INTRODUCTION:

Syngas is used as an intermediate in the industrial synthesis of ammonia and fertilizer. During this process, methane (from natural gas) combines with water to generate carbon monoxide and hydrogen.

The gasification process is used to convert any material that has carbon to longer hydrocarbon chains. One of the uses of this syngas is as a fuel to manufacture steam or electricity. Another use is as a basic chemical building block for many petrochemical and refining processes.

The general raw materials used for gasification (creation of syngas) are coal, petroleum based materials, or other materials that would be rejected as waste. From these materials, a feedstock is prepared. This is inserted to the gasifier in dry or slurry form. In the gasifier, this feedstock reacts in an oxygen starved environment with steam at elevated pressure and temperature. The resultant syngas is composed of 85% carbon monoxide and hydrogen and small amounts of methane and carbon dioxide.

The syngas may contain some trace elements of impurities, which are removed through further processing and either recovered or redirected to the gasifier. For example, sulfur is recovered in the elemental form or as sulfuric acid and both of these can be marketed. Syngas is a primary source of sulfuric acid. If syngas contains a considerable quantity of nitrogen, the nitrogen must be separated to avoid production of nitric oxides, which are pollutants and contribute to acid rain production. Both carbon monoxide and nitrogen have similar boiling points so recovering pure carbon monoxide requires cryogenic processing, which is very difficult.

OBJECTIVE:

Hazards of synthesis gas and and its mitigation measures

Syngas is an abbreviation for synthesis gas. It refers to a mixture primarily of hydrogen (H₂) and carbon monoxide (CO) which may also contain significant but lower concentrations of methane (CH₄) and carbon dioxide (CO₂) as well as smaller amounts of impurities such as chlorides, Sulfur compounds, and heavier hydrocarbons. There are many similarities between H₂ and CO as well as some important differences. The requirements for Syngas are a combination of safe practices for H₂ and CO, since both are present in significant proportions. Therefore there is a need for familiarization with the mitigation measures of syngas to deal with hazards related to it.

MATERIAL AND METHOD:

The syngas is produced by gasification of a carbon containing fuel to a gaseous product that has some heating value. Some of the examples of syngas production include steam reforming of petcoke, gasification of coal emissions, and waste emissions to energy gasification.

The name syngas is derived from the use as an intermediate in generating synthetic natural gas and to create ammonia or methanol. It is a gas that can be used to synthesize other chemicals, hence the name synthesis gas, which was shortened to syngas. Syngas is also an intermediate in creating synthetic petroleum to use as a lubricant or fuel.

Figure 1: Composition of synthesis gas

Properties of synthesis gas:

Flammability limits:

The flammability limit is the experimentally-determined minimum concentration of fuel (lean limit) or oxidant (rich limit) required for self-sustaining flame propagation at a specified initial pressure and temperature. The flammability limit is of primary interest in safety assessments as an absolute indication of the existence of a combustion hazard and the main reference point in defining a safety margins for a combustion hazard.

Safety requirements for Syngas are a combination of safe practices for H₂ and CO, since both are present in significant proportions.

Table 1: Properties of H2 & CO

LFL%	4	12.5
UFL%	75	74
Auto Ignition Temp Deg C	500	630
Ignition Energy mJ for 100 %	0.2	0.3

The flammability limits of wet CO in air is known to be 12.5% CO at the lean limit and 74% CO at the rich limit. A pure CO/air mixture may not burn at NTP because of the absence of chain carriers and chain branching reactions essential for flame propagation. A small amount of water vapour or hydrogen will ensure production of chain carriers; when present in small quantities, the effects of hydrogen and water vapour have about the same effect on CO oxidation kinetics. In containment, water vapour and, possibly, hydrogen are essentially assured so the dry flammability limit of CO is not relevant.

The flammability limit of H₂ in air is 4% H₂ at the lean limit for upward propagation and 75% H₂ at the rich limit for both upward and downward propagation. Addition of up to 12.5% CO to a lean-limit H₂-air mixture is not expected to change the flammability limits of H₂-air mixtures. Thus all mixtures containing >4% H₂ or >12.5% CO will burn provided the oxygen limit is not reached. The oxygen limit is the same, about 5%, for both CO/air (with traces of H₂ or H₂O) and H₂/air mixtures. Flammable range of most mixtures widens with increasing temperature. The flammability limits are also influenced by the amount and type of diluent. CO₂ has a greater thermal effect on the flammability limits.

Table 2: Flammability limits for H2-CO mixtures

1:3	8.16	2.04	6.12
1:1	6.06	3.03	3.03
3:1	4.82	3.61	1.20

Auto-ignition:

Auto-ignition temperature is the temperature (at a given pressure and mixture composition) at which a combustible gas mixture will spontaneously ignite. It is of interest in safety assessments in the evaluation of mechanisms for initiating combustion.

Auto-ignition temperatures of H₂ and CO are separately known and are on the order of 500^oC and 630^oC, respectively. There is no unique auto-ignition temperature for CO-O₂ or CO-air mixtures. Even small amounts of moisture or hydrogen can drastically alter the auto-ignition temperature of CO-O₂ or CO-air mixtures.

