Life-cycle assessment

Life-cycle assessment is where claims about carbon technologies are settled or exposed. A process that captures CO2 is not thereby beneficial, and one that emits CO2 is not thereby harmful; what matters is the change in total emissions across the whole system relative to what would otherwise have happened. Getting that comparison right is harder than it looks, and the ways it goes wrong are well documented and repetitive [1].

Where techno-economics asks what a technology costs, this page asks whether it helps at all.

The comparison is the method

An LCA result is meaningless without three declarations, and most disputes trace back to one of them being left implicit.

The functional unit is what the comparison delivers: a tonne of methanol, a kilometre driven, a megawatt-hour. Comparing a CO2-derived product to a fossil one only works if both deliver the same function, and products with different properties or lifetimes are not automatically interchangeable.

The system boundary decides what counts. A CO2 utilization process that looks beneficial within a gate-to-gate boundary can look quite different once the capture step, the hydrogen supply, and the fate of the product are included [2].

The reference system is what would have happened otherwise, and it is a choice rather than a fact. This is the same sensitivity that makes cost of CO2 avoided depend on the reference plant, appearing again in a different currency.

Multifunctionality, and where double counting enters

CO2 capture and utilization is structurally awkward for LCA because the CO2 is simultaneously a waste from one process and a feedstock for another. Two systems have a claim on the same tonne, and unless the allocation between them is stated explicitly, both can book the benefit. Handling that multifunctionality correctly is the central methodological problem of CCU assessment [1][2].

The practical failure is easy to describe. An emitter claims a reduction for capturing CO2 rather than venting it. A producer claims a reduction for making a product from captured CO2 rather than from fossil feedstock. If nobody subtracts, the same tonne has been counted twice, and the sum of the parts overstates the whole.

When is a removal actually a removal

The most useful discipline imposed on this field is a checklist of minimum qualifications a negative emission technology and its accounting must satisfy. There are four, and all must hold: physical greenhouse gases are removed from the atmosphere; the removed gases are stored out of the atmosphere in a manner intended to be permanent; upstream and downstream emissions across the whole system life cycle are comprehensively estimated and included in the balance; and the total quantity removed and permanently stored exceeds the total quantity emitted [3].

Most things described as carbon removal fail at least one of these, usually the second or the fourth. Applying the list is the fastest way to evaluate a claim. It also explains cleanly why chemical and fuel products are emissions reductions rather than removals: they fail the permanence condition, because the carbon returns when the product is used [5].

Open challenges

• Boundary shopping. Because the boundary is a choice, a favourable result can be produced without any dishonest arithmetic, which is why the boundary has to be reported prominently rather than in supplementary material [1].
• Attributional versus consequential. Assessing what a process is responsible for and assessing what changes if it is deployed are different questions with different answers, and the two are frequently conflated [2].
• Temporal accounting. Emissions now and removals decades later are not equivalent, and standard LCA has no native way to express that difference [3].
• Coupling assessment to chemistry. Catalysis and life-cycle assessment belong in one analysis rather than in two separate literatures, because a catalytic improvement can shift the life-cycle result and often does not [4].