[sseagull]

This is a really good question, and a bit deeper than it first appears. So here is some semi-educated spitballing (I'm a chemist, but thermodynamics was a while ago):

1. Immediately after ignition, you have a low-volume, high-pressure, high-temperature amount of gas. Sequestration does not aim to turn CO2 back to this exact same state, but only a high-ish, average-temperature state.

2. Combustion often evolves more molecules of gas (look at the formula for the combustion of octane, and remember that water after combustion will be a gas). This increases the pressure, but is not something that needs to be reversed during sequestration.

3. Carbon dioxide isn't bad, but having too much in the atmosphere is. Sequestration doesn't aim to completely reverse the reaction in the first place, it just aims to remove it from the atmosphere so that it can't act as a greenhouse gas.