1. Biomass gasification in an autothermal pilot-scale bubbling fluidized bed for fuel gas production
Daniel Torrão Pioª,
Luís Tarelhoª
ª Department of Environment and Planning, CESAM, University of Aveiro
Abstract
The use of biomass for useful energy production has been increasing in the recent years and makes part of the energy strategies of developed and developing countries. The driving force for this interest in biomass has been the search for alternatives to fossil fuels that allow the use of established energy conversion technologies and promote the decrease of greenhouse gases emissions.
Direct (air) biomass gasification was demonstrated in a pilot-scale bubbling fluidized bed reactor and the influence of the process parameters analyzed. For the operating conditions used, namely equivalence ratio between 0.17 and 0.36 and bed temperature between 700 and 850 ºC, the process was demonstrated as autothermal and operating under steady-state conditions. The dry gas produced showed the following composition (volumetric basis): % CO, % CO2, % CH4, % C2H4, % H2 and % N2. The lower heating value of the dry gas was between 4.4 and 6.9 MJ/Nm³, with the highest values observed during the experiments with lower equivalence ratio.
Introduction
Several thermochemical processes are available for heat and power production from biomass, being that combustion is the most widely used. However, biomass gasification is gaining interest worldwide due to the process flexibility and the need of renewable fuels that can replace gaseous fossil fuels in distinct applications.
Experimental work
The experimental facility used (see Fig. 1) for the gasification experiments includes a thermally insulated pilot-scale 80 kWth bubbling fluidized bed (BFB) reactor made of AISI 310 SS with a reaction chamber of 0.25 m internal diameter and 2.3 m height, developed at the Department of Environment and Planning in the University of Aveiro (Portugal).
The fuel used in the gasification experiments included commercial pine pellets and different types of residual forest biomass (RFB) derived from pine (pinus pinaster) and from eucalyptus (eucalyptus globulus).
The results from the biomass gasification experiments includes temperature along the BFB reactor (see Fig. 2), concentration of gaseous compounds in the producer gas (see Fig. 2 and 3) and efficiency parameters (Fig. 3).
Conclusion
The pilot scale BFB reactor was successfully operated under autothermal gasification conditions at bed temperatures between 700ºC and 850ºC, without the need of auxiliary external heat sources. The temperature at several locations along the reactor and the properties of the produced gas were very stable along time reflecting the existence of steady-state operating conditions and the adequacy of the BFB reactor, and its inherent characteristics as reactive system, to this thermochemical process. For the experimental operating conditions used in the pilot-scale BFB, namely equivalence ratios in the range , the dry gas composition (volume basis) was % CO, % CO2, % CH4, % C2H4, % H2 and % N2. The lower heating value of the dry gas was found between 4.4 and 6.9 MJ/Nm³, the specific dry gas production between 1.2 and 2.2 Nm³/kg dry biomass, the cold gas efficiency between 41.1 and 62.6% and the carbon conversion efficiency between 60.0 and 87.5%. Concerning the BFB gasifiers, the experimental results obtained in this work are in the medium to upper range of values found in the literature regarding direct biomass gasification. This work supports the adequacy of the direct (air) gasification of distinct types of residual forest biomass in BFB as an autothermal and steady state process to produce a low heating value combustible gas.
References
Pio D.T., Tarelho L.A.C., Matos M.A.A. (2017) Characteristics of the gas produced during biomass direct gasification in an autothermal pilot-scale bubbling fluidized bed reactor”, Energy, Vol. 120, February, pp ,
Fig. 1 - Layout of the experimental gasification facility with the pilot-scale bubbling fluidized bed reactor
Fig. 2 - Typical performance of the pilot-scale bubbling fluidized bed reactor in terms of temperature and dry gas composition (CO2, CO CH4 and C2H4).
Fig. 3 - Influence of equivalence ratio on the average dry gas composition (CO2, CO, H₂, CH4 and C2H4), lower heating value (LHV), cold gas efficiency (CGE) and carbon conversion efficiency (CCE) for the gasification experiments performed in the pilot-scale BFB with RFB from pine (BPE), wood pellets (WPE) and RFB from eucalyptus (BEE).
Thanks are due for the financial support to CESAM (UID/AMB/ POCI FEDER ), to FCT/MEC through national funds, to the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020, and to RAIZ for the PhD scholarship co-funding.