The Gasification Step

The produced gas contains a mixture of H2, CO, CO2, H2O, and CH4, coming mainly from the combination of hydrocarbons reforming and the reversible water-gas shift reactions. 

Optionally, N2 (up to 50% in the product gas) can be present if O2 from air is used as (co)gasification agent without the implementation of Air Separation Unit (ASU) technology however, N2 content higher than 3% should be avoided [10] in order to prevent dilution of the Bio-SNG, which results in a reduced size of the facility, and, eventually, high heat 

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The processes that have been developed for the production of synthetic natural gas are often configured to produce as much methane in the gasification step as possible thereby minimizing the need for a methanation step. In addition, methane formation is highly exothermic which contributes to process efficiency by the production of heat in the gasifier, where the heat can be used for the endothermic steam—carbon reaction to produce carbon monoxide and hydrogen. 

Ammonia production by partial oxidation of hydrocarbon feeds depends to some degree on the gasification step. The clean raw synthesis gas from a Shell partial oxidation system is first treated for sulfur removal, then passed through shift conversion. A Hquid nitrogen scmbbiag step follows. 

The first step, the gasification step, can be made thermally neutral by setting a target of AH = 0. The second step, the synthesis step, is exothermic and therefore produces heat. We can now solve for a, b, c, and d. 

These results are important since they highlight that once biomass has been gasified to synthesis gas, the usual processing technologies can be used for chemical synthesis, which are well understood by the industry. As the efficiency of the gasification step is fairly competitive with coal, this may make the gasification of biomass a fairly viable option, but the other metrics developed in this book must be applied as well. 

The filter water from the MARS is recycled to quench and venturi. Surplus of water, formed the gasification step has to be rejected. It is fed to a stripper to remove traces of... 

In the CASST process (see figure I) bitemperature pyrolysis at 350-400°C, into volatiles and charcoal. The diarcoal is gasified with steam at about 00-900°C to produce fiiel or synthesis gas. The volatiles are combusted with air at 1000-1100°C to supply the heat required for the endothermic gasification step. A circulating inert heat carrier, e.g. sand, transports the heat between the combustion and the gasification step. 

Since the CASST process is an indirectly heated process, no air is used in the gasification step. Therefore, the product gas will have a low nitrogen content giving a medium calorific fuel gas and oiabling use of the product gas as synthesis gas. 

At a carbon conversion of 80% in the gasification step, the performance of a CASST/CC system is lower than for air-blown gasification/combined cycle system (39.0 vs. 41.7%). The lower gross power output of the CASST/CC system is only partly compensated by a lower internal power use. Inaeasing the carbon conversion to 95% almost eliminates the difference in the performance of twth systems. 

Table II notes that in the process two important equilibria in acceptor reactions are involved. These were checked experimentally since the literature data are contradictory. No data at all were available on the H2S acceptor reaction, which is important in the gasification step, and the equilibrium was also determined for Reaction 5.

A block flow diagram for production of fuel grade methanol from biomass is depicted in Figure I. The gasification step is based upon the Purox process and is followed by shift conversion and gas purification steps. The clean gas, which is shifted to a H2/CO ratio of approximately 2/1, is converted to methanol in the ICI low-pressure methanol synthesis process. The process yields approximately 98% pure methanol with the remaining 2% consisting of water and some higher carbon number alcohols. 

Oxidation can also be realized in two steps in which the excess air ratio of the first step is below 1.0. Under these conditions the reaction products can be further oxidized in a second step releasing the rest of the available energy. Carbon monoxide and/or hydrocarbons are typically produced at the first step and transported to another device for full oxidation. Within such processes the procedure described above is paused for example after the gasification step e.g. within a gasifier) is performed, the oxidizing step is realized at another time and at another place e.g. within the engine). 

It should also be pointed out that gasifiers operated at steam to oxygen ratios as low as possible provide the thermally most efficient and lowest cost route to the production of medium BTU gas (1). The co-production of pipeline gas in the gasification step improves the overall thermal efficiency of various liquefaction processes (Flscher-Tropsch, Methanol, Mobils MTG route) by 13 to 18 % (2). 

In the long-term research perspective, hydrogen is produced from electrolysis via solar or wind power and this hydrogen reacts with CO2 to either form synthesis gas, methane, or (biologically) converted into ethanol (the Ineos process). In conclusion, it can be said that methanol production via the synthesis gas route will remain the dominant route for the next 10—20 years, but greening this process is being introduced via the use of bio-based feedstocks in the gasification step. 

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