Risk Analysis, Vol. 34, No. 12, 2014
DOI: 10.1111/risa.12273
Early Warning Indicators for Challenges in Underground Coal Storage ¨ 1,∗ Pertti Auerkari,2 Stefan Holmstrom, ¨ 3 and Iris Vela4 Juha Sipila,
Early warning or leading indicators are discussed for unexpected incidences in case of large-scale underground coal storage at a power plant. The experience is compared with above-ground stockpiles for which established procedures are available but where access for prevention and mitigation are much easier. It is suggested that while the explicit organization, procedures, and the general safety systems aim to provide the targeted levels of performance for the storage, representing new technology without much precedence elsewhere in the world, the extensive experience and tacit knowledge from above-ground open and closed storage systems can help to prepare for and to prevent unwanted incidents in the underground storage. This kind of experience has been also found useful for developing the leading or early warning indicators for underground storage. Examples are given on observed autoignition and freezing of coal in the storage silos, and on occupational hazards. Selection of the leading indicators needs to consider the specific features of the unique underground facility. KEY WORDS: Fire; freezing; risk indicator; safety; underground storage
1. INTRODUCTION
tent, and saving (urban) space and real estate. Also, an above-ground stockpile is rather unsightly and moving it underground will definitely help the city aesthetics. The cost of constructing the Salmisaari underground facility was about half of the proceeds from selling the released real estate and building rights.(1) The underground storage also carries some limitations, such as tightened ATEX (environment with an explosive atmosphere, from the French original ATmosph`eres EXplosibles) requirements, added requirements for ventilation and monitoring, and reduced access for any major action such as firefighting. However, the limitations were (and are) thought to be minor in comparison to the advantages. As there is only recent experience on the risks related to operation of a large underground storage, it appears important that such experience is systematically mapped for any significant emerging risk issue. For the same reason, well-selected leading or early warning indicators are suggested for early
The underground coal storage at the Salmisaari CHP power plant has been operational since 2004, and replaces an earlier above-ground open stockpile (Fig. 1). The storage has a total capacity of 250,000 tons, corresponding to about half of the yearly fuel consumption of the plant. The storage consists of four rock silos, ø 40 m × 65 m each, with the silo bottom at a depth of −120 m. The Salmisaari coal storage represents relatively new technology as similar facilities have not been previously used. Specific advantages of such storage in comparison to an aboveground stockpile include reduced dust, noise, and greenhouse gas emissions, slower loss of heat con1 Helsingin
Energia, Helsinki, Finland. Technical Research Centre of Finland, Espoo, Finland. 3 Joint Research Centre–IET Petten, The Netherlands. 4 BAM, Berlin, Germany. ∗ Address correspondence to Juha Sipila, ¨ Helsingin Energia, FI0090 HELEN, Finland; [email protected]. 2 VTT
2089
C 2014 Society for Risk Analysis 0272-4332/14/0100-2089$22.00/1
Sipila¨ et al.
2090
Fig. 1. Salmisaari power station (a) before and (b) after construction of underground coal storage.
