Producción de bio-hidrógeno mediante gasificación catalítica de biomasa con captura integrada de CO2

Abstract

Hydrogen has been proposed as the energy carrier of the future. However, for the Hydrogen Economy to be feasible, hydrogen pro-duction must be cheap, accessible and sustainable. Nowadays, more than 96% of H₂ is produced from fossil fuels, contributing to increase the CO₂ concentration in the atmosphere and leading to a progres-sive Climate Change. The most widespread method for hydrogen production consists in the steam reforming of methane (SMR), which employs several catalytic reactors at different temperatures to pro-duce gas with a 70% H₂ content that should be later purified. In this context, integration of steam reforming and CO₂ capture in a single step process called Sorption Enhanced Steam Reforming (SESR) is considered a promising technology. This process allows to reduce the number of required reactors and the reaction tempera-ture, increasing yield and H₂ purity at the same time. The ultimate consequence is to achieve lower costs of installation and operation. Additionally, if a renewable fuel like biomass is fed to the reactor, production of cheaper, accessible and sustainable hydrogen is possi-ble, as biomass has zero net impact in the atmospheric CO₂ concen-tration, thus reducing the contribution of hydrogen to Climate Change. To optimize the SESR process, an appropriate selection of catalyst and sorbent for CO₂ must be made, together with a proper optimiza-tion of the operating conditions. A Pd/Ni-Co-hydrotalcite derived material has been chosen as catalyst, while dolomite has been select-ed as CO₂ sorbent. All experiments were preceded by thermodynam-ic equilibrium calculations to estimate the maximum limits of yield, selectivity and concentration of products. This results were later compared with the experimental results obtained. A fluidized bed reactor was employed to assess the optimal condi-tions for the SESR process. The three main variables modified during these experiments were temperature, steam to carbon ratio (S/C) and weight hourly space velocity (WHSV). Acetone was employed as model compound to simulate the behaviour of biomass pyrolysis de-rived bio-oil. The fluidized bed reactor was also employed to investigate the effect of bio-oil composition on the performance of the SESR process. The behaviour of blends with different proportions of acetic acid and acetone was tested along a wide range of temperatures. The values of steam proportion and WHSV were chosen from the optimized parameters obtained in the previous work. Thereafter, to reach a deeper knowledge of the influence of composition, several blends of phenol, acetic acid and acetone were prepared at similar proportions to those found in pyrolysis bio-oil, and tested for the SESR process. Sorption Enhanced Catalytic Steam Gasification (SECSG) was car-ried out employing two solid lignocellulosic biomasses. During these experiments, a fixed bed reactor was fed semi‑continuously with the biomasses in order to evaluate the effect of temperature and biomass composition on yield and H₂ production. The present work has demonstrated that temperature is the most influent factor in the SESR process, followed by S/C and WHSV. It is possible to achieve a H₂ purity higher than 99.5% between 525 and 625 °C, for both the model compounds and the lignocellulosic bio-mass. However, other process variables such as H₂ yield and selec-tivity are greatly affected by the nature of the fuel and the reactor employed, needing higher temperatures to reach the same values. The concentration of some by-products, like CO and CH₄, is also very sensitive to changes in temperature and feed, increasing when the process moves away from the optimal conditions.