World Bunkering > News > Winter 2009 > H2S! Take care!

The presence of hydrogen sulphide (H2S) in marine fuels poses a dual danger through the storage and handling of fuels. Not only must we be aware of the associated health and safety risks but also of the damage that this corrosive gas can cause to tanks and fuel lines. If we are aware of danger we can act to mitigate the risks. Historically there has been no routine test to identify this gas under ISO8217. Instead, the policing of H2S was left with the refinery. But does the testing and indeed the specification limit adopted downstream fully assist in quantifying the potential risk? This article aims to highlight the issues surrounding H2S and the developments with the testing of H2S in bunker fuel oils.

Just what is H2S?

Hydrogen sulphide is a colourless chemical compound that is both toxic and flammable. It is often responsible for the foul odours of rotten eggs in volcanic gas, stagnant water or flatulence! Although very pungent at first, it quickly deadens the sense of smell so that potential victims may be unaware of its presence until it is too late. This poisonous gas can attack several different systems in the body, although the nervous system is most affected.

It forms a complex bond with iron blocking oxygen from binding and stopping cellular respiration. Exposure to lower concentrations will result in nausea, shortness of breath, eye irritation, sore throat and fluid in the lungs. Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability, poor memory, and dizziness. H2S is heavier than air and will settle and accumulate at the bottom of tanks and poorly ventilated spaces. To demonstrate its lethal ability we can list the following affects of exposure:

• One ppm is the level at which most people will detect the characteristic odour;
• 10 ppm is the occupational health exposure limit for eight working hours per day;
• 20 ppm causes eye irritation;
• 200 ppm paralyses the olfactory nerve: the sense of smell disappears;
• 350–550 ppm leads to pulmonary oedema with the possibility of death;
• 550–1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading eventually to loss of breathing
• 800 ppm is a lethal concentration to humans with five minutes exposure;
• Concentrations, over 1,000 ppm causes immediate collapse with loss of breathing, even after a single breath.

How is it created?

Crude oil is a complex mixture of hundreds of hydrocarbons, including many which contain sulphur. Refining the crude oil includes converting most of that sulphur into gaseous hydrogen sulphide. The hydro-desulphurisation process liberates sulphur from petroleum by the action of hydrogen and the resulting H2S is converted to elemental sulphur by partial combustion. Raw natural gas also contains gaseous hydrogen sulphide and mercaptans (sulphur-containing organic chemical substances) which are removed in natural gas processing. The hydrogen sulphide removed in the refining and processing of crude oil and natural gas is subsequently converted into by-product elemental sulphur.

H2S and bunkers

H2S is undesirable in marine fuels and should not be present. However, as there are no current limits set for the accepted levels of H2S at the point of delivery of bunkers, the adoption of an agreed level is being considered. Storage tanks and fuel systems which use bunkers containing H2S have been known to encounter high levels of corrosion. Sulphate reducing bacteria (SRB) in the oil have been said to cause microbial induced corrosion to the tank bottom plating. In addition, the H2S gas released from the bulk liquid during storage will build up in the tank head space to create an ideal environment for pyrophor (pyro-phoric iron sulphide).

In basic terms this means that sulphide will react with iron oxide (rust) without oxygen to create iron sulphide in an exothermic reaction, resulting in substantial heat, which for obvious reasons is not desirable!. Wet cracking is the occurrence of blistering to exposed steel in an aqueous environment that contains hydrogen sulphide. This hydrogen related corrosion results in blisters or blister cracks to the plate surfaces. Fuel containing entrained H2S gas will release some of that gas throughout its storage. The rate at which the gas will evolve will depend on many varying factors, including:

• The quantity of H2S in the liquid phase;
• The length of time the fuel is stored;
• The temperature of the oil;
• The way in which fuel is agitated, pumped or transferred;
• Bio-chemical decomposition.

