Biomass blend effect on energy production in a co-gasification-CHP system

Abstract

Biomass is recognized as a reliable renewable energy source because its supply can be planned and it overcomes the problems related to the intermittency of other renewables. Furthermore, sustainable biomass supply chain allows considering this primary energy source as carbon neutral. To this regard, the use of local bio-residues from agro-industries can have a significant impact on the local economies and the local production of renewable energy. In a previous research study, the authors investigated the impact of the energy integration of citrus peel gasification in a citrus juice factory. The study showed that using citrus peel waste to produce heat and power through the biomass gasification-CHP system, the whole factory's energy demand cannot be covered. Also, due to the high ash content of citrus peel, it is recommended to operate the gasifier at moderated temperatures. In order to increase the production of electricity through the gasification-CHP system, it was decided to integrate the orange peel waste of the factory with a different kind of biomass. To this aim, this work studies the effects of citrus peel-woody biomass co-gasification on the process efficiency, using a thermodynamic simulation method. Indeed, after the validation of both woody biomass and citrus peel gasification models, the analysis of the co-gasification process has been developed employing a simulation model. The variation in terms of syngas composition, efficiency and yields of the process has been evaluated at different wood mass fraction (from 0.1 to 0.4 wt/wt) in the mix of wood/citrus peel biomass blend at 750°C, ER = 0.3 and S/B = 0.5. The central element in the simulation flowsheet is the ideal Gibbs reactor, which makes use of different approach temperature in the single feedstock simulation. Two different model's configurations have been considered. The main differences consisted of the use of one or two Gibbs reactors. In the first case, the approach temperature was weighted according to the concentration of the two feedstocks, showing negligible differences compared with the other configuration with two independent Gibbs reactors at different temperatures approach. The results of the simulation model showed that both syngas yield and gasification efficiency showed slight variations as the woody fraction increases. In particular, the Cold Gas Efficiency (CGE) is around the values 0.59-0.60 at the wood mass fraction that were investigated, while the syngas yield varies from 2.3 to 2.4 Nm3/kg, respectively. The LHV of syngas increases as the wood fraction increases, varying from 4.35 MJ/Nm3 (0.1 wood mass fraction) to 4.54 MJ/Nm3 (0.4 woody mass fraction). In conclusion, the results allow determining the influence of mixing biomasses with different reactivity, in terms of conversion and thermal efficiency, in a gasification reactor operated at a moderate temperature, as well as the impacts on a possible gasification-CHP system.