Tar removal is considered to be one of the biggest obstacles in biomass gasification processes. Various catalyst activity and stability are studied to find an economically feasible catalyst. In this study syngas is produced from tar reforming using Ni-Olivine Catalyst. Ni/Olivine catalyst is prepared by thermal impregnation and wet impregnation methods and their performance are compared using various characterization techniques like x-ray diffraction (XRD), BET surface analysis, x-ray photo electron spectroscopy (XPS). It was determined that the thermal impregnation catalyst provides better stability to the catalyst and also had improved high temperature activity and decreased coke deposition than the catalyst prepared by wet impregnation. Further, the effect of different catalyst compositions on the activity and selectivity were analyzed using 3.0% NiO/Olivine, 3.0% NiO/Olivine doped with 1.0%CeO2 and 6.0% NiO/Olivine catalyst. From these different catalysts it was determined that the cerium oxides promoted the catalytic activity of nickel and resist the deposition on the carbon.
Biomass Gasification is a process of converting biomass into syngas (CO, H2 CH4) at high temperature (500-900°C) in the presence of gasifying agent such as air, oxygen, steam or mixtures of the components (Wang, Weller, Jones, & Hanna, 2008). The general reaction for biomass gasification is shown below:
Biomass Gasification is considered as one of the best way to produce syngas that could be processed to synthesize fuels (Fischer Tropsch Process) and different chemicals that are useful to human beings in creating a sustainable world. However, there are several challenges associated with producing syngas and one of them is maximizing the gas yield by lowering the tar formation. Tars are the condensable fraction of aromatic compounds like benzene, toluene and naphthalene which can present a number of process challenges like coking of catalyst condensation on downstream piping, filters and other equipment's (Milne 2009). Catalyst plays significant role in maximizing the yield by lowering down the tar formation. Hence the selection of catalyst should be done wisely in order to make the process economically feasible.
Tar cracking can be done by several different methods like steam reforming, dry reforming and partial oxidation. Among these methods petrochemical Industries had advanced their research and technology in steam methane reforming and naphtha steam reforming. Various catalyst have been studied based upon their activity, stability and among them Nickel/Alumina (Ni/Al2O3) has been found to be widely used for tar reforming. However, catalyst deactivation was found to be a major problem due to harsh chemical physical processes associated with the harsh reaction conditions and impurities in the feed stream (Yung, Jablonski, & Magrini-Bair, 2009).
Gas phase reaction are very slow below 500oC therefore increasing the temperature would increase equilibrium rate fast enough that thermodynamic calculations can predict important trends about gas composition. However, a reliable prediction of product composition is only possible with a detailed kinetic model for that reactor incorporating global reaction rates and the effects of thermodynamic parameters temperature, pressure and initial composition. The equilibrium is greatly affected by parameters like residence time, and gas solid contacting methods employed in gasification methods (Noyes Data Corporation, 1981).
A complete gasification involves oxidation, reduction, pyrolysis and drying of biomass. In an updraft gasifier, the oxidation of char with oxygen occurs in a lowest zone resulting into heat release which is a driving energy for subsequent processes. This zone is rich in oxygen to favor CO2 formation. Probably being a mass transfer limited the reaction (e) is very fast and thickness of this zone varies from one to tens of centimeters. This gas stream is rich in CO2 and H2O. If blast contains steam which favors the Boudouard (d) and water-gas (c) reaction which are highly endothermic reaction. This controls the upper limit of temperature reduction zone. However, the rise in temperature depends upon the blast condition as an example the presence of only oxygen in the blast wouldn't increase the temperature by more than hundred degrees. The exothermic methanation reaction can provide temperature floor for the reduction zone, however, the process is too slow to control bed temperatures. As the gas rises beyond the reduction zone the temperature falls below 900K due to the contact with cooler feed stocks which stops reductions and shift reactions.
The partially dried feed above the char bed is pyrolyzed by the rising gas stream, resulting into low molecular weight hydrocarbons, alcohols, acids, oils, and tars, as well as CO, H2 , CO2, H20 and CH4. The important reactions involved in the gasification process are (Noyes Data Corporation, 1981):
Table . Reactions involved in Gasification and the enthalpy associated with it
Reaction
H (KJ/mole)
298K
a
CO + H2O = CO2 + H2
-41.2
b
C+2H2 = CH4
-74.93
c
C + H2O = CO + H2
131.4
d
C + CO2 = 2CO
172.6
e
C + O2 = CO2
-393.8
The use of catalyst depends upon its functionality and cost. Though Rh, Pt catalyst have higher activity than Ni catalyst their cost limits their use in the industries. Further, the type of support that is used to prepare the catalyst can play significant role in the production of syngas. These supports provide mechanical strength to the catalyst and some might also have chemical role during the reaction. As different supports have different functionality, determining the right kind of support will increase the stability of the catalyst and saves millions of dollar. According to researchers Ni/Al2O3 showed the highest activity among Ni/Al2O3>Ne/ZrO2>Ni/CeO2>Ni/La2O3>Ni/MnO and Ni/MnAl2O4. However, significant coking caused reactor to plug which did not happen with La2O3 catalyst. (Seok,S; Choi, S.;Park, E.; Han, S; Lee, J, 2002). However, there might be much other combination of catalyst and support that can play significant role in coke reduction and maintain its stability.
