Article pubs.acs.org/IECR
Study of Silane Decomposition during the Combustion of Renewable Natural Gas Aydin Jalali,† Fokion N. Egolfopoulos,† and Theodore T. Tsotsis*,‡ †
Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California 90089-1453, United States ‡ Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1211, United States ABSTRACT: The decomposition of silane (SiH4), as a model silicon-containing trace compound in renewable natural gas (RNG), has been studied during RNG combustion at ambient pressure conditions, using the opposed-jet, flat-flame experimental configuration. Silane flame concentration profiles were obtained, which indicate that complete SiH4 oxidation occurs in the preflame and luminous flame regions. The laser-extinction technique was used to measure particle volume fractions in the flame, and scanning electron microscopy and energy dispersive analysis by X-ray were used to study the morphology (size and shape) and the elemental composition of the particles formed. It was shown that pure solid silica (SiO2) particles are generated and carried out with the gaseous combustion products into the postflame region. The experimental data were modeled using two detailed SiH4 decomposition models, and the major SiO2 production channels have been identified for both these models via sensitivity and reaction path analyses. It was determined further that a first-order global decomposition reaction rate can describe adequately the experimental data.
1. INTRODUCTION Biogas is produced in wastewater treatment plants (WWTP) and in landfills (known then as landfill gas or LFG). It is a promising renewable energy source, a large fraction of it being CH4, ∼40−70 vol %, the rest mostly CO2, but also O2, N2, and Ar in smaller amounts.1−3 Though promising for energy and power generation,4−6 biogas contains more than 140 adverse trace contaminants7−9 including siloxanes, a problematic constituent.10,11 During biogas combustion, siloxanes are oxidized and produce silica microparticulates,11 necessitating frequent maintenance of the electricity generators and reducing their operating life.12−14 In addition, unless adequate precautions are taken, these fine silica microparticles escape into the atmosphere, where they may cause air pollution problems and pose a risk to human health. Because of the challenges siloxanes present to the use of biogas, they have attracted attention in recent years. There are numerous studies, for example, on techniques for removing siloxanes from biogas.15−18 Absorption and adsorption are most commonly utilized, but they are not all that effective, because common adsorption/absorption media are not uniquely selective toward the siloxanes in the presence of the other biogas contaminants. Today, biogas, either as a medium-BTU or high-BTU fuel (after contaminants are removed and its CH4 content is upgraded to meet natural gas (NG) pipeline standards), also known as renewable natural gas (RNG), finds increasing use in the USA and in several European countries. As a result, the potential of siloxanes finding inadvertently their way, as RNG impurities, into the NG supply system, makes clear the need to understand fundamentally how the silica microparticulates are generated from siloxanes during combustion, and how they deposit on various combustion equipment surfaces. © 2014 American Chemical Society
Few investigations have been carried out to date on siloxane decomposition kinetics, mostly focusing on deriving global decomposition rates.19−22 No rigorous siloxane oxidation mechanism has been reported to date, even for the simplest linear siloxane, hexamethyldisiloxane (L2). For L2, but also for all other larger siloxane molecules in biogas,22 there is complete lack of information regarding the decomposition channels leading into SiO and eventually SiO2 formation. Considering the lack of reliable siloxane decomposition mechanisms, silane (SiH4) was chosen in this research as a model Si-containing biogas contaminant to study silica particulate formation during RNG combustion. The reason for the choice is that SiH4, among the simplest Si-containing compounds, has received the most attention in the past, with several investigations focusing on its decomposition kinetics, thus providing reliable pathways toward SiO/SiO2 formation. Moreover, SiH4 is important, in its own right, in the powder industry, where flame aerosol technology is used commercially to manufacture nanoparticles from SiH4.23 Gas-phase synthesis of nanopowders offers advantages over other synthesis processes, due to its simplicity as a one-step process, and the fact that it uses machinery with no moving parts.24,25 In an early study of SiH4 thermal decomposition, Hogness et al.26 showed it to obey a first-order global reaction rate and concluded that SiH2 (silylene) production is the first step of the process. However, more recent investigations27−29 reported that SiH3 (silyl) production is more plausible as the main channel for SiH4 decomposition, especially at the higher Received: Revised: Accepted: Published: 12993
May 18, 2014 July 25, 2014 July 31, 2014 August 11, 2014 dx.doi.org/10.1021/ie502039x | Ind. Eng. Chem. Res. 2014, 53, 12993−13005
Industrial & Engineering Chemistry Research
Article
Figure 1. Schematic of the experimental configuration.