warning in contrast to longer term (lagging) indicators or risk evaluation. Here, a brief review is presented of the current understanding of what have been considered potentially relevant, covering particularly incidents of autoignition and freezing of coal, and safety issues of the storage facility.(2–4) The indicators are discussed with reference to other performance measures that are applied to manage the risks. 2. EXPERIENCE ON THE CHALLENGES 2.1. Self-Heating and Autoignition Exothermic oxidation of carbon in coal can generate significant heating when the surrounding coal bed is a sufficient thermal insulator but does not exclude air (oxidant) ingress for continuing oxidation. Resulting self-heating up to ignition and smoldering fire is not uncommon in above-ground coal piles, but the hazard must also be accounted for in closed storage that has naturally reduced access for preventive or mitigating action. Extinguishing such fires in closed storage can be somewhat complicated, as has been learned in the course of operating the underground storage, in spite of precautions in the design.(1–3,5,6) The critical parameters for spontaneous combustion include age (rank), composition (volatile and water content), and particle size of coal. Efficient fire prevention and suppression require airtight storage, as leaks will feed oxygen (air) to the combustion process.(7,8) For timely action, monitoring of gas emissions and temperature at key locations is very important. In addition, scheduling for filling
and discharging the silos can be used to limit the likelihood of spontaneous combustion. A case example of a persistent smoldering fire in the storage silo in 2008(3) resulted in damage of hopper bellows and silo wall in the fire that required intermittent nitrogen injection for a total period of four months. Improved bottom sealing against air ingress has later helped to prevent fires of a similar scale, in spite of no change in sensor and alarm systems, other equipment, or storage operation. The causative factors and potential event chains for the smoldering fire hazard were reviewed in multiple sessions of a dedicated expert group including plant personnel. A resulting simple fault and event tree presentation of the 2008 incident is shown in Fig. 2. The consequences can include impact of noxious flue gases. Fires in a new type or design of a closed storage may in this way introduce new (emerging) risk for the operator personnel. Another effort of the expert group was to develop a tentative risk matrix related to storage fire incidents, as outlined in Fig. 3. The scales, limits, and matrix size (3 × 4) were adjusted to cover the new experience on the incident frequency and consequences, both on logarithmic scale, and extending the frequency and cost at least one order of magnitude beyond the observed extremes. So far, the experience suggests that by proper attention to coal grade, coal handling, suitable order and timing of silo filling and discharge, and properly maintained capabilities of both equipment and personnel, the fire risk has been contained to a significantly reduced level in comparison to the time of early operation of the storage.
Challenges in Underground Coal Storage Primary event
Intermediate
Top event
Ignition
2091 Alarm Coal removal Extinguishing Repeat
Consequence
Removal of burning coal Safe state or some smoke
High volatiles, moisture, rich in fines
Operator alerted
Self-heating coal in silo
Burning coal not removed
Autoignition in silo
Extinguished with water
Smoke, dirty belt (see case 1)
Extinguished with N2 Failed extinction
Coal removal & cyclic inertisation
Extinguished with water
Extended storage time of a batch
Extinguished with N2
Air ingress to silo/coal
Late alarm & major fire
Failed extinction
Intermittent emission of smoke & harmful gases Emission of smoke & harmful gases
Smoke, dirty belt, water gas Emission of smoke & harmful gases Extensive emission of smoke & harmful gases
Coal removal & cyclic inertization
Fig. 2. Fault and event trees for smoldering fire incidents; thick line shows the event path of the example case.
Fig. 3. Suggested risk matrix for self-heating and spontaneous combustion incidents in closed coal storage (colors visible in online version): red = immediate action required; orange = action required within defined time; yellow = tolerable; green = minor to negligible risk; the marker shows an estimate of the example case.
2.2. Coal Freezing Subzero winter temperatures can challenge the operation of handling, transport, storage, and end use of coal. This can happen when freezing water in coal forms ice that binds coal particles together and to external surfaces. Intensive studies in the 1970s
to 1980s resulted in the development of, e.g., freeze conditioning agents as additives to prevent clumping. However, these have been mainly used for transport by open space rail cars and storage in large open piles with much easier access than in case of closed storage. Also, freezing that occurs in open coal stockpiles is rarely severe, and is also fairly well accounted for by the current experience and operational procedures.(9–11) In practice, the process of coal freezing is largely driven by the extent and time at subzero ambient temperature. Cold winter spells may promote such challenges not only at the site of end use, but also at the origin of coal and along the route of transport. An example at the receiving end in Salmisaari was in December 2010, when a batch of coal with 12% total moisture was partly frozen at the harbour hoppers during discharge from ship. Operating coal feed to crusher and further to the underground storage became too slow at an ambient temperature of −19 ºC, even when attempting to assist coal flow by manual clearance and steam thawing (Fig. 4). In this case the coal batch was apparently already including frozen clumps that could not pass the hopper grilles, and the incident represented conventional coal freezing due to heat flow from coal to the environment. However, cold coal may freeze seepage water from the silo drains, producing clumps of ice or frozen coal that can block the coal flow at discharge
Sipila¨ et al.
