  The interplay between quantum-mechanical properties, such as coherence, and
classical notions, such as energy, is a subtle topic at the forefront of
quantum thermodynamics. The traditional Carnot argument limits the conversion
of heat to work; here we critically assess the problem of converting coherence
to work. Through a careful account of all resources involved in the
thermodynamic transformations within a fully quantum-mechanical treatment, we
show that there exist thermal machines extracting work from coherence
arbitrarily well. Such machines only need to act on individual copies of a
state and can be reused. On the other hand, we show that for any thermal
machine with finite resources not all the coherence of a state can be extracted
as work. However, even bounded thermal machines can be reused infinitely many
times in the process of work extraction from coherence.