updated thermochemical data was proposed by Babushok et al.41 aiming to capture the upper SiH explosion limit. In the model,41 reactions of the SiH2 radical dominate at high temperatures, whereas at low temperatures, the SiH3 kinetics are the controlling ones. In the most recent SiH4 combustion model by Miller et al.,50 previous advancements on SiH3 production and consumption51,52 as well as solid silica polymerization reactions53 were considered, and a more realistic decomposition model was developed, which includes 69 species and 201 elementary reactions. In a subsequent study by Donovan et al.54 OH concentrations were measured to identify different ignition stages in SiH4 combustion, and this model was evaluated with experimental data obtained in their study. Because of its industrial importance, particle formation from SiH4 and other Si-containing precursors (e.g., siloxanes) has also received its share of attention. Some of the early studies55−57 addressed particle coalescence and growth and the important parameters controlling particle synthesis, and reported that the formation of condensing species, followed by nucleation, and growth are affected greatly by the flame characteristics; such conclusion was also reached by Wooldridge58 in a paper on the fundamental processes governing gas-phase combustion synthesis of particles, and discussing the parameters influencing particle morphology and composition. Precursor concentration, as well as the temperature field and particle residence time in the synthesis environment are the key factors influencing particles morphology.23,59,60 Chung and Katz61 studied SiO2 particle formation from SiH4 in an opposed-jet, flat H2/O2/Ar diffusion flame, while monitoring temperature and the SiO and oxygen partial pressures. Using the light scattering technique, they showed that SiO and SiO2 nucleation directly depends on the SiH4 feed flow rate; supersaturation, resulting in the nucleation and formation of solid silica particles, does not occur below a certain SiH4 flow rate. Assuming that chemical equilibrium prevails, they were able to simulate the gas species and SiO2 particle concentrations using phase diagrams and chemical equilibrium equations reported in Schick.62 Zachariah et al.63 reported that during SiO2 particle formation, increased flame temperatures enhance the chemical kinetics and homogeneous nucleation, leading into higher numbers of particles that are
temperatures. Furthermore, more comprehensive studies of global decomposition rates27,30 indicate that the rate order is 3/ 2 for SiH4 conversions below 3%, and first order for higher conversions (3−30%). A number of SiH4 decomposition mechanisms have been proposed based on the measurement of fundamental combustion properties, e.g., ignition delay times. Some of the early research was done31−33 with SiH4/H2 mixtures. In subsequent studies, the models were improved further, and more advanced mechanisms were proposed.34,35 As noted above, a key focus for most studies was whether the first key step is H abstraction to form SiH3 or H2/SiH2 production from SiH4. Britten et al.36 proposed one of the first comprehensive SiH4 decomposition models, incorporating 70 elementary reactions and 25 chemical species. The key model feature, postulated earlier by Hartman et al.,37 involves the competition between thermal stabilization and decomposition of the excited state xH3SiOO (silyl-peroxy) radical formed by the reaction of SiH3 and O2;38 however, the nucleation of SiO2 to form solid particles was not considered. Another model of SiH 4 decomposition, reported by Fukutani and co-workers39,40 during the same year, is less comprehensive, consisting of 45 elementary reactions. This model concludes that at low temperatures, SiH4 decomposes first into SiH3, whereas at high temperatures, it decomposes into SiH2.41 Koda42 published a comprehensive review on the pyrolysis of SiH4 at different conditions, Murakami et al.38 and Kondo et al.43 carried out a comprehensive study of the reaction kinetics of SiH3 and related compounds, and Jasinski and Chu44 and Moffat et al.45 investigated the reaction of SiH2 with hydrogen and calculated reliable rate constants. The existence of SiO, OH, and SiH gas species was confirmed via emission spectroscopy by Koda and Fujiwara46 during SiH4 combustion in an opposed-jet diffusion flame. They reported that SiO2 particles nucleation intensifies when the SiH4 flow rate increases beyond a critical value. The rate constants of the reaction of SiH4 with atomic hydrogen and oxygen to produce SiH3 were measured by Goumri et al.47 and Ding and Marshall,48 respectively, and