2092
Fig. 4. (a) Thawing of harbour hopper blocked by frozen coal at −18 ºC; (b) a smallish, about 400 × 500 mm piece of frozen coal that was forming a part of blockage of coal flow. Primary event
Subzero coal in silos
Intermediate
Frozen drains, water to coal
Top event
No fuel
Clearing Thawing Reserve fuel 1 and 2
Safe state
No supply of reserve fuel
Short break or restriction
No fuel supply
No coal due to freezing Not cleared
Waterflow in drains Sufficient time to freeze
Consequence
Icy coal blocks, no coal from silos
(manually) No thawing (warm air/ Failed fuel steam) supply (reserve)
Short break or restriction Short break or restriction
Shutdown
Failed coal transport from other location
Fig. 5. Fault tree (left) and consequence tree for lost fuel supply due to frozen coal; thick line shows the event path of the example case.
bins and conveyors, and here the direction of heat flow was exclusively from outside toward the coal bed. This happened in the winter 2009–2010 with subzero temperatures from December to March, and high demand by the district heating service and corresponding coal consumption. At the time, two silos were not available, and cold Siberian coal was introduced to the remaining silos when the outside temperature in Helsinki was well below −10 ºC. After a while, disturbance was observed in coal discharge from the silos due to large ice clumps that blocked the discharge bins. Manual unblocking and heating were insufficient to restore coal flow to the power plant, and additional truck transport from another above-
ground stockpile was initiated. Heavy fuel oil as reserve fuel was also unavailable due to cold weather that made the necessary fuel heating system insufficient. The disturbance resulted in power derating and added cost of external energy supply, additional labor and maintenance cost, and cost of transport of replacement fuel from another plant. The disturbance was only ended with the arrival of a shipment of unfrozen Polish coal. The causative factors and potential event chains for the freezing hazard were reviewed by a dedicated expert group including plant personnel, and the resulting fault and event trees for this case are shown in Fig. 5, including the event path in the example case. The expert group also developed
Challenges in Underground Coal Storage
Fig. 6. Suggested risk matrix for in-silo coal freezing incidents (color visible in online version): red = immediate action required; orange = plan/implement preventive/mitigating action; yellow = tolerable; green = negligible risk; the marker shows an estimate of the example case.
a tentative risk matrix related to the freezing incidents, outlined in Fig. 6. Again, the scales, limits, and matrix size (3 × 3) were adjusted to cover the new evidence on incident frequency and consequences, and extending the frequency and cost at least one order of magnitude beyond the observed extremes. The corresponding risk matrix is outlined in Fig. 6. 2.3. Safety Issues As the storage facility is under normal operation automatic and remotely controlled, it generally avoids exposing the personnel to safety hazards except at times of, e.g., maintenance and other times requiring direct human attention. Hence the root causes of unwanted incidents and risks to personnel are expected to concentrate to relatively short periods of disturbance, maintenance campaigns, or other exceptional circumstances.(12,13) The characteristics of the storage are such that most safety concerns are expected to be related to “slips, trips, and falls from height” or “caught between” or “struck by” type of events, leading to injury or death of one or a few individuals, i.e., not to wider-scale damage or catastrophe.(14) An accident can be roughly modeled as an event or hazard (or series of them) penetrating the existing safety barriers, as outlined in the so-called Swiss cheese model (Fig. 7) to visualize the
