Saturday, May 16, 2009

General hazards of carbon dioxide

This information is from the following web site:

"At room temperature and ambient pressure CO2 is a colourless, odourless gas that will not support combustion or human life. CO2 has been recognised as a workplace hazard for over a century. It is significantly heavier than air and many fatalities from asphyxiation have resulted from entry into pits, tanks, sumps or cellars where CO2 has accumulated and displaced oxygen.

It is also possible for dangerous levels of CO2 to form out-of-doors in trenches, depressions or valleys. This is particularly likely when the gas is colder than the surrounding air, which may occur following pressurised release.

In 2000, a US Environmental Projection Agency study on CO2 related incidents in fire scenarios reported that since 1975 there were 51 recorded incidents involving the discharge of CO2 fire extinguishing systems resulting in 72 deaths and 145 injuries.
There is no significant inherent human response to CO2 that could be useful as a detection mechanism. Human response to hydrogen sulphide by smell occurs at very low (ppm) concentrations, similarly with ammonia and sulphur dioxide.

In contrast, CO2 is present in the air we breath (0.037%). This may cause problems with instrumented detection because the 'background' CO2 levels are so high. In addition, the cooling effects of a pressurised CO2 leak may have an adverse effect on the accuracy and operability of CO2 gas detection systems.

The recognition of the dangers of CO2 has prompted much research into its toxicity in both human volunteers and animals. It is now known that, in addition to the problem of asphyxiation due to the displacement of oxygen, the inhalation of elevated concentrations of CO2 can increase the acidity of the blood triggering adverse effects on the respiratory, cardiovascular and central nervous systems. Data from published research reports has been used by HSE to quantify the toxicity of CO2 in the form of Dangerous Toxic Load (DTL)1 values.

The DTLs have been used in calculations by the Health and Safety Laboratories (HSL) to demonstrate that CO2 exhibits major accident potential, when transported by pipeline in large quantities at ambient temperature and at a pressure of 7 bar or more, well below the dense phase or supercritical region. It is not yet clear whether controls should be applied to the transport of CO2 in this context but pending further research, it is possible that HSE will propose amending the Pipelines Safety Regulations to include CO2 as a dangerous fluid.

Additional hazards of dense phase or supercritical carbon dioxide

For economic and technical reasons it is likely CO2 will be handled close to or above its critical pressure (73.82 bar) where many of its properties are similar to that of a liquid. In this state it is often referred to as a dense phase fluid, whereas above critical temperature (31.04oC) and pressure it is referred to as supercritical. Most of the additional hazards associated with dense phase or supercritical CO2 arise when this pressure suddenly falls or is lost completely.

Scale of the thermal cooling envelope

In the event pf a major pressure loss, e.g a pipe rupture or containment failure, the depressurisation will result in an increase in the volume occupied by the CO2 of several hundred fold as the escaping fluid undergoes a rapid expansion (and phase change) as a proportion essentially 'boils' and becomes a gas while the remainder forms solid particles. This rapid, violent expansion causes the temperature of escaping CO2 to fall very rapidly, frequently below -80°C. while the particles of solid CO2 formed (dry ice) will result in projectiles expelled at very high velocities.

Cryogenic burns and impact injuries from extremely cold jet of gas and entrained missiles are serious hazards to personnel. Cryogenic embrittlement of structural steelwork and adverse effects from the impingement of extremely cold gas jets on safety-critical equipment are major threats to the structural and functional integrity of nearby plant unless appropriately designed or protected.

Toxic contamination effects

Supercritical CO2 is a highly efficient solvent. When supercritical CO2 undergoes a significant pressure reduction it moves from its supercritical state with super solvent properties to a gaseous state with virtually no solvent capability. In any environment where other substances are present with supercritical CO2 their solvation will occur resulting in fluid medium or "solution" containing various compounds or elements many of which may be extremely toxic. Any toxic substance held in such a pressurised 'solution' will 'precipitate' out on loss of pressure or containment and is likely to result in harmful human exposure or environmental damage due to the contamination of the area of deposition unless appropriate measures are taken.