Hydrogen:

Hydrogen is lighter than air, highly flammable, easily ignited, heats up when reduced in pressure, does not support breathing and is one of the most difficult gases to prevent from leaking. In the pure state, it presents some unique corrosion mechanisms and when combined with even small impurities (ppm), the corrosion concerns can multiply.

Hydrogen burns in air with a pale blue, almost invisible flame which increases the risk of injury in case of fire.The maximum flame propagation rate is up to 3 m/s in air. Although auto-ignition of leaks and atmospheric vents is always a possibility with any flammable gas, hydrogen is especially prone to this phenomenon. This is due to hydrogen’s low ignition energy and the fact that, unlike most gases, hydrogen increases in temperature when it expands from a higher to a lower pressure. This tendency towards auto-ignition of leaks and atmospheric vents, combined with the difficulty in seeing the flame make small leaks a serious potential personnel injury risk.

Carbon monoxide:

Carbon monoxide is a flammable gas. Ignited in air, it burns with a little illuminating blue flame. Carbon monoxide is a toxic gas. For this very reason, any CO or Syngas project should be considered as “critical” in the sense of requiring a detailed risk management review under the responsibility of the owner.

Note: Carbon monoxide is quickly fixed on haemoglobin, causing a decrease in cellular respiration, which is particularly harmful to the central nervous system .Therefore, it is important to understand the potential effect of exposure to various concentrations of carbon monoxide, which may be encountered so as to provide a safe environment.A threshold limit value of 50 ppm is recommended for carbon monoxide, as a concentration in air to which nearly all workers may be exposed during an 8-hour workday and 40-hour workweek, without adverse effects.

RESULT AND DISCUSSION:

While studying the hazards of synthesis gas, following mitigation measures are identified as :

• Control of third party interference.

• Ensuring that the gas feed remains dry.

• Increased thickness of the pipe.

• Installation in pipe ways or corridors.

• Nondestructive test on welds.

• Inerting of the pipe.

• Pipeline marking.

• Specific pipelines warning devices: marking mats or tapes.

• Isolation valves.

• Excess flow or low pressure shut down valves.

• Physical protections: concrete coating or casing, concrete slabs.

• Operating procedures, including: inspection programmes, leak detection surveys, corrosion control programmes, emergency plan, personnel training.

• Information of third parties, collaboration with local authorities.

• Fire and gas detection systems for early detections.

• Fire Protection systems for the process equipment.

•

CONCLUSION:

Syngas has 50% of the energy density of natural gas. It cannot be burnt directly, but is used as a fuel source. The other use is as an intermediate to produce other chemicals. The production of syngas for use as a raw material in fuel production is accomplished by the gasification of coal or municipal waste. In these reactions, carbon combines with water or oxygen to give rise to carbon dioxide, carbon monoxide, and hydrogen.

There are many similarities between H₂ and CO as well as some important differences. The requirements for Syngas are a combination of safe practices for H₂ and CO, since both are present in significant proportions.

Syngas can represent a significant hazard and needs to be handled carefully to prevent high consequence events from occurring. The gas, although lighter than air, disperses quickly and the net effect of buoyancy is small inside the flammable boundaries. Incident histories indicate that major hydrogen release incidents result in delayed ignition in about 4–5% of the cases. The vast majority of high pressure releases promptly ignite, which precludes formation of a flammable cloud and, therefore, a VCE. Experiments with synthesis gas in a shock tube indicated that high pressure releases would ignite at temperatures below the auto ignition temperature of hydrogen, which is helpful to allow use of hydrogen data to infer syngas ignition.

REFERENCES:

1. Beychok, M.R., Process and environmental technology for producing SNG and liquid fuels, U.S. EPA report EPA-660/2-75-011, May 1975

2. Syngas in Gas Engines, www.clarke-energy.com, accessed 15.11.11

3. Syngas used in IC engines VOL-1

4. Syngas used in IC engines 2

5. Beychok, M.R., Coal gasification and the Phenosolvan process, American Chemical Society 168th National Meeting, Atlantic City, September 1974

6. "Syngas using metal catalyst". University of Minnesota. Retrieved 25 August 2011.

7. NWT magazine 6/2012

8. "Sunshine to Petrol". Sandia National Laboratories. Retrieved April 11, 2013.

9. "Integrated Solar Thermochemical Reaction System". U.S. Department of Energy. Retrieved April 11, 2013.

10. Matthew L. Wald (April 10, 2013). "New Solar Process Gets More Out of Natural Gas". The New York Times. Retrieved April 11, 2013.

11. Frances White. "A solar booster shot for natural gas power plants". Pacific Northwest National Laboratory. Retrieved April 12, 2013.

12. Syngas production with solar energy

13. No use of fossil fuels with production of syngas using solar power

14. Goldstein. "Beyond electricity generation: airborne wind energy system for synthetic fuel production and energy storage". Presentation at Airborne Wind Energy Conference, 2013.

15. Emmanuel O. Oluyede. "Fundamental impact of firing Syngas in gas turbines". Clemson/EPRI. Retrieved 2012-11-10.

Received on 17.09.2014 Accepted on 30.09.2014

Research J. Engineering and Tech. 5(3): July-Sept. 2014 page 144-146