Dolomite and Olivine are being popular these days as they are cheaper than any other supports, they are easily available in the nature and have a good tar reforming activity with capability of adjusting syngas composition. Calcined Dolomite and Ni-Olivine catalyst were tested on similar conditions and was found that dolomite was 1.4 times more active than olivine in biomass gasification with air. However, dolomite generated 4-6 times more particulates or dust and some extra NH3 in the gasification gas than olivine (Corella, Jose; Toldedo Jose M; Padilla, Rita, 2004). After years of research it has been found that the Ni based catalyst to be very effective in tar removal, and ammonia removal. However, at high temperatures nickel is deposited on support like Alumina and hence the metallic particle tends to migrate and form larger aggregates, reducing the dispersion and consequently the catalyst activity. Hence the selectivity of a right kind of support is essential in maintaining the stability and activity of a catalyst.
Olivine is a natural occurring mineral containing magnesium oxide, iron oxide and silica which have high attrition resistance in compared to dolomite. Strong nickel-olivine interaction prevents nickel sintering and attrition of the active phase in compared to other supports (Zhang, Ruiqin; Wang, Yanchang; Brown, Robert C, 2007).
Three different type of Ni/Olivine catalyst was prepared by wet impregnation. Type A (3% NiO on olivine), type B(3%NiO, 1% CeO2 on Olivine) and type C (6%NiO on olivine). Natural olivine was crushed and sieved to particle sizes between 20 and 30 mesh. Solution of Ni(NO3)3.6H2O and Ce(NO3)3.6H20 was made in di-ionized water where Nickel and Cerium were loaded onto supports by weight impregnation followed by drying in a vacuum at 1050C for 8h. Samples were then calcined in air at a low heating rate until a final calcination temperature of 8000C was achieved and maintained for 2h.
Before the test the catalyst was reduced at 7000C by passing a mixture of 50% H2 and 50%N2 through the reactor at a rate of 80ml/min for 2.5h. After getting the catalyzed bed at desired temperature the model tar compound either benzene or toluene and steam were injected into the preheater where they were rapidly vaporized. Space velocities were chosen high enough to minimize external mass transfer limitations while holding conversion less than 80%.During the characterization technique it was confirmed that metallic nickel was the active component of steam reforming catalyst.
The catalytic activity for the steam reforming was evaluated by carbon-based conversion to gas, which was calculated as the fraction of the carbon contained in benzene and toluene that was converted to gaseous products (CO, CO2, CH4, and H2). The low performance of type A and C catalyst was supported by the results which indicated the presence of higher surface carbon of used catalyst in compare to type B. The presence of CeO2 improved the properties and increased the crystal oxygen on the surface benefiting the redox reaction during the steam reforming process.
Table Gaseous product from steam reforming of Benzene and Toluene at 750C
Catalyst
Benzene steam reforming
H2(%)
CH4(%)
CO(%)
A
61.26±0.37
0.01±0.00
32.47±1.10
B
63.61±0.56
0.01±0.01
23.82±0.29
C
61.94±0.37
0.01±0.00
31.64±1.10
Catalyst
Toluene steam reforming
H2(%)
CH4(%)
CO(%)
A
61.32±0.21
0.16±0.02
28.85±0.14
B
63.61±0.35
0.11±0.01
22.49±0.65
C
61.15±0.41
0.22±0.02
29.29±0.48
From the above table we can see that the production of Hydrogen for benzene and toluene were higher for type B catalyst than for type A and C catalyst. This supports the fact that the CeO2 acted as a promoter in Ni/Olivine catalyst increasing the conversion. However, as the temperature increased from 750 to 830oC the difference in conversion between the catalysts were not very significant. At 830oC the difference was less than one half percent which indicated that the reaction was controlled by chemical equilibrium. Though trend of benzene and toluene conversion are similar the toluene conversion was found to be higher than that of benzene at comparable temperature because of benzene stable chemical structure.
The author characterized their catalyst by using powder X-ray diffraction (XRD) for both fresh and used catalyst. BJH mode performed with a US NOCA 1000e surface and Pore Analyzer helped to analyze the specific area and pore structure of the catalyst. The fresh NiO/Olivine catalyst for type A and C had the peak of NiO/Olivine catalyst where as for the type B had CeO2 peak in addition. In the reduced and expended samples, nickel appeared instead on NiO. This confirms that the metallic nickel was an active component for steam reforming catalyst. Scanning electron microscope graphs of reduced catalyst and expended catalyst looked similar before and after testing which further suggests that the principal effect of the catalytic test is the reduction of NiO to Ni.
