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The uncertainty in the energy systems due to the continuously growing share of variable renewable energy sources (VRES) increases the requirements for flexibility. From the multiple mechanisms to improve flexibility, biomass-fired combined heat and power plants (CHPs) have gained attention due to their sustainability and high efficiency. CHPs with thermal energy storage (TES) can provide flexibility by adjusting their power to heat ratio. The aim of this thesis is to analyse the transient operation of the boiler and the TES tank in biomass CHP, based on dynamic modelling. The commonly applied proportional integral (PI) controller has numerous limitations for complex multivariable processes like boilers. Advanced controller (model predictive control (MPC)) and additional sensor (feed-forward (FF)) are proposed to improve the control of the process. For the TES tank, the dynamic model is used to investigate the influence of the state of charge, flowrates and temperatures used on its charge and discharge rates. Results show that FF MPC can allow for more accurate transient control of the boiler, which will result in a faster response rate of the CHP to changes in supply and demand. The assessment of the TES tank operation enabled a mapping of the range of charge and discharge rates and the factors that can influence it. This knowledge can be utilised to conduct accurate additional studies on the role of TES in a CHP flexible operation.