Setting limits

ISGOTT (International Safety Guide for Oil Tankers and Terminals) states in its guidelines for crew entering pump rooms and enclosed spaces, a limit of 10ppm TLV (Threshold Limiting Value) and OSHA (Occupational Health & Safety Administration) has guidelines for H2S exposure also set at 10ppm. The crux of the matter is understanding the potential risk that this gas poses. Overexposure can kill. So what limit should we apply? And should the limit be applied to H2S in the vapour phase or the liquid phase? From a health and safety perspective, it is the gas phase which is most important, as significant concentrations of the gas are known to accumulate in the head space of storage tanks.

From an operational perspective, the liquid phase is considered more important. However, the concentration of gas in the vapour phase can only be measured at a given moment of time, whereas the measurement of H2S in the liquid phase gives a total concentration and therefore indicates the potential of the fuel to emit the gas in the vapour phase. Unfortunately, the correlation between H2S in the liquid phase and in the vapour phase is far from simple. The amount of gas emitted by a liquid to the atmosphere depends largely on a “partition coefficient”. In simple terms, this is the amount of gas that is entrained in a liquid phase that would be expected to be released to the atmosphere (the equilibrium point).

The problem with marine bunker fuel is that it is not a “pure distillate” or “pure chemical”, and as such each batch could have a different chemical make-up and therefore a different coefficient. Various studies have been carried out in an effort to determine the partition coefficient of residual oil. Depending upon which study is referred to, the findings vary. However, in general, it is estimated that every 1 mg/kg H2S in liquid phase represents anything between 80 and 400 ppm in the vapour phase.

Test methods

Historically the industry has relied on two established test methods for the detection of H2S, but both need to be put into the context of the appropriate application. To measure H2S in the liquid phase of the oil (by IP 399) we use a traditional wet chemistry test which involves lengthy preparation. This is considered a “complex” test and measures the trapped (entrained) gas in the liquid which can be released over time. The test, once complete, will quote a result in milligrams per kilogramme (mg/kg or ppm). Refinery (downstream) specifications may vary but generally a limit of between 2 to 3 mg/kg is applied prior to the sales of the stock.

To measure H2S in the vapour phase requires the measurement of evolved gas into the head space of a sample container. This test is carried out using gas detection tubes. The laboratory test (ASTM D5705) involves heating a known quantity of oil in an enclosed vessel for a set period of time, after which a gas detection tube in the inert head space measures the H2S in the gas phase. Additionally, these gas detection tubes can be used to check H2S levels in tank tops or worn continuously by barge or vessel crew to calculate the exposure over time.

This onboard vapour phase test is best suited for use to quantify and assess occupational health risks. In reality the only semi-accurate way of measuring the evolved gas is to carry out onboard measurements in the tank tops or the affected confined working areas. These onboard tests will give a “snap shot” of the level of H2S at that given time. However, the onboard test is not a standard test method, it is simply a means to measure the levels of exposure, if you remember ISGOTT recommend exposure be limited to 10ppm TLV (Threshold Limiting value) Reports of increased cases of H2S in bunker fuels has prompted the ISO working group to add a new test and specification limit to the proposed (draft) of the ISO8217, expected to be finalised and published in 2010.

The new test method (IP 570) has been adopted by ISO in an attempt to make routine analysis faster and more accessible. But there are problems. Due to poor historical data on the occurrences of H2S in bunker fuels, the inclusion of the parameter has been delayed until 2012 to allow time for data to be collated and an appropriate specification to be concluded.

The way forward

It is clear that the presence of H2S in marine fuel is undesirable. Defining an acceptable limit within the specification is something that we believe will require considerably more debate. The limit of 2 mg/kg put forward by the ISO working group has been questioned because an acceptable level of 2 mg/kg in the liquid phase could give rise to as much as 800 ppm in the vapour phase; a lethal concentration with five minutes exposure and well in excess of the occupational health recommendation of 10 ppm value

The fact that the ISO working group have added H2S to the latest draft of ISO8217 is in our opinion a very positive step as it serves to heighten the profile of what is a very important debate. Currently there is little research on H2S in marine fuels and it seems that there is some way to go before an acceptable level will be agreed.