2093 preconditions of an incident due to aligned defects in independent barriers. In reality, the safety barriers may not be fully independent and their performance can be time dependent, as the holes move, open, and close to allow for opportunity of penetration, especially under exceptional circumstances.(15–19) During normal operation without intensive human involvement in the automated and remotely controlled storage the safety risk is obviously much lower than during short periods or events of disturbance or maintenance requiring human intervention. A few published safety-related incidents have been reviewed from the underground coal storage. An example case(20) concerns an incident that occurred when coal was repeatedly clumping on a horizontal conveyor belt, resulting in sticky coal accumulating on the supporting roller, belt misalignment, and conveyor stop by limit switch action. Working alone, a well-experienced operator of the fuel supply system went to clean the roller with a steel bar from below the moving conveyor. He was found fatally injured after having been caught between the roller and the belt. According to an operating guideline, the conveyor must be stopped for cleaning. There was a protective steel grid fence in front of the rollerbelt gap, but it was possible to bypass it (Fig. 8). An emergency stop line runs along the conveyor but is not accessible from under the belt. Coal particles can accumulate on the rollers from residual water and coal sludge on the belt, e.g., after extinguishing selfignited coal on belt or in silos. The causative factors and potential event chains for the safety hazards were reviewed by a dedicated expert group including plant personnel, and the resulting fault and event tree presentation of the incident is outlined in Fig. 9. Additional safety measures such as an improved protective fence and further training of storage operators on safe operational practices were implemented, and no similar or comparable incident has since occurred. Note that a recent study on coal mines also suggests that although accidents at roller conveyors are relatively infrequent, they tend to be severe.(21) The case example represents a relatively infrequent event. Rarity of events may reduce the alertness and ability of organizations to notice and respond to the initiating events that may be small and not very clearly symptomatic at first. Therefore, it is important to be proactive in striving for safety by reducing exposure to hazard. The rate of smallerscale incidents that may not be easily spotted without effort, such as minor disturbances, injuries not requiring sick leave, or other comparable symptoms
Sipila¨ et al.
2094 Events / hazards Safety barriers Event causes
Organization: - management, principles, standards - motivation, rewarding Safety system: - training, audits, instructions - other safety measures Human behavior : - proficiency, education, experience - cultural & personal features Technical barriers to: - failures, disturbances, exceptional circumstances - poor design, aging, other degradation
Fig. 7. Swiss cheese model on the conditions of an incident.(15–19)
can be useful indicators of existing risk just below the current safety radar. Such information can help to combat less frequent but more severe manifestations of the associated risk.(22) Early warning indicators of safety concerns can be substandard adherence to safety norms of the company and potential exposure to hazards (e.g., as monitored in safety surveys).
3. EARLY WARNING AND PERFORMANCE INDICATORS It should be noted that although the issues of self-heating, fires, and freezing of stored coal are not unknown,(10,23–25) published experience that would refer to underground coal storage is very rare. The observed incidences may therefore represent emerging risk for the operator, and while the recent experience suggests means to foresee or mitigate such incidents,(3) there are also some threats. For example, low incidence frequency may reduce alertness and capability of the organization to respond to the infrequent initiating events. Such an event could be freezing by extreme winter cold spells that do not seem to disappear even in case of a generally warming world.(26) Also, favoring renewable solid fuels with high contents of volatiles and moisture may increase the risk of self-heating and freezing, exacer-
