Carbon dioxide (CO2) quality begins with the raw feedstock, and even the ingredients and agents which go into this feedgas or feedstock – these agents can be substances such as the natural gas or coal which is combusted for flue gas, then downstream liquefaction and purification occurs.
The lion’s share of CO2 source types are chemical by-product in nature, most of which are a handful of source types, including a highly-concentrated by-product from reformer operations in oil refineries and by-product from anhydrous ammonia operations. There is also an ever-important number of fermentation by-product plants in the US, distilled ethanol from (usually) corn and grain.
Then we have natural sources from mother nature; which are wells dating back before the dawn of man. These major sources are often the cleanest and easiest to recover and refine, and typically these sources are very high in raw CO2 content, in a water-saturated stream. Once the water is knocked out, then the CO2 content alone can be up to or beyond 99% by volume. In some cases, as with the natural sources, impurities are a challenge; for example, there have been cases containing high levels of hydrocarbons, including methane and heavier hydrocarbons, too much sulfur content, and of course the case of benzene, which is particularly difficult.
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