Novel nickel ceramic filter for hot gas cleanup of tars from syngas
Introduction
Renewable energy and fuels have been a topic of particular interest recently with the growing awareness of climate change and the harmful effects that the use of fossil fuels has on the environment. Attention towards alternative energy production has also been spurred on by volatility of the fossil fuel market and the devastating impact it has brought on local and global economies that are reliant upon a single source for energy and jobs. In order to diversify the energy portfolio, the U.S. investment in alternative energies such as wind, solar, hydroelectric, and biomass has risen from $11.3 billion in 2005 to $45 billion in 2015 to a record $59 billion in 2019 [1]. Globally, investments in clean energy reached a record $301.7 billion in 2019 with the U.S. planning to invest additional $2 trillion over the next four years [2]. This comes at a time when energy demand is ever increasing as evidence by total U.S. energy consumption is at an all-time high of 100.1 quadrillion BTUs with renewable energy sources comprising 11%. Of these sources, energy from biomass leads the way totaling 43.5% with a variety of conversion routes including thermal, chemical, biochemical, and thermochemical [3].
A promising route within thermochemical conversion of biomass is gasification. There are 238 commercial gasification plants globally with 33 located in the U.S. Amongst these 33 facilities, eight use biomass/waste as a feedstock [4]. Traditionally coal has been used as the feedstock in the gasification process, however the source of energy has been expanded to natural gas, petroleum, petcoke, and even MSW (municipal solid waste)/plastics. Gasification has several advantages over combustion as the produced syngas can be used for a variety of applications including combined heat and power, generation of electricity, conversion into transportation fuels, production of general and specialty chemicals, and further separation to produce hydrogen. Gasification involves the partial oxidation of a carbonaceous feedstock to produce synthesis gas (syngas) composed mostly of carbon monoxide, hydrogen and carbon dioxide [5]. Syngas also contains several unwanted impurities including ammonia, H2S, tars and particulates. Downstream syngas applications require contaminants to be below certain levels in order for the process to be effective. For downstream usage of syngas in methanol synthesis, particulate and tar concentrations must be below 0.1 mg/m3 and 1 mg/m3, respectively [6]. While particulates, responsible for equipment damage, can be separated through high temperature filtration, tar formation is the most cost-inducing problem whereby their condensation clogs downstream equipment presenting a major hurdle limiting biomass gasification commercialization. Existing tar removal technologies include physical methods such as scrubbing or thermal cracking, however catalytic tar reforming is a cost-effective strategy for tar abatement whereby conversion of tars into syngas not only eliminates tars but improves the overall efficiency of the gasification process [7]. Further, integrating tar and particulate removal into one unit could be realized by using a ceramic filter as a catalyst support. This approach will result in process intensification and will improve the overall process economics of the gasification process. Ceramic catalyst supports come in a variety of geometries and compositions including candles, monoliths, and foams [[8], [9], [10], [11]]. Traditionally ceramic filters are made of SiC, however there have been studies using α-Al2O3, ZrO2, CeO2, and CaSiO3 for boosting material flexibility and decreasing the density [[12], [13], [14], [15], [16]]. The open cell high porosity format of the ceramic foams gives advantages including minimal external mass transfer limitations and low fluid flow resistance creating a lower pressure drop [17]. All of these characteristics in addition to the natural high temperature resistance of ceramics are advantageous for continuous filtration of particulate matter and as a catalyst support material in hot syngas cleanup.
Nickel (Ni) was chosen as a catalyst owing to its low cost, high effectiveness towards tar reforming, and ease in regeneration [[17], [18], [19], [20], [21]]. The relatively high activity of nickel compared to other less economic high activity transition metal catalysts such as Pt, Pd, Ru, Rh has made Ni-based catalysts the subject of many steam reforming studies [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. It has also been theorized that the high activity towards tar compounds that nickel possesses may in part be due to its ability to use H2O and CO2 for the purpose of breaking CH and CC bonds [32]. Ni-based catalysts have been shown to reduce tar by >99%, however the challenge has been in maintaining this activity by overcoming challenges such as metal sintering and carbon deposition [31]. Coking of catalysts has been one of the challenges in tar reforming applications as it is a major contributor leading to the loss of catalytic activity over time due to its deposition on the nickel metal as well as support material. Deposition of coke on catalyst surface will lead to blocking active sites and blocking of pores thus restricting access to active sites present within the pores [33].
Limited number of studies have been performed investigating the use of ceramics as a catalyst support for the purposes of process intensification for simultaneous filtration and tar reforming [10,18,19,[34], [35], [36], [37]]. Zhao et al. [38] used nickel impregnated on a ceramic disc (surface area 0.33 m2/g) for preliminary studies on catalytic tar removal. At 1% nickel, the catalyst was able to achieve nearly 100% naphthalene (tar model compound) removal at 900°C, a gas hourly space velocity (GHSV) of 2088 h−1. They were able to achieve nearly complete naphthalene removal at a velocity of 2.5 cm/s where filtration velocity is typically 1–4 cm/s for candle filters [39]. A study by Straczewski et al. [40] compares ceramic filter discs impregnated with different catalysts including metals for their tar reforming capabilities. The Al2O3 (44)/SiO2 disc had a surface area of only 1.9 m2/g. The catalytic filter was able to achieve over 90% naphthalene conversion at a GHSV of 8400 h−1 and 20,700 h−1. In a review of studies using catalytic ceramic filters, the pressure drop was found to be in the range of 2.5–6.8 kPa when operating within the normal filtration velocity of 1–4 cm/s [41]. The tar conversion ranged from 57% to ∼100%.
These studies highlight the fact that in order to achieve high tar conversions it is necessary to provide certain compensation to make up for limitations in the material properties of the catalyst support. The surface area of the filters seen in these studies are extremely low and thus do not provide satisfactory support area for catalyst deposition. To compensate for this, the addition of noble metals such as platinum or ruthenium are added to boost catalytic activity. Furthermore, the catalysts are only able to achieve a high conversion by operating at a low GHSV. The ceramic filter used in this study boasts a surface area significantly higher than those found in previous studies so as to allow for easier access to a plethora of active sites for tar conversion. Additionally, the pressure drops observed are at an order of magnitude less than those seen in previous studies. This is achieved at a high GHSV using a filter with mesopores. These mesopores are able to reduce the internal diffusion limitations while also allowing a high porosity for low pressure drop. The material properties of the filter in question give several advantages that give way to high tar conversions at operating conditions necessary for high throughput steam reforming processing.
In this study, a Ni-based catalyst was prepared using incipient wetness method on a ceramic filter support. Additionally, proof of concept was tested by performing preliminary steam reforming experiments using naphthalene as a tar simulant molecule due to its high abundance in biomass syngas, high stability, and the ease of comparison due to the plethora of literature [21,31,[42], [43], [44], [45], [46], [47]].
Section snippets
Catalyst preparation methods
Porous ceramic high temperature filters were supplied by Borla Composites (Formerly CerX Filters) based out of Johnson City, Tennessee, USA. They specialize in the manufacture of ceramic filters for a wide range of high temperature applications. The filter materials have a thickness of 1.27 mm (0.05″) and were cut into 1.588 cm (0.625″) diameter discs for further testing. Incorporation of nickel into the ceramic discs was done using incipient wetness impregnation technique. The discs were
BET
Surface measurements were run on the ceramic filter used in this study and are displayed along with properties of ceramic filters used in other steam reforming studies in Table 1. The table shows that the filter used in this study has a much greater surface area compared to those used in other studies (< 2 m2/g). The primary composition of ceramic fiber filter is α-Al2O3, which was then wash coated with γ-Al2O3 and heat treated at 500 °C. γ-Al2O3 primarily contributes to the higher surface area
Conclusions
A novel ceramic filter support was impregnated with nickel for the purposes of catalytic tar reforming. Naphthalene was used as a simulant molecule for tar in order to evaluate the effectiveness of the catalytic filter in terms of tar conversion. Based on the results of the steam reforming experiments, several conclusions were made and are as follows:
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The incipient wetness impregnation technique was effective at depositing nickel particles on the surface of an alumina ceramic filter support as
CRediT authorship contribution statement
Devin Peck: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing, Visualization. William Holmes: Resources, Investigation. Mark Zappi: Resources, Writing – review & editing. Rafael Hernandez: Resources, Writing – review & editing. Daniel Gang: Resources, Writing – review & editing. Prashanth Buchireddy: Conceptualization, Methodology, Formal analysis, Resources, Supervision, Writing – review & editing, Visualization, Project
Declaration of Competing Interest
Prashanth Buchireddy reports equipment, drugs, or supplies was provided by CerX Filters.
References (93)
Recent advances in thermodynamic analysis of biomass gasification: a review on numerical modelling and simulation
J. Energy Inst.
(2022)- et al.
5.10 - Biomass Gasification and Pyrolysis
- et al.
A review of cleaning technologies for biomass-derived syngas
Biomass Bioenergy
(2013) Corrosion of silicon carbide hot gas filter candles in gasification environment
J. Eur. Ceram. Soc.
(2014)Promoting hydrogen-rich syngas production from catalytic reforming of biomass pyrolysis oil on nanosized nickel-ceramic catalysts
Appl. Therm. Eng.
(2017)- et al.
Preparation and performance of V-Wreparation and performance of V-W/x(Mn-Ce-Ti)/y(Cu-Ce-Ti)/cordierite catalyst by impregnation method in sequence for SCR reaction with urea
J. Fuel Chem. Technol.
(2014) - et al.
Mixed zirconia–alumina supports for Ni/MgO based catalytic filters for biomass fuel gas cleaning
Powder Technol.
(2008) - et al.
Ceramic foams coated with Pt Ni/CeO2ZrO2 for bioethanol steam reforming
Int. J. Hydrog. Energy
(2016) Hydrogen production from catalytic steam reforming of benzene as tar model compound of biomass gasification
Fuel Processing Technology
(2016)Syngas conditioning by ceramic filter candles filled with catalyst pellets and placed inside the freeboard of a fluidized bed steam gasifier
Fuel Process. Technol.
(2019)