bated by high volumes needed to compensate for the low heating value. The suggested early warning indicators on spontaneously igniting fires, freezing, and safety for the underground coal storage are summarized in Table I, with suggested limit criteria and some notes on practical application. Note that the lead time of the indicators varies from actual alarm to about one year. In case of incidents and risk of self-heating and spontaneous combustion of coal in storage, early warning indicators include a storage time exceeding about one year, as assessed for the particular storage silos of the present case. This time is a function of the ratio of stored coal volume to surface area, and reactivity of coal (see, e.g., the standard EN 15188). One apparently useful index on reactivity, describing the propensity of bituminous coal to self-heating, is the modified Smith-Glasser index.(27) Leading indicators that provide less warning time include indicated odor (at more or less any detected level by the operators) and CO level exceeding a limit of about 5–15 ppm, depending on coal bed thickness, ventilation, and other storage-specific details. Yet another shorter term warning criterion is coal temperature exceeding about 40 ºC; this can also be applied for coal entering or exiting storage on conveyor belts. A suggested early warning indicator for coal freezing is subzero weather (about one year SGI > 0.42 Odor detection Indicated CO > 5–15 ppm Elevated coal temperature (>40 ºC) Subzero weather in filling (
DOI: 10.1111/risa.12273
Early Warning Indicators for Challenges in Underground Coal Storage ¨ 1,∗ Pertti Auerkari,2 Stefan Holmstrom, ¨ 3 and Iris Vela4 Juha Sipila,
Early warning or leading indicators are discussed for unexpected incidences in case of large-scale underground coal storage at a power plant. The experience is compared with above-ground stockpiles for which established procedures are available but where access for prevention and mitigation are much easier. It is suggested that while the explicit organization, procedures, and the general safety systems aim to provide the targeted levels of performance for the storage, representing new technology without much precedence elsewhere in the world, the extensive experience and tacit knowledge from above-ground open and closed storage systems can help to prepare for and to prevent unwanted incidents in the underground storage. This kind of experience has been also found useful for developing the leading or early warning indicators for underground storage. Examples are given on observed autoignition and freezing of coal in the storage silos, and on occupational hazards. Selection of the leading indicators needs to consider the specific features of the unique underground facility. KEY WORDS: Fire; freezing; risk indicator; safety; underground storage
1. INTRODUCTION
tent, and saving (urban) space and real estate. Also, an above-ground stockpile is rather unsightly and moving it underground will definitely help the city aesthetics. The cost of constructing the Salmisaari underground facility was about half of the proceeds from selling the released real estate and building rights.(1) The underground storage also carries some limitations, such as tightened ATEX (environment with an explosive atmosphere, from the French original ATmosph`eres EXplosibles) requirements, added requirements for ventilation and monitoring, and reduced access for any major action such as firefighting. However, the limitations were (and are) thought to be minor in comparison to the advantages. As there is only recent experience on the risks related to operation of a large underground storage, it appears important that such experience is systematically mapped for any significant emerging risk issue. For the same reason, well-selected leading or early warning indicators are suggested for early
The underground coal storage at the Salmisaari CHP power plant has been operational since 2004, and replaces an earlier above-ground open stockpile (Fig. 1). The storage has a total capacity of 250,000 tons, corresponding to about half of the yearly fuel consumption of the plant. The storage consists of four rock silos, ø 40 m × 65 m each, with the silo bottom at a depth of −120 m. The Salmisaari coal storage represents relatively new technology as similar facilities have not been previously used. Specific advantages of such storage in comparison to an aboveground stockpile include reduced dust, noise, and greenhouse gas emissions, slower loss of heat con1 Helsingin
Energia, Helsinki, Finland. Technical Research Centre of Finland, Espoo, Finland. 3 Joint Research Centre–IET Petten, The Netherlands. 4 BAM, Berlin, Germany. ∗ Address correspondence to Juha Sipila, ¨ Helsingin Energia, FI0090 HELEN, Finland; [email protected]. 2 VTT
2089
C 2014 Society for Risk Analysis 0272-4332/14/0100-2089$22.00/1
Sipila¨ et al.
2090
Fig. 1. Salmisaari power station (a) before and (b) after construction of underground coal storage.
