Scavengers: Doomsday

Direct injection of chemical scavengers is being proposed as a way to avoid corrosion in CO₂ pipelines. Injecting chemicals is an operation that is done in oil & gas. The concept is really simple: inject a chemical that reacts with the problematic impurities, thus avoiding the formation of strong acids. But what if the scavenger introduces more problems than it solves?

Some proposed approaches draw on chemicals familiar to other industries, for example, from the semiconductor industry. Experience handling these substances is valuable, but does not establish their suitability for high pressure CO₂ transport.

The chemistry may work. Autoclave experiments might demonstrate impurity consumption and negligible corrosion. Such results matter, provided the untreated conditions would actually cause corrosion. But in closed experiments, reactive impurities may become depleted. In operating pipelines, they are continuously replenished. Even successful laboratory tests do not necessarily establish performance under continuous flow.

A pipeline is not a controlled chemical reactor. A scavenger must disperse through dense or gas phase CO₂, encounter impurities at parts per million concentrations and react sufficiently quickly. Mixing, mass transfer, reaction kinetics and competing reactions determine if favourable chemistry translates into effective corrosion control.

Then, it comes dosing. Too little scavenger may leave the corrosion risk unresolved. Too much could introduce reactive residuals into the pipeline. What happens during impurity spikes, poor mixing or injection skid failure? How is the correct dose established and maintained?

Even if the scavenger works, the impurities do not simply disappear. Depending on the chemistry, reaction products may include solids, liquids or aerosols. These could require filtration or separation to prevent accumulation or transport into downstream equipment, injection wells and geological storage. Pigging might remove some deposits, but this may be impractical if the operation needs to be done frequent.

Some of the chemicals, such as SiH₄, used to manufacture semiconductors are pyrophoric gases. For that industry, the reactions are very controlled in clean rooms and in the small scales in comparison to an industrial CCS system. Safely storing large amounts of those chemicals, pressurising, metering and injecting them into dense phase CO₂ presents considerable engineering and process safety challenges. Dilution in a carrier/inert gas might facilitate delivery, but introduces additional flow and changes the composition of the transported CO₂ stream.

A chemical intended to prevent corrosion could instead leave operators managing the original reactive impurities alongside hazardous chemical inventories, reactive residuals, unwanted deposits and additional equipment failures. These outcomes are not inevitable, but neither can they be ruled out by demonstrating a successful reaction in an autoclave.

Chemical scavengers may have a role in CO₂ corrosion control. However, their qualification requires evidence of reliable delivery, effective mixing, controlled dosing, manageable reaction products and acceptable consequences throughout the transport and storage system.

An in-depth critical review of direct in-line chemical injection for corrosion control in CO₂ transport systems is in preparation to be presented in a conference.