Dry Ice 'grit blasting effects'

Where captured CO2 may be present with solid particles such as reservoir-derived sand and other solid debris, loss of containment may result in these combining with the dry ice formed to produce particles of a much greater abrasive capability than dry ice alone. This would enhance the erosion effects on process pipework and vessels adjacent to the leak which could lead to further damage to equipment and hence risk to people.

Specific challenges associated with dense phase or supercritical carbon dioxide

Whilst the processes that make up Carbon Capture and Storage (CCS) are not novel in themselves there is relatively little experience worldwide in managing the risks associated with CO2, compared with oil and gas. The major accident hazards presented by handling high pressure CO2 offshore or onshore need to be considered in the context of about 10,000 years' operating experience in managing hazards associated with hydrocarbon processing offshore alone, and probably much more if onshore processes are included2. In comparison there are probably less than 100 operating years for handling CO2 and significantly less in dealing with supercritical CO2.

Modelling dense phase/supercritical CO2 releases

The ability to anticipate foreseeable major accident scenarios and accurately predict the consequences of these hazardous events is a fundamental element in the assessment of the risk. A lack of substantial operation experience in a novel process or technology generally leads to significant difficulties in identifying accurately the hazards associated with that process or technology.

We do not yet fully understand the behaviour of CO2 when released from dense phase. Industry is researching appropriate models which will need to be validated. There is a need for appropriate scale experimental work to provide HSE and duty holders with a thorough understanding of how CO2 behaves during foreseeable large releases.

Containment and integrity

Whilst there are applicable general engineering standards, there is a lack of internationally recognised standards and codes of practice specifically for dense phase or supercritical CO2 plant and equipment. When designing, fabricating and maintaining plant for handling and transporting CO2 it is important that the full significance its physical properties, at the temperatures, pressures and inventories required are fully recognised and managed accordingly. Where applying standards developed for other substances including hydrocarbons, such as natural gas, extreme caution is advised as even the highest standards for many other substances may not be sufficient to ensure adequate containment for CO2 under the expected, and unexpected operating envelope(s).

Deep-Sixing CO2 Emissions

Until recently, however, few cared so much about how long the carbon dioxide stayed put.

That is starting to change. Since 2000, the North American energy company EnCana Corp. has boosted oil production 50% at Weyburn, Canada, by injecting millions of tons of surplus CO2 from North Dakota. Plans call for at least 20 million tons in all to be sequestered permanently there in coming decades -- an amount equal to the annual emissions of 6.8 million cars.

So far, monitoring indicates that most of it will stay underground but, by one report, about 2,500 tons a day bubble to the surface where it must be recaptured and re-injected.

Critics of the storage operations worry about the long-term safety of the reservoirs. No one knows whether excess carbon dioxide will remain stable underground for hundreds or thousands of years.

"If it can find any well, crack or conduit in the rock, it will escape," said Harvard carbon storage researcher Kurt Zenz House.

With more than 3.5 million oil wells drilled in the U.S. since petroleum exploration began in earnest 150 years ago, there is no shortage of potential leaks.

Experts also worry how so much carbon dioxide will alter the chemistry of the storage formations themselves. Bubbles composed of millions of tons of sequestered CO2 could form an acid that could etch away the confining rocks or erode the concrete caps on well heads.

To test the effects of carbon dioxide storage, researchers funded by the U.S. Department of Energy recently injected 2,000 tons -- about half a day's power plant emissions -- into a mile-deep well northeast of Houston.

After monitoring the site for two years, researchers at the U.S. Geological Survey found no leaks.

But in a study made public in July, they did discover that the buried CO2 increased the acidity of the saltwater in the rock enough to dissolve the surrounding minerals. Should enough minerals be eaten away, the gas could seep slowly into the atmosphere again, they reported. The acidic solution also could combine with trace metals and organic compounds to contaminate groundwater."