warning in contrast to longer term (lagging) indicators or risk evaluation. Here, a brief review is presented of the current understanding of what have been considered potentially relevant, covering particularly incidents of autoignition and freezing of coal, and safety issues of the storage facility.(2–4) The indicators are discussed with reference to other performance measures that are applied to manage the risks. 2. EXPERIENCE ON THE CHALLENGES 2.1. Self-Heating and Autoignition Exothermic oxidation of carbon in coal can generate significant heating when the surrounding coal bed is a sufficient thermal insulator but does not exclude air (oxidant) ingress for continuing oxidation. Resulting self-heating up to ignition and smoldering fire is not uncommon in above-ground coal piles, but the hazard must also be accounted for in closed storage that has naturally reduced access for preventive or mitigating action. Extinguishing such fires in closed storage can be somewhat complicated, as has been learned in the course of operating the underground storage, in spite of precautions in the design.(1–3,5,6) The critical parameters for spontaneous combustion include age (rank), composition (volatile and water content), and particle size of coal. Efficient fire prevention and suppression require airtight storage, as leaks will feed oxygen (air) to the combustion process.(7,8) For timely action, monitoring of gas emissions and temperature at key locations is very important. In addition, scheduling for filling
and discharging the silos can be used to limit the likelihood of spontaneous combustion. A case example of a persistent smoldering fire in the storage silo in 2008(3) resulted in damage of hopper bellows and silo wall in the fire that required intermittent nitrogen injection for a total period of four months. Improved bottom sealing against air ingress has later helped to prevent fires of a similar scale, in spite of no change in sensor and alarm systems, other equipment, or storage operation. The causative factors and potential event chains for the smoldering fire hazard were reviewed in multiple sessions of a dedicated expert group including plant personnel. A resulting simple fault and event tree presentation of the 2008 incident is shown in Fig. 2. The consequences can include impact of noxious flue gases. Fires in a new type or design of a closed storage may in this way introduce new (emerging) risk for the operator personnel. Another effort of the expert group was to develop a tentative risk matrix related to storage fire incidents, as outlined in Fig. 3. The scales, limits, and matrix size (3 × 4) were adjusted to cover the new experience on the incident frequency and consequences, both on logarithmic scale, and extending the frequency and cost at least one order of magnitude beyond the observed extremes. So far, the experience suggests that by proper attention to coal grade, coal handling, suitable order and timing of silo filling and discharge, and properly maintained capabilities of both equipment and personnel, the fire risk has been contained to a significantly reduced level in comparison to the time of early operation of the storage.
Challenges in Underground Coal Storage Primary event
Intermediate
Top event
Ignition
2091 Alarm Coal removal Extinguishing Repeat
Consequence
Removal of burning coal Safe state or some smoke
High volatiles, moisture, rich in fines
Operator alerted
Self-heating coal in silo
Burning coal not removed
Autoignition in silo
Extinguished with water
Smoke, dirty belt (see case 1)
Extinguished with N2 Failed extinction
Coal removal & cyclic inertisation
Extinguished with water
Extended storage time of a batch
Extinguished with N2
Air ingress to silo/coal
Late alarm & major fire
Failed extinction
Intermittent emission of smoke & harmful gases Emission of smoke & harmful gases
Smoke, dirty belt, water gas Emission of smoke & harmful gases Extensive emission of smoke & harmful gases
Coal removal & cyclic inertization
Fig. 2. Fault and event trees for smoldering fire incidents; thick line shows the event path of the example case.
Fig. 3. Suggested risk matrix for self-heating and spontaneous combustion incidents in closed coal storage (colors visible in online version): red = immediate action required; orange = action required within defined time; yellow = tolerable; green = minor to negligible risk; the marker shows an estimate of the example case.
2.2. Coal Freezing Subzero winter temperatures can challenge the operation of handling, transport, storage, and end use of coal. This can happen when freezing water in coal forms ice that binds coal particles together and to external surfaces. Intensive studies in the 1970s
to 1980s resulted in the development of, e.g., freeze conditioning agents as additives to prevent clumping. However, these have been mainly used for transport by open space rail cars and storage in large open piles with much easier access than in case of closed storage. Also, freezing that occurs in open coal stockpiles is rarely severe, and is also fairly well accounted for by the current experience and operational procedures.(9–11) In practice, the process of coal freezing is largely driven by the extent and time at subzero ambient temperature. Cold winter spells may promote such challenges not only at the site of end use, but also at the origin of coal and along the route of transport. An example at the receiving end in Salmisaari was in December 2010, when a batch of coal with 12% total moisture was partly frozen at the harbour hoppers during discharge from ship. Operating coal feed to crusher and further to the underground storage became too slow at an ambient temperature of −19 ºC, even when attempting to assist coal flow by manual clearance and steam thawing (Fig. 4). In this case the coal batch was apparently already including frozen clumps that could not pass the hopper grilles, and the incident represented conventional coal freezing due to heat flow from coal to the environment. However, cold coal may freeze seepage water from the silo drains, producing clumps of ice or frozen coal that can block the coal flow at discharge
Sipila¨ et al.
