Introduction
A challenge of this century is to develop industrial processes using renewable energies to replace fossil fuels and thus to reduce our impacts on the environment. High energetic gas called “syngas” produced from the thermochemical conversion of biomass and waste is considered as environmental-friendly energy. This syngas composed of CO and H2 at convenient ratio, could be used for production of fuels or chemicals (ex. diesel, methanol, ethanol), for steam generation via combustion boilers, or for electricity production in a gas turbine or fuel cell. However the industrial development of this gasification process is hold back by the production of unwanted by-products: tars and char/ash (aromatic hydrocarbons and solid residues respectively) which decrease the global efficiency yield and require very costly treatments [1, 2]. At gasification temperature (ca. 700 to 900°C) all co-products are in their gaseous forms while downstream the reactor some co-products called tars start to condense which leads to fouling and blocking process equipments as turbines and engines [3]. The gas phase is also contaminated by co-products and pollutants which may contribute up to 20% of the volatile material [4]. Therefore some recent studies investigated potential applications to recycle these by-products and thus to reduce their economic impacts. The challenge was to rely on the fact that these biochars from natural feedstock already contain organic, minerals and more specifically metals which make them very attractive for catalytic or sorbent applications. In this context, the main options are to catalytically reform tar over biochars, or to use biochars for hot gas cleaning. Biochars from gasification have recently been recognized to be cheap candidates for catalytic applications related to tar reforming and hot gas cleaning [4-10].These microporous materials of high specific surface area (570 m2/g) contain active sites, such as endogenous Alkali (Li, Na, K..), Alkaline Earth Minerals (Mg, Ca…), namely AAEM, and oxygen atoms. Functionalization of the biochars surface is an opportunity for further development as an inexpensive catalyst from renewable resources. The functionalization may impact the carbonaceous matrix structure, the content of O-groups as well as the increase in mineral content [9-10]. This study is focused on the impact of AAEM content towards the reactivity of the functionalized biochars in methane cracking.
Materials and Methods
Chips of poplar wood have been impregnated with Ca, K and a mixture of both of them to increase their concentrations. The impregnation was carried out at room temperature with concentrated aqueous solutions using KNO3 and Ca(NO3)2. Biochars from gasification were obtained using raw poplar wood and impregnated wood in a steam atmosphere (90 vol% H2O / 10 vol% N2).
The mineral composition in biochars was measured using X-ray fluorescence. Biochars were also characterized using Scanning Electron Microscopy (Philips XL30 FEG) to observe the texture and the distribution of minerals at the surface. The porosity was measured with BET equipment (ASAP 2010, Micromeritics).
Methane cracking experiments have been performed at 700°C in flow through micro-reactor (ChemBet Pulsar-model 05090) under a mixture of 10 vol% CH4 / 90% N2 and a flow rate of 80 mL/min. prior to methane cracking, biochar was heated to 700°C at 20°C/min in N2 atmosphere.
Results and discussion
The impregnation led to the increase of K and Ca concentrations from 0.6 wt% in the raw biochar to 5 wt.% in the K-char and Ca-char respectively. In the sample where Ca and K are impregnated simultaneously, the concentration of K and Ca reached 2.2 wt.% in the biochar (Ca+K-char). The results show that the biochar surface is coated with K and Ca resulting to the decrease of its porosity and specific surface area. Indeed the specific areas are of 574, 504, 413 and 322 m2/g for raw char, Ca-char, K-char and Ca+K-char respectively. It has been also seen that the affinity of K towards the biochar surface is stronger than that of Ca as the coating is more efficient (Figure 1). Indeed, the bright spots which are due to minerals are strongly linked to the surface for K-char (b). The presence of K in the impregnated Ca+K-char significantly improve the bounding of Ca at the surface as shown in Figure 1 (c). These results are in agreement with BET measurements as the specific surface area has been decreased by 12, 38 and 44% for the Ca-char, K-char and Ca+K-char respectively.
