Partial oxidation of biogas to hydrogen rich gas

The conversion of gaseous hydrocarbons can be achieved in many ways. Partial oxidation with air is one of the options. In this process methane in the biogas with oxygen in air to form hydrogen in a bed of catalyst.
i. Principle
This commercial process route is basically the result of sequential combustion reactions in which the gas is burnt with deficit oxygen (nearly 30 % of stoichiometric requirement). Depending on the feed -stock the product gas may require purification of sulphur compounds and CO2 - removal. The advantage of dispensing with an external heat source favours the partial oxidation step, since the oxidation of CO supplies the necessary heat.
ii. Process flow
The biogas containing 55-60 % methane , 40 % CO2 and traces of H2 S is first dehydrated and then purified from H2 S. The gas is then sent to the partial oxidizers to form hydrogen. Depending on the composition of the outlet gas from the oxidizer methanation of residual carbon oxides can be incorporated. Finally a Co2 scrubber may be added depending on the type of fuel cell to be used in the power plant.
iii. Process
The process occuring in the partial oxidizer is

CH4 + 0.5 O2 = Co + 2H2
The process requires oxygen, which may be separated and supplied from air and is favoured by moderately high pressure. It is understood that due to residence time limitations, the process approaches equilibrium leaving some residual methane and carbon in the product gas. Hence CO2 needs to be scrubbed and recycled in the plant (Balthasar).
iv.Need for pure oxygen
If air is used instead of oxygen the separation of hydrogen from nitrogen is difficult. Hence oxygen at a purity of at lease 95 % and has to be used in large scale systems. Pressure of notrogen could be accepted in small scale fuel cell applications. After all the cathode gas (air) contains 80% nitrogen. The process has thus to be modified for operation on air in this particuler study.
v.Factors controlling the process
As the partial oxidation proceeds through a flame reaction, it is necessary to moderate the flame temperature, preferably by means of steam. The raw gas composition is controlled by the oxygen to methane ratio and by the steam addition. In order to reduce the oxygen consumption for the oxidation step the biogas and steam has to be preheated and have to be metered precisely to the reactor.
The operating conditions vary with the non-catalytic reactors.

Pressure: 60 to 90 bar, Reaction temperature: 1200 to 1370 C.
Under catalytic partial oxidation a commercial process, topsoe SBA has specified the following conditions.
Pressure: up to 30 bar or more. Temperature: 90C.
Catalyst: NiFeed-stock gas and super heated steam are mixed and preheated to 60c and mixed with oxygen .
vi.Reactor
The reactor may be a refractory lined stainless steel vessel with one or more burners. In order to initiate the reaction part of the gas has to be burnt inside the reactor. Johnson Matthey Ltd. report in their patent a reactor made of SS tube with entry for gas at the middle of the reactor and exit for product gas from the end. As gas entry is made near the middle of catalyst bed, they have observed a higher temperature of 450 c at the tip and a uniform temperature of 280 c surrounding the hot zone. This is claimed to be superior to at once through reactor. The reactor may then have multiple (4 or 5 entries from the sides).
A theoretical model developed by Opris et. al describes the temperature and product distribution profiles along the length of the reactor. This fits well with the commercial data. The reactor requires a minimum length for complete partial oxiation of methane leading to a continuous supply of hydrogen at a fixed concentration.
vii.Catalysts
The catalyst used by Johnson Matthey let. in their Hot Spot TM reactor contained 0.01 to 5 wt % platinum and from 1 to 15 wt % chromium oxide supported on a refractory solid such as silica. The support may be monolith honeycomb or particles with a maximum size of 1.5 mm.
viii.Catalyst deactivation
The successful economic and technical utilization of the process depends on the avoidance of free carbon deposition which decreases the catalyst surface area resulting in lower reaction rates. It is suggested that carbon deposition shall be avoided by operational techniques rather than by inhibition.
ix.Product gas
The hot product gas is expected to have the following composition on a dry basis.Hydrogen and CO, 93 % by volumeCarbon dioxide, 5 % by volumeNitrogen and argon, 1.5 % by volume Methane, 0.6 % by Volume. The reactor effluent needs rapid cooling to freeze the gaseous equilibria established at the high temperature reactor by a direct water quench or by heat exchange. The product gas then has to be washed free of carbon-sulphur compounds, carbon dioxide and inert gases.

