Model development to calculate explosion and combustion limits in gasification reactors

Abstract

The combined heat and power (CHP) plants used for district heating and electricity supply has seasonal and daily variations in demand. The thermal capacity of the boilers is thus under-utilized which is tapped into by Bioshare AB. They develop fluidized bed reactors, a well-established technology in Sweden’s energy sector. These reactors can be integrated with the boiler plant to transform them into polygeneration units. Liquid and gaseous biofuels, biochemicals and biochar derived from biomass during the pyrolysis reaction within the reactor, extends the product portfolio of the CHP plants making it a financially attractive and a sustainable option. Thus, Bioshare aims to transform the existing co-generating CHP plants into polygeneration units.

While integrating the reactor with the boiler, the fluidized bed is split and exchanged between the two units. The boiler has an oxygen rich environment, and the reactor is oxygen deficient. Both environments are separated by particulate fluidized bed material that is exchanged. This poses a safety risk as oxygen could leak along with the transfer of the bed material from the boiler into the reactor. During the turn down operation of the reactor, the remaining products from the pyrolysis reaction on reacting with oxygen can cause a spontaneous explosion. This thesis work studied the ignition limits of this explosion. Using Cantera software, a numerical model was built to study the ignition delay time under varying conditions. The model presents a 0-dimensional transient problem. By using a surrogate pyrolysis oil composition, the input reaction mechanism file was given from Bioshare. The results from the analysis align with the expected behavior of ignition delay as the pressure, temperature and equivalence ratio varies.