  The goal of this paper is to review and critically assess different methods
to monitor key process variables for ethanol production from lignocellulosic
biomass. Because cellulose-based biofuels cannot yet compete with
non-cellulosic biofuels, process control and optimization are of importance to
lower the production costs. This study reviews different monitoring schemes, to
indicate what the added value of real-time monitoring is for process control.
Furthermore, a comparison is made on different monitoring techniques to measure
the off-gas, the concentrations of dissolved components in the inlet to the
process, the concentrations of dissolved components in the reactor, and the
biomass concentration. Finally, soft sensor techniques and available models are
discussed, to give an overview of modeling techniques that analyze data, with
the aim of coupling the soft sensor predictions to the control and optimization
of cellulose to ethanol fermentation. The paper ends with a discussion of
future needs and developments.