2092
Fig. 4. (a) Thawing of harbour hopper blocked by frozen coal at −18 ºC; (b) a smallish, about 400 × 500 mm piece of frozen coal that was forming a part of blockage of coal flow. Primary event
Subzero coal in silos
Intermediate
Frozen drains, water to coal
Top event
No fuel
Clearing Thawing Reserve fuel 1 and 2
Safe state
No supply of reserve fuel
Short break or restriction
No fuel supply
No coal due to freezing Not cleared
Waterflow in drains Sufficient time to freeze
Consequence
Icy coal blocks, no coal from silos
(manually) No thawing (warm air/ Failed fuel steam) supply (reserve)
Short break or restriction Short break or restriction
Shutdown
Failed coal transport from other location
Fig. 5. Fault tree (left) and consequence tree for lost fuel supply due to frozen coal; thick line shows the event path of the example case.
bins and conveyors, and here the direction of heat flow was exclusively from outside toward the coal bed. This happened in the winter 2009–2010 with subzero temperatures from December to March, and high demand by the district heating service and corresponding coal consumption. At the time, two silos were not available, and cold Siberian coal was introduced to the remaining silos when the outside temperature in Helsinki was well below −10 ºC. After a while, disturbance was observed in coal discharge from the silos due to large ice clumps that blocked the discharge bins. Manual unblocking and heating were insufficient to restore coal flow to the power plant, and additional truck transport from another above-
ground stockpile was initiated. Heavy fuel oil as reserve fuel was also unavailable due to cold weather that made the necessary fuel heating system insufficient. The disturbance resulted in power derating and added cost of external energy supply, additional labor and maintenance cost, and cost of transport of replacement fuel from another plant. The disturbance was only ended with the arrival of a shipment of unfrozen Polish coal. The causative factors and potential event chains for the freezing hazard were reviewed by a dedicated expert group including plant personnel, and the resulting fault and event trees for this case are shown in Fig. 5, including the event path in the example case. The expert group also developed
Challenges in Underground Coal Storage
Fig. 6. Suggested risk matrix for in-silo coal freezing incidents (color visible in online version): red = immediate action required; orange = plan/implement preventive/mitigating action; yellow = tolerable; green = negligible risk; the marker shows an estimate of the example case.
a tentative risk matrix related to the freezing incidents, outlined in Fig. 6. Again, the scales, limits, and matrix size (3 × 3) were adjusted to cover the new evidence on incident frequency and consequences, and extending the frequency and cost at least one order of magnitude beyond the observed extremes. The corresponding risk matrix is outlined in Fig. 6. 2.3. Safety Issues As the storage facility is under normal operation automatic and remotely controlled, it generally avoids exposing the personnel to safety hazards except at times of, e.g., maintenance and other times requiring direct human attention. Hence the root causes of unwanted incidents and risks to personnel are expected to concentrate to relatively short periods of disturbance, maintenance campaigns, or other exceptional circumstances.(12,13) The characteristics of the storage are such that most safety concerns are expected to be related to “slips, trips, and falls from height” or “caught between” or “struck by” type of events, leading to injury or death of one or a few individuals, i.e., not to wider-scale damage or catastrophe.(14) An accident can be roughly modeled as an event or hazard (or series of them) penetrating the existing safety barriers, as outlined in the so-called Swiss cheese model (Fig. 7) to visualize the
2093 preconditions of an incident due to aligned defects in independent barriers. In reality, the safety barriers may not be fully independent and their performance can be time dependent, as the holes move, open, and close to allow for opportunity of penetration, especially under exceptional circumstances.