The operation of a micro-scale cogeneration system prototype – A comprehensive experimental and numerical analysis

Abstract

This paper shows the results of experimental and numerical investigations related to the energy use of straw from the point of view of its preparation (drying), combustion, energy production and emissions. Experimental works on straw drying were conducted on a specially developed installation based on dedicated batch straw dryers, which use flue gas, coming from a 500 kWth boiler, as a drying medium. The further part of the work was conducted on a prototype of a micro-scale cogeneration system that operates according to a low temperature Rankine cycle and it is powered by a 100 kWth biomass-fired batch boiler. Conducted so far research allowed to analyse selected operation aspects of the developed system. An analysis of straw drying, proximate, ultimate, and heating value analysis, heat generation process in the boiler, steam generation process in the evaporator and superheater, steam engine operation as well as dust emissions during straw combustion related to the boiler return temperature were presented. Energy transferred from flue gases to straw with a high value of moisture content was 5.75 kW, which allowed to evaporate about 18.6 kg of water from 192 kg wheat straw bale. The moisture content was reduced from 36.9 to 27.2%. The power generated in the system was lower than the one expected, since only 1.15 kWel was achieved which was about 1% of the boiler’s heat capacity. Such result was caused by construction restrictions of the installation. Experimental results were supported by dynamic simulations performed in TRNSYS software. The simulations were performed for both the current configuration of the system and suggested modifications. Simulation results showed, that the expected power generation in the final version of the system should be at a level of 10 kWel. Results obtained during the analysis of multi-cyclone operation showed, that the concentration of dust was reduced in a range from 55% to even 85% depending on the temperature of the boiler return and flue gas flowrate in multi-cyclone.