BIOMASS NON-REHEAT STEAM CYCLE POWER PLANTS. PART 2: INFLUENCE OF WATER AND STEAM PARAMETERS ON THE EFFICIENCY OF FLUIDIZED BED STEAM GENERATORS

The paper refers to the biomass steam cycle power plants of medium capacity (12 to 40 MW at generator clamps), which burn wood vegetal biomasses. The first part highlights the specific features of this kind of biomass: the high organic Oxygen content in the dry mass (which reduces the amount of air for combustion), and the high hygroscopicity (which reduces the calorific values of the wet biomass, compared to the dry state). Further on we model numerically, using well-known methods, the termochemical processes for burning wood biomass with different diffuse and hygroscopic water quotas. Next, we preliminary compute the energy performances of the fluidized bed steam generators with 40 to 125 MJ/s heat flow rate, which burn wood vegetal biomasses. For the water-steam circuit we use the rational correlations between the parameters of the main steam, the feeding water temperature and the water preheating scheme determined in the previous paper: Biomass non-reheat steam cycle power plants. Part 1: Technical assessment of schemes and water-steam parameters". Knowing the heat quotas transmitted in the Steam Generator (SG) sections allows to construct the heat transfer diagram in the SG area of high and medium temperatures and to determine the gas temperature when entering the air preheater, which depends mainly on the feeding water temperature. On increasing the feeding water temperature the SG losses associated to the gas heat evacuated on the stack increase, and the SG energy efficiency decreases. On the other hand, the gross electric efficiency of the Thermal Power Plant (TPP) is a product of the SG efficiency and of the thermodynamic conversion circuit. The development of the regenerative preheating leads to two adverse effects : SG efficiency decrease and the thermal cycle efficiency increase. The technically optimal solution is the one for which the product of the two efficiencies is maximized. Due to the high number of variables, the problem can only be solved by case analyses. In the paper we analyze the TPP with the steam cycle without reheat with two variants of thermal circuit design type: with 5 preheat stages up to 190 degrees C, respectively with 6 preheat stages up to 220 degrees C. The results show that, for a high biomass humidity, an advanced preheating could be unfavorable.