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Figure 1: SEM pictures of Ca-char (a), K-char (b) and Ca+K-char (c)
The biochars produced have been used as a catalyst for methane cracking. The efficiency of the conversion was evaluated following the production of hydrogen. Figure 2 shows the cumulative H2 production during the CH4 cracking over the 4 biochars. Impregnated biochars demonstrated a catalytic activity as the methane cracking was improved. The reactivity of the K-char towards the conversion of methane into hydrogen is 4 times higher compared to raw biochar. The Ca+K-char was more efficient than the Ca-char and the raw-char, but less efficient than the Ca+K-char. The total content of K in the Ca+K-char was lower than in the K-char as mentioned previously. In the literature, the reactivity of AAEM in biochars was studied and 3 groups were established [4]. Group I corresponds to a biochar where the K+Na molar concentration is higher than that of Ca. The reverse situation corresponds to the group II while group III highlights high concentrations of Si. Literature shows that group I is more efficient than the other ones towards catalytic efficiency [4]. The results obtained are in agreement with the literature as the K-char belongs to group I while the other ones should be considered in group II [4].
Figure 2: Cumulative hydrogen production over raw and impregnated chars during methane cracking.
Conclusions
The goal of the work was to use biochars from gasification for functionalization using minerals and to compare their reactivity for methane cracking at 700°C. The impregnation led to the increase of K and Ca concentrations from 0.6 wt% in the raw biochar to 5 wt.% in the K-char and Ca-char respectively. In the sample where Ca and K are impregnated simultaneously, the concentration of K and Ca reached 2.2 wt.% in the Ca+K-char. The results show that the biochar surface is coated with K and Ca resulting to the decrease of its porosity and the specific surface area decreased by 12, 38 and 44% for the Ca-char, K-char and Ca+K-char respectively. Although the specific surface area of the K-char was significantly reduced the reactivity was the highest towards methane cracking. The rank of efficiency is as follow K-char>Ca+K-char>Ca-char>raw char. These results highlight the role and the type of metal ions on the catalytic activity and are in agreement with literature where the high content of K compared to Ca improves the catalytic reactivity.
References
[1] de Wild PJ, den Uil H, Reith JH, Kiel JHA, Heeres HJ. Biomass valorization by staged degasification. A new pyrolysis-based thermochemical conversion option to produce value-added chemicals from lignocellulosic biomass. J Anal Appl Pyrolysis 2009;85:124-133.
[2] Demirbaş A. Gaseous products from biomass by pyrolysis and gasification: effect of catalyst on hydrogen yield. Energy Convers Manage 2002;43:897-909.
[3] Kiel JHA et al. Primary measures to reduce tar formation in fluidised-bed biomass gasifiers. Final report SDE project P1999-012, 2004.
[4] Nzihou A, Stanmore B, Sharrock P. A review of catalysts for the gasification of biomass char, with some reference to coal, Energy, 2013;58:305–317.
[5] Moliner R, Suelves I, Lazaro M, Moreno O. Thermocatalytic decomposition of methane over activated carbons: influence of textural properties and surface chemistry. Int. J. Hydrogen Energy 2005;30:293–300.
[6] Muradov N, Smith F, T-Raissi A. Catalytic activity of carbons for methane decomposition reaction, Catal. Today, 2005;102–103:225–233.
[7] Serrano DP, Botas JA, Fierro JLG, Guil-López R, Pizarro P, Gómez G. Hydrogen production by methane decomposition: Origin of the catalytic activity of carbon materials, Fuel, 2010;89:1241–1248.
[8] Min Z, Yimsiri P, Asadullah M, Zhang S, Li C-Z. Catalytic reforming of tar during gasification. Part II. Char as a catalyst or as a catalyst support for tar reforming. Fuel 2011;90:2545–2552.
[9] Klinghoffer NB, Castaldi MJ, Nzihou A. Catalyst Properties and Catalytic Performance of Char from Biomass Gasification. Ind. Eng. Chem. Res. 2012;51:13113-13122.
[10] Klinghoffer NB, Castaldi MJ, Nzihou A. Influence of char composition and inorganics on catalytic activity of char from biomass gasification. Fuel 2015. doi:10.1021/ie3014082.
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