The one your last audit already flagged. Failing that: heat losses and steam traps — shortest payback (often under a year), no process risk, and they're what ESOS/ISO 50001 assessors list first. Every practice here links to the evidence behind its range.
From our guide: Industrial Decarbonization Practices — full context, charts and sources there.
Boilers, kilns, heat exchangers, valves and steam lines lose energy continuously. Inzonex makes patented (UK GB2508992.1) removable modular insulation — snap-fastened covers engineered per temperature tier, not generic off-the-shelf jackets: