From Waste to Gas: Biogas Valorization
From waste to gas: biogas valorizationV.L. Barrio , U. Izquierdo , J. Requies , M.B. Güemez , J.F. Cambra and P.L. Arias Faculty of Engineering (UPV/EHU) , c/ Alameda Urquijo s/n , 48013 Bilbao , Spain IntroductionIn this work biogas valorization – a renewable resource – for synthesis gas and hydrogen generation through reforming processes is studied using a conventional fixed bed reactor and an advanced microreactor system. It is time for hydrogen as an energy vector and this opportunity should be seized by doing improvements and innovations in the existing technologies and processes as well as for developing new ones. This renewable hydrogen can also be used for the Power to Gas technology. The behaviour of two different catalytic systems was studied in this work. On the one hand , Ni-based catalysts and a bimetallic Rh–Ni catalyst supported on magnesia or alumina modified with oxides like CeO2 and ZrO2were used comparing conventional and microreactor reaction systems. On the other hand , Ni monometallic and Rh-Ni bimetallic based catalysts were prepared supported on three different Zeolites L. For all the experiments , a synthetic biogas (molar composition: 60% CH4 and 40% CO2) was fed and the catalytic activities were measured in two different experimental facilities: a bench-scale fixed bed reactor system and a microreactor reaction system , at 1073 K and atmospheric pressure. Physicochemical characterization of catalyst samples by ICP-OES , N2 physisorption , H2chemisorption , TPR , SEM , TEM , XRD and XPS showed differences in chemical state , metal–support interactions , average crystallite sizes and redox properties of nickel and rhodium metal particles , indicating the importance of the morphological and surface properties of metal phases in driving the reforming activity. Results & DiscussionMagnesia – Alumina modified catalystsSeveral Ni-based catalysts and a bimetallic Rh–Ni catalyst supported on magnesia or alumina modified with oxides like CeO2 and ZrO2were used. In this sense , nickel monometallic and rhodium-nickel bimetallic based catalysts were prepared , characterized and tested in dry reforming (DR) , steam reforming (SR) , oxidative reforming (OR) and tri-reforming (TR) processes for syngas and/or hydrogen generation. The catalysts which achieved higher activity and stability in the fixed-bed reactor were impregnated in a microreactor to explore possible process intensification. For TR processes , different steam to carbon ratios , S/C , from 1.0 to 3.0 , and O2/CH4ratios of 0.25 and 0.50 were used. For the Ni-based catalysts and the bimetallic Rh–Ni catalyst supported on magnesia or alumina modified with oxides like CeO2 and ZrO2 , high methane and carbon dioxide conversions were reached for DR reaction in the fixed bed reactor system. However , this process requires huge energy supply and carbon filaments growth was observed by SEM. Regarding biogas TR process , different O2/CH4 and S/C ratios were tested. For this process , at O2/CH4= 0.25 and S/C = 1.0 the highest carbon dioxide conversions and hydrogen yields were reached. The Rh–Ni/Ce–Al2O3 catalyst achieved the highest hydrogen production yield in DR reaction and also in the biogas TR at the following experimental conditions: O2/CH4= 0.25 and S/C ratio of 1.0. These results can be due to its very high active metal dispersion as well as the reached small particle size measured by XPS and XRD. Operating with microreactors similar conversions were achieved being Ni/Ce–Zr–Al2O3 catalyst the one reaching the highest hydrogen production yield for biogas TR at O2/CH4= 0.25 and S/C = 1.0. Attending to the TOF and productivity values , the catalytic activity measured using microreactors was one order of magnitude higher than the achieved one using fixed bed reactor. In addition , process intensification improved catalysts stability. Zeolite supported catalystsFor the Ni-based catalysts and the bimetallic Rh–Ni catalyst supported on zeolites , three different Zeolites L were synthesized to be used as catalyst supports. A disc-shape Zeolite L (length around 0.2-0.4 µm and diameter of approximately 0.5 µm) and two Zeolites L with a cylindrical morphology (smaller particle size around 30-60 nm and bigger particle size around 1-3 µm) were tested. The operation conditions for the experiments carried out with the Zeolites L were chosen taking into account the previous results obtained for the alumina supported catalysts , that correspond to the following: for biogas SR steam to carbon (S/C) of 1.0 and 2.0 , for biogas OR the O2/CH4 ratio of 0.25 and 0.50 , and S/C=1.0 and for TR reactions O2/CH4=0.25. Focusing on the rest of catalysts and according to the morphological difference between the Disc and Cylindrical (30-60 nm) Zeolites L , different activities were measured. For the SR processes , the Disc catalysts were more active at the lowest S/C ratio and the activity decreased when the S/C ratio was increased to 2.0. By the contrary , the Cylindrical (30-60 nm) catalysts are more active in the OR at O/C=0.25 and decreased when the S/C ratio was increased to 0.5. In all tested processes the Ni and Rh-Ni catalysts based on the Zeolite L (1-3μm) achieved the lowest activity values. This could be associated to the presence of Merlinoite zeolite –detected by XRD- which contributes in around the 25% of the sample. With the exception of the Ni Disc catalysts for SR at S/C=1.0 and DR processes , the bimetallic catalysts showed much better reforming capacities in the tested processes and conditions. Thus , Rh incorporation increased catalytic activity in all the reforming processes tested. The Rh-Ni catalysts based on Disc and specially the Cylindrical (30-60 nm) Zeolite L seemed to be a very promising catalyst due to its very high metallic dispersion –measured by H2chemisorption– and excellent activities. Regarding the processes , for biogas SR and TR processes , H2/CO ratios near to 2.0 were obtained and this is an appropriate ratio for the Fischer Tropsch synthesis. Finally , attending to the CO2 reforming capacity , DR is the most appropriate process for the conversion of almost all the CO2that makes up the biogas. . References1) Winter CJ. Int. J. Hydrogen Energ.34 (2009) S1. 2) Rasi S , Veijanen A , Rintala J. Energy32 (2007) 1375. 3) Kolbitsch P , Pfeifer C , Hofbauer H. Fuel87 (2007) 701. 4) Dehmer , Dagmar. The Electricity Journal26(1) (2013) 71. 5) Gahleitner , Gerda. Int. J. Hydrogen Energ.38 (2013) 2039. 6) Murphy , Jerry D.; James Browne , Eoin Allen , Cathal Gallagher. Renew. Energ. 55 (2013) 474.
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