Tar removal during biomass gasification

One of the major issues in the biomass gasification process is how to deal with the tar formed during the process. Tars can be easily defined as undesirable and problematic organic products of biomass gasification. There are a large number of different operational parameters that define composition and quantity of the produced tar, concerning both mentioned methods, such as temperature, pressure, gasifying medium, catalyst and additives used, equivalence ratio (ER), gasification ratio (GR), steam-to-biomass ratio (SB), gas residence time (or space time) etc. The tars can cause quite a few problems in the different applications such as cracking in the pores of filters, forming coke and causing plugging of the filters, condensing in the cold spots and plugging the cold spots; all this resulting in serious operational interruptions and maintenance costs. Another vital issue regarding tars is that they contain carcinogenic compounds that have to be removed to achieve health and environmental demands. Both physical and chemical treatment processes can reduce the presence of tar in the product gas.
The physical processes are classified into wet and dry technologies depending on whether water is used. Various forms of wet or wet/dry scrubbing processes are commercially available, and these are the most commonly practiced techniques for physical removal of tar. Wet physical processes work via gas tar condensation, droplet filtration, and/or gas/liquid mixture separation. Cyclones, cooling towers, venturis, baghouses, electrostatic precipitators, and wet/dry scrubbers are the primary tools. The main disadvantage to using wet physical processes is that the tars are just transferred to wastewater, so their heating value is lost and the water must be disposed of in an environmentally acceptable way. Wastewater that contains tar is classified as hazardous waste; therefore, its treatment and disposal can add significantly to the over-all cost of the gasification plant.

Dry tar removal using ceramic, metallic, or fabric filters are alternatives to wet tarremoval processes. However, at temperatures above 150°C, tars can become “sticky” causing operational problems with such barriers. As a result, such dry tar removal schemes are rarely implemented. Injection of activated carbon in the product gas stream or in a granular bed may also reduce tars through adsorption and collection with a baghouse. The carbonaceous material containing the tars can be recycled back to the gasifier to encourage further thermal and catalytic decomposition.Chemical tar treatment processes are the most widely practiced in the gasificationindustry. They can be divided into four generic categories: thermal, steam, partially oxidative, and catalytic processes.

Tars can be removed from the gas stream in the fuel reformer or by separate hot gas tar removal catalysts. Thermal destruction has been shown to break down aromatics at temperatures above 1,000 degC. However, such high temperatures can have adverse effects on heat exchangers and refractory surfaces due to ash sintering in the gasification vessel. The introduction of steam does encourage reformation of primary and some secondary oxygenated tar compounds, but has little effect on tertiary aromatics.There are two methods that have been used in research on catalytic tar conversion in laboratories worldwide.

The first method is with catalyst mixed with the feed biomass in so called catalytic gasification or pyrolysis (in situ). In this case tar is removed in the gasifier itself (usually in a fluidized bed gasifier). In the second method tar is treated downstream of the gasifier in a secondary reactor, outside of the gasifier (fixed bed catalytic reactor).There are a large number of different catalysts that have been used to eliminate the tars in the product gas from the gasification process. The two most researched groups are Ni-based catalysts and dolomites. When Ni-based catalysts are used, tar concentration in the product gas can be reduced significantly by means of reforming but since this process is endothermic, a part of the chemically bound energy of the gas has to be burned to sustain this process. This effect leads to a decreased efficiency of the gasification process.In contrast, when so called tar cracking catalysts such as dolomite are used, the only thing that is reformed is the tar itself while low hydrocarbons e.g. methane, ethane and propane are left intact. Simultaneously with this transformation of tar, the gas composition (CO2, CO, H2 etc.) changes as a consequence of reactions that will be described later in the text. Tar cracking can be defined as a process that breaks down the larger, heavier and more complex hydrocarbon molecules of tar into simpler and lighter molecules by the action of heat and aided by the presence of a catalyst but without the addition of hydrogen.

Dolomite is a calcium magnesium ore with the general chemical formula CaMg(CO3)2 with some minor impurities. In order for dolomite to become active for tar conversion, it has to be calcined. Calcination involves decomposition of the carbonate mineral, eliminating CO2 to form MgO-CaO, at high temperatures (usually 800-900 degC). The effective use of dolomite as a catalyst is restricted by relatively high temperatures and the partial pressure of CO2. When it comes to the importance of dolomites composition for catalytic activity, it has been shown that an increased content of iron in dolomites, i.e. Fe2O3, can raise its activity towards tar elimination by 20%.