Natural gas, biogas, biomass gas and landfill gas all are the sources for methane rich gas. However, the percentage of its content varies. Each source gas arbitrarily contains other components like hydrogen sulphide, nitrogen, carbon dioxide and water. Well-engineered technologies are established to remove these components and produce rich methane content gas. Scientifically improved technologies can convert the upgrades methane gas into synthesis gas with different ratios of carbon monoxide and hydrogen. Some of the famous technologies are
- Steam reforming
- Partial oxidation
- Autothermal reforming
- Dry reforming
- Combination of steam reforming and dry reforming
- Tri reforming
Synthesis gas by steam reforming
In production of chemical compounds from a natural gas, steam reforming is used as intermediate step from several years. This process does not use air or oxygen and avoids nitrogen content fraction problems. The steam-reforming reactor operates at low temperatures and pressures. Overall cost is much higher than other methods. Various developers like Lurgi, Haldor topsoe, Foster Wheeler Crop, Kinetics Technology, and International BV and Uhde Gmbh developed and established the process. They are best in steam reforming technology. All over the world, many plants are operated and commissioned by these licensors.
The governing chemical reaction is methane + water ↔ carbon monoxide + hydrogen. Its heat of reaction value at 298 K is about 206000kJ/kmol of methane molecule. An efficiently designed tube reactor with external heating operation used in most industries. Operating temperatures of complete unit range from 500 to 780 oC. Carbon formation does not happen even at temperature 800 oC and reaching the conversion of 0.99.
Partial Oxidation Technology to Produce Synthesis gas from Methane Gas
Controlled reactions take place by releasing heat energy that produces carbon monoxide and hydrogen from methane with less quantity of oxygen supply is scientifically explained by partial oxidation mechanism. Engineering model of partial oxidation equipment adapts technology from fixed bed reactor and combustion system. In overall, methane molecule splits into hydrogen and carbon monoxide on a catalyst that control the combustion reaction mechanism. Oxygen from air reacts with methane naturally in combustion engine when used as fuel in the form of CNG.
This spontaneous reaction generates heat and does mechanical work on piston. The engine produces flue gas of carbon dioxide and water but in synthesis gas technology, this spontaneous reaction is controlled to produce CO and hydrogen along with fewer quantities of CO2 and H2O. Technologies developers from Royal Dutch and Texaco convert the methane gas into synthesis gas for valuable chemical products production plants. Desulphurization of methane gas is not required in many cases because partial oxidation reactor can handle less content of sulphur. The sulphur is converted into sulphur dioxide. However, oxygen for the process must be enough to prevent nitrogen content. An oxygen separation plant is required to produce oxygen from air. More over efficient heat exchange is required to utilize the heat energy released due to highly exothermic reaction, CH4 + 0.5O2 ↔ CO + 2H2.
Types in partial oxidation
- Catalytic (at less temperature and pressure)
- Non-catalytic (old method at high temperature and pressure):
Sulfur in the form of hydrogen sulphide and other organic compounds line mercaptans make the natural gas ineligible for catalytic processing. Therefore, an ideal of partial oxidation without using catalyst in the reactor could eliminate the desulfurization process step. However, sulfur oxide is generated in the combustion process. By process intensification and integration, SO2 can be reacted with water to produce sulphurous acid. By introducing the water scrubbing process step SO2 can be removed from synthesis gas.
Catalyst list used for catalytic reaction
- Nickel, Al2O3, Zinc, Rhodium/ alpha- Aluminia, Platinum metal gauze
Reactor types that handle partial oxidation
- Adiabatic or isothermal, fixed bed model, plug flow reactor.
An advanced process operates on the combination of endothermic steam reforming and exothermic partial oxidation. By controlled mechanism, heat generated from combustion of a fraction of methane with oxygen is used for reforming the remaining fraction to CO and H2. The technique eliminates the requirement of secondary reformer. However, pure oxygen should be used in the process for high conversions and controlling the heat transfer between the beds.
Lugri and Haldor Topsoe are top developer of ATR model. This is the only model that can used to adjust ratios of synthesis gas components by temperature control.
Dry reforming technique
Developed by Carbon Sciences it is an upcoming technology. In this model, carbon dioxide reacts with methane and so low emission of CO2 can be achieved. However the bottleneck is carbon formation, it is advantages as per the green technologies. Endothermic reaction occurs while synthesis gas processing.
The idea of combing the DR and SMR led to the positive results for hydrogen and carbon monoxide ratio in synthesis gas. The composition it contains is just the requirement for producing liquid fuels.
Haldor Topsoe developed the technology that uses the flue gas carbon dioxide and converts it to synthesis gas. It is a competition to dry reforming process. CO2 separation step can be eliminated which save more energy. It is a model designed based on the dry reforming, steam reforming and partial oxidation methods. A promising process use flue gas (CO2, H2O and O2) directly with methane and conduct above three reaction simultaneously. A nickel-based catalyst engineered for this model operates thermodynamically at 850 oC and 1 atm.