and defined performance indicators.
Acknowledgement Technical support by the partnership of the European project iNTeg-Risk, and financing by the 7th Framework Programme (FP7/ 2007-2013, under Grant Agreement no. 213345) of the European Union, Helsingin Energia and VTT are gratefully acknowledged.
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