Synthesis Gas. James G. Speight

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      Although these reactions may be represented very simply using equations of this type, the reactions can be complex and result in carbon deposition on parts of the equipment thereby requiring careful inspection of the reactor.

      The gasification process is the culmination of five processes: (i) feedstock preparation, (ii) gasification, (iii) steam generation, (iv) air separation, and (v) synthesis gas cleaning, each of which will be described in detail in the subsequent chapters. Briefly, and at this point, a brief mention of the processes is warranted.

      The feedstock preparation section will, more than likely, include unit operations such as (i) size reduction, (ii) screening, and (iii) slurrying, the extent of each segment is feedstock dependent and also dependent on the properties of the raw material feed and on the requirements of the gasification technology that is selected.

      The gasifier is a reactor for the partial oxidation of the carbon-based raw material to produce raw synthesis gas. This is a high temperature conversion process, and in the gasification zone of the gasifier the temperature is in the range 900 to 1600^oC (1650 to 2910^oF). The conversion of the raw material feed to synthesis gas is endothermic. Part of the feedstock is completely oxidized to carbon dioxide and water to provide the energy for syngas production. The carbon monoxide, hydrogen, carbon dioxide, and water equilibrate at reaction conditions so that the composition of these compounds in the raw synthesis gas product is governed by the water gas shift equilibrium at outlet conditions. All elements that can be volatilized at the gasification temperature will end up in the raw synthesis gas. For example, if the raw material contained sulfur or nitrogen, the raw synthesis gas will also contain hydrogen sulfide (H2S) and ammonia (NH3). Non-volatilized carbon matter and mineral matter are rejected from the gasification process either as soot, ash, or slag. Another important aspect of gasification is heat recovery, which determines the thermal efficiency of the gasifier.

In the steam generation section, the synthesis gas composition can be manipulated by the ratio of steam to the feedstock that is co-fed to the gasifier. The steam is a source of hydrogen and is essential for feed materials with a low effective hydrogen-to-carbon ratio.

      Gasification is a partial oxidation and pure oxygen is usually the most efficient oxidant. It is possible to use air as oxidant, but air should not be considered as an oxidant in a gasification process where the energy is supplied by direct combustion of part of the process feed. When air is mixed with the process feed, a large volume of inert gases, mainly nitrogen, is also introduced to the process. Once the inert gases enter the process stream, they remain in the process stream until after the Fischer-Tropsch process. Thus, most gasification processes require an air separation unit to produce pure oxygen as the oxidant for the gasification process.

      The raw synthesis gas produced by the gasifier contains contaminants that are poisons for the Fischer-Tropsch catalyst. Contaminants, such as hydrgoen sulfide and halogenated compounds (that can arise when plastic waste – such as from PVC plastic waste – is part of the feedstock), must be removed from the synthesis gas. It may also be beneficial to remove carbon dioxide and water, depending on the requirements of the design of the Fischer-Tropsch gas loop.

2.3 Feedstocks

      Gasification technology offers the important ability to take a wide range of feedstocks and process them into synthesis gas, from which a similarly diverse number of end products are possible. In fact, the technology has advanced to such an extent that gasifiers have been developed to suit all different types of carbonaceous materials such as highly carbonaceous products from crude oil refining – such as crude oil resids, heavy crude oil, tar sand bitumen, and coke from thermal processes – and biomass, as well as various type of waste such as agricultural waste, industrial waste, and municipal waste (Chapter 1). The flexibility stems from the ability of gasification to take any carbon and hydrogen containing feedstock and then thermochemically break down the feedstock to a gas containing simple compounds which are easy to process into several marketable products.

      2.3.1 Natural Gas

      Synthesis gas can be produced from natural gas, or virtually any hydrocarbon feedstock, by reaction of the feedstock with steam or oxygen. The reaction of the hydrocarbon with oxygen can be represented by the simple equations:

      The formation of synthesis gas is strongly endothermic and requires high temperatures. Steam reforming of natural gas (or shale gas) proceeds in tubular reactors that are heated externally. The process uses nickel catalyst on a special support that is resistant against the harsh process conditions. Waste heat from the oven section is used to preheat gases and to produce steam. This plant generates synthesis gas with hydrgoen-carbon monoxide (H₂/ CO) ratios in the range of 3 to 4, and is suitable for hydrogen production.

      The partial oxidation of methane (or other hydrocarbon derivatives) is a non-catalytic, large-scale process to make synthesis gas and yields synthesis gas with a hydrogen-carbon monoxide (H₂/CO) ratio on the order of 2. This is an optimal ratio for gas-to-liquids plants. A catalytic version of partial oxidation that is based on short-contact time conversion of methane, hydrocarbon derivatives or biomass on, for example, rhodium catalysts, is suitable for small-scale applications (Zhu et al., 2004; De Campos Roseno et al., 2018).

      2.3.2 Crude Oil Resid, Heavy Crude Oil, Extra Heavy Crude Oil, and Tar Sand Bitumen

      Gasification is the only technology which makes possible a zero residue target for refineries, contrary to all conversion technologies (including thermal cracking, catalytic cracking, cooking, deasphalting, hydroprocessing, etc.) which can only reduce the bottom volume, with the complication that the residue qualities generally get worse with the degree of conversion (Speight, 2014a, 2017). While it is unlikely that any members of the crude oil family (Chapter 1) with substantial amounts of volatile components will be used directly as a gasification feedstock, the potential is high for the use of the resids from these crude oil as gasification feedstocks.

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