Steam Reforming of Bio-Compounds with Auto-Reduced Nickel Catalyst
Feng Cheng, Valerie Dupont
As an extension of chemical looping combustion, chemical looping steam reforming (CLSR) has been developed for H 2 production. During CLSR, a steam reforming (SR) process occurs following the reduction of catalysts by the reforming feedstock itself (termed "auto-reduction"), as opposed to a separate, dedicated reducing agent like H 2 . This paper studied SR performances of four common bio-compounds (ethanol, acetone, furfural, and glucose) with a nickel catalyst that had undergone
... . A packed bed reactor was used to carry out the experiment of auto-reduction and subsequent SR. The effects of temperature and steam to carbon ratio (S/C) on the carbon conversions of the bio-compounds to gases and yields of gaseous products were investigated. The carbon deposition on spent catalysts was characterized by CHN elemental analysis and Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDX). The SR performance with the auto-reduced catalyst was close to that with the H 2 -reduced catalyst. In general, an increase in temperature or S/C would lead to an increase in H 2 yields. The dependence of SR performance on temperature or S/C was specific to the type of bio-compounds. Accordingly, the main bottlenecks for SR of each bio-compound were summarized. A large amount of CH 4 existed in the reforming product of ethanol. Severe carbon deposition was observed for SR of acetone at temperatures below 650 • C. A high thermal stability of furfural molecules or its derivatives restricted the SR of furfural. For SR of glucose, the main problem was the severe agglomeration of catalyst particles due to glucose coking. Catalysts 2017, 7, 114 2 of 18 of bio-oil leads to a complicated reaction network and coking mechanism during SR of bio-oil. In order to better understand this process, SR of single bio-oil components (termed "bio-compound") was investigated by experiments        or simulation calculations [11-14]. Bio-compounds that have been subjected to SR studies include ethanol [15-19], acetic acid [7,13,20], ethylene glycol , glycerol , hydroxyacetaldehyde , acetone , acetol , ethyl acetate , glucose [5,10], xylose , sucrose , m-cresol [5,22], m-xylene , di-benzyl ether , methanol , phenol , etc. The SR performance (normally indicated by carbon conversion of feedstock to gases, H 2 yield, and carbon deposition) varies with the catalyst and the condition applied. Therefore, it is not easy to find consistency between the results from different research groups despite the use of the same bio-compound. High H 2 yields and thus the feasibility of these bio-compounds for SR were usually reported, but less attention has been paid to process characteristics that are specific to individual bio-oil components. Hu and Lu  investigated the effects of molecular structure on the SR performance of bio-oil components. The type of functional group significantly affected steam reforming. Alcohol steam reforming tended to produce a significant amount of CH 4 , which was influenced by the length of the carbon chains, and the number and the location of hydroxyl groups. Severe coke deposition was encountered in the steam reforming of ketone compounds such as acetone. Hu and Lu  also compared the coking rate of different bio-oil components during a SR process and discussed the coking mechanism of different bio-oil components. They reported that decomposition or polymerization of the feedstocks were main routes for coke formation in glucose, m-xylene, and acetone reforming. The large amounts of by-products such as ethylene, CO, or acetone were coke precursor in the SR of acetic acid, ethyl acetate, and ethylene glycol. Remon et al.  studied the influence of bio-oil composition on the SR result. A strategy was established to identify the chemical compounds that were responsible for the most significant variations observed during SR of the bio-oil. The SR results were greatly affected by the proportion of acetic acid and furfural in the bio-oil. Compared to acetic acid, furfural had a high tendency to produce coke in a reforming process. Chemical looping steam reforming (CLSR, also termed unmixed steam reforming) is an advanced steam reforming technology      . It has a similar basic principle as chemical looping combustion (CLC), whereas the main difference is that the target product of CLSR is syngas rather than heat in CLC . High-purity H 2 could be obtained if passing the syngas from CLSR through a water gas shift (WGS) reactor. During a CLSR process, either a supported metal oxide circulates between an air reactor and a fuel reactor (recirculating fluidised bed configuration), or an air/fuel feed flows alternately over a packed bed reactor, in which case at least two reactors are needed for the production of a continuous H 2 output. In a fuel reactor or under a fuel feed flow, the metal oxide is reduced by the fuel first and then catalyses the subsequent SR reaction of the fuel. In the air reactor or under an air flow, the reduced metal oxide is oxidized by air. Obviously, the supported metal oxide performs both the roles of oxygen carrier and SR catalyst. The reduction of the metal oxide by fuel is a necessary and key step for a CLSR process. If H 2 is used to reduce the catalyst instead of fuel during a CLSR process, the operation complexity will increase although better reduction kinetics and free carbon deposition can be achieved. Compared with conventional SR, CLSR has several advantages [31,33]: (a) A more uniform supply of heat to the endothermic SR reaction can be achieved by the internal unmixed combustion. (b) The coked catalyst can be cyclically regenerated by combusting the coke in the air reactor. (c) No external heat is needed if the process is designed properly (i.e., autothermal CLSR). (d) The CLSR configuration is easy to integrate with in situ CO 2 adsorption by adding sorbents such as CaO in the bed materials. The CLSR system integrated with in situ CO 2 adsorption is termed "sorption enhanced chemical looping steam reforming" (SE-CLSR), a process that can generate H 2 with >90% purity [34, 35] . Extensive studies have been devoted to CLSR of natural gas. It is generally believed that supported NiO is a promising oxygen carrier for CH 4 CLSR due to its high reduction reactivity and good catalytic activity for SR once NiO is reduced to Ni [32, 36] . Actually, alumina supported NiO (NiO/α-Al 2 O 3 ) is a common reforming catalyst in industry  . Similar to other transition metal catalysts, NiO catalysts require reduction to yield the active phase (i.e., metallic Ni) prior to their use Catalysts 2017, 7, 114 3 of 18 Catalysts 2017, 7, 114 4 of 18 Catalysts 2017, 7, 114 4 of 19 furfural might fall on the catalyst bed directly before they were completely vaporized. As a result, the gas stream was disturbed and went through the gas analyser like pulses.