The reaction mechanism in catalyzing the benzene (a model tar compound) involved the absorption of the target molecules and water molecules on the catalyst surface where they reacted until all carbon atoms are converted into CO or CO2. Nickel dissociates the H2O in .OH radicals which opens or cuts the rings of the polyaromatic hydrocarbons the most typical and most-abundant species in tar. Methanation production was less than 0.2% of the product gas and was discovered that it was formed from the methanation of CO but not as a reaction intermediate or primary product. Out of three different catalyst type it was found that the type B had the higher H2 yield. This suggests that the doped CeO2 act as a promoter in the catalytic activity of NiO/Olivine catalyst. Whereas A and C had almost the same catalytic activity despite of the 3.0% difference in their catalyst composition. Also it was found that the toluene conversion was higher than that of benzene at comparable reaction conditions because of the stable chemical structure of benzene than toluene.
The carbon content by elemental analysis of type A and C were on the order of 15wt% was much higher than the amount of carbon found on catalyst B. This indicated that the doped CeO2 had greater resistance to carbon deposit on Ni/Olivine catalyst.
In the other study of Ni-Olivine catalyst tar removal by steam reforming (Zhao, Kuhn, Felix, Slimane, Choi, & Ozkan, 2008) it was determined that Ni/olivine catalyst prepared by thermally impregnated process had improved activity and decreased coke deposition rate compared to the catalyst prepared by incipient wetness method. In this method Ni was integrated into the olivine structure (5% by mass) by heating in an Ar environment at 14000C for 4h and was then grounded and sieved to 50-100mesh. The TI preparation lead to a better coke resistance due to the fact that the larger amount of reducible species for the IWI catalyst led to higher initial activity, but also likely contributed to deactivation at longer TOS due to agglomeration of metal species and possible coke deposition.
Characterization technique like BET surface Area, XRD and XPs were used to evaluate the catalyst prepared from two different techniques. Surface areas for the catalyst prepared by TI and IWI were 0.06 and 0.25m2/g, respectively. Difference in the catalytic activity and coke formation for two different methods were evaluated by CH4-H2O reforming reaction as shown in figure below.
Figure (a) CH4 and H2 signals in CH4-H2O reforming TPRxn. (b) CO2
signal from TPO following TPRxn over olivine catalysts prepared with TI
Researcher showed that the IWI catalyst has high conversion of CH4 and increase production of H2 at low temperature, however, the TI has higher conversion of methane and higher production of H2 during the hold at 9000C. So, it proves that despite of having higher surface area for the fresh IWI catalyst, the TI catalyst demonstrated higher surface area normalized reaction rates. The CO2 signal intensity shows that the TI led to a better coke resistance than IWI and the difference can be the Ni-Olivine interaction while preparing the catalyst. (Zhao, Kuhn, Felix, Slimane, Choi, & Ozkan, 2008). Also in other research the BET surface area of the TI catalyst spent sample was found to be 0.05m2/g in compare to fresh catalyst of 0.055 m2/g where as that of the catalyst prepared by IWI was found to be 0.25 m2/g for the fresh and 0.10 m2/g for the spent catalyst. These facts clearly indicate that the TI catalyst is better in compare to IWI catalyst in maintaining its stability (Kuhn, Zhao, Senefeld-Naber, Felix, Choi, & Ozkan, 2008).
In order to do kinetic study TPR experiments were performed on the catalyst. For the first reduction cycle it was determined that IWI catalyst was more reducible than TI catalyst as expected since NiO formed during the calcination whereas metallic Nickel was formed for TI catalyst. However, when it was reduced for the second time the profiles were very similar.
Temperature (k)
H2 consumption (a.u)
In order to make the biomass to syngas economically feasible, significant improvement has been made using the Ni/Olivine catalyst. The short syngas catalytic conditioning catalyst lifetime an obstacle, was slightly improved by this research by increasing the activity of the catalyst be decreasing coke formation and minimizing attrition, and also the cost of the materials was reduced by using Olivine a cheap and easily available support. Further a combined method of preparing a catalyst with CeO2 as a promoter and applying TI method to prepare a catalyst is believed to be the best way to enhance the quality. The low valence state cerium oxide was thought to adsorb water and dissociate it resulting in species -O or -OH transferring to nickel and reacting with surface carbon species to form CO, CO2 and H2, whereas the stronger Ni/Olivine interactions in a catalyst prepared by TI is believed to be the main reason for its high activity. Though the mentioned properties seems to be making a better catalyst, however it is yet to be experimentally proven and would be an interest for further research.
