1- Calculation : Temperature of the Crucible should be close to adiabatic combustion temperature of the biomass which could be calculated if you have basic composition of biomass material (in terms of raw fuel - H2,C, CnHm, O2, S...) and through heat balance of the furnace: Temperature of Furnace is result between energy flux (enthalpy) provide by biomass combustion minus heat loss (fatal heat loss due to waste flue gas exhaust and heat losses from furnace walls)
2 Or You can do measurements : 2.1 Direct measurement with IR camera of the furnace walls temperature or with thermocouple place through the wall just under internal surface of wall refractories and 2.2 A high temperature thermocouple into combustion crucible or Suction pyrometer prob juste under Biomass combustion