(15–19) During normal operation without intensive human involvement in the automated and remotely controlled storage the safety risk is obviously much lower than during short periods or events of disturbance or maintenance requiring human intervention. A few published safety-related incidents have been reviewed from the underground coal storage. An example case(20) concerns an incident that occurred when coal was repeatedly clumping on a horizontal conveyor belt, resulting in sticky coal accumulating on the supporting roller, belt misalignment, and conveyor stop by limit switch action. Working alone, a well-experienced operator of the fuel supply system went to clean the roller with a steel bar from below the moving conveyor. He was found fatally injured after having been caught between the roller and the belt. According to an operating guideline, the conveyor must be stopped for cleaning. There was a protective steel grid fence in front of the rollerbelt gap, but it was possible to bypass it (Fig. 8). An emergency stop line runs along the conveyor but is not accessible from under the belt. Coal particles can accumulate on the rollers from residual water and coal sludge on the belt, e.g., after extinguishing selfignited coal on belt or in silos. The causative factors and potential event chains for the safety hazards were reviewed by a dedicated expert group including plant personnel, and the resulting fault and event tree presentation of the incident is outlined in Fig. 9. Additional safety measures such as an improved protective fence and further training of storage operators on safe operational practices were implemented, and no similar or comparable incident has since occurred. Note that a recent study on coal mines also suggests that although accidents at roller conveyors are relatively infrequent, they tend to be severe.(21) The case example represents a relatively infrequent event. Rarity of events may reduce the alertness and ability of organizations to notice and respond to the initiating events that may be small and not very clearly symptomatic at first. Therefore, it is important to be proactive in striving for safety by reducing exposure to hazard. The rate of smallerscale incidents that may not be easily spotted without effort, such as minor disturbances, injuries not requiring sick leave, or other comparable symptoms
Sipila¨ et al.
2094 Events / hazards Safety barriers Event causes
Organization: - management, principles, standards - motivation, rewarding Safety system: - training, audits, instructions - other safety measures Human behavior : - proficiency, education, experience - cultural & personal features Technical barriers to: - failures, disturbances, exceptional circumstances - poor design, aging, other degradation
Fig. 7. Swiss cheese model on the conditions of an incident.(15–19)
can be useful indicators of existing risk just below the current safety radar. Such information can help to combat less frequent but more severe manifestations of the associated risk.(22) Early warning indicators of safety concerns can be substandard adherence to safety norms of the company and potential exposure to hazards (e.g., as monitored in safety surveys).
3. EARLY WARNING AND PERFORMANCE INDICATORS It should be noted that although the issues of self-heating, fires, and freezing of stored coal are not unknown,(10,23–25) published experience that would refer to underground coal storage is very rare. The observed incidences may therefore represent emerging risk for the operator, and while the recent experience suggests means to foresee or mitigate such incidents,(3) there are also some threats. For example, low incidence frequency may reduce alertness and capability of the organization to respond to the infrequent initiating events. Such an event could be freezing by extreme winter cold spells that do not seem to disappear even in case of a generally warming world.(26) Also, favoring renewable solid fuels with high contents of volatiles and moisture may increase the risk of self-heating and freezing, exacer-
bated by high volumes needed to compensate for the low heating value. The suggested early warning indicators on spontaneously igniting fires, freezing, and safety for the underground coal storage are summarized in Table I, with suggested limit criteria and some notes on practical application. Note that the lead time of the indicators varies from actual alarm to about one year. In case of incidents and risk of self-heating and spontaneous combustion of coal in storage, early warning indicators include a storage time exceeding about one year, as assessed for the particular storage silos of the present case. This time is a function of the ratio of stored coal volume to surface area, and reactivity of coal (see, e.g., the standard EN 15188). One apparently useful index on reactivity, describing the propensity of bituminous coal to self-heating, is the modified Smith-Glasser index.(27) Leading indicators that provide less warning time include indicated odor (at more or less any detected level by the operators) and CO level exceeding a limit of about 5–15 ppm, depending on coal bed thickness, ventilation, and other storage-specific details. Yet another shorter term warning criterion is coal temperature exceeding about 40 ºC; this can also be applied for coal entering or exiting storage on conveyor belts. A suggested early warning indicator for coal freezing is subzero weather (about one year SGI > 0.42 Odor detection Indicated CO > 5–15 ppm Elevated coal temperature (>40 ºC) Subzero weather in filling (
