Marginal Abatement Cost Curve: Which Decarbonisation Projects, and in What Order

How to read this sheet

The curve is built one project at a time, each priced against what the earlier ones leave behind, and that is the only sense in which a decarbonisation plan can be costed. Adding a list up counts the same tonne twice wherever two projects touch the same load, and the error is not small: on the seeded mill it is a tenth of everything claimed. Split the loads finely enough for this to work, because two projects only overlap here if they name the same load - a compressed air project and a motor project charged against one line called electricity will appear to overlap completely, and charged against separate lines will appear not to overlap at all. The load table is where the honesty of the answer is decided. Capital does not shrink when the load does, which is why the ranking moves. A project installed fifth is spread over whatever abatement is left, so its cost per tonne rises while its capital does not, and a project that pays for itself on the untouched plant can cost money in the plan. Read the two cost columns together rather than the second alone. The most useful line on the sheet is where the reduction stops being free. Everything above that point is capital with a return attached and needs no carbon argument at all; everything below it is a cost that has to be justified by a buyer requirement, a levy or a commitment. Mills routinely discover that the free part reaches twenty per cent and the target is forty, which is a financing conversation and not an engineering one. What is not here: the projects are treated as independent except through the load they share, so a project that changes another project percentage rather than its baseline is outside the model. Nothing is scheduled - two projects wanting the same shutdown, the same crew or the same wall are all just a list here. Grid emission factors fall over time and are held constant, which flatters electrical projects taken late and penalises them taken early. Scope 3 is absent entirely, and for a mill selling to apparel brands it is usually the larger number. And the abatement is engineering abatement, not a certificate: what a buyer will accept, what an accounting standard will recognise, and what a certification body will verify are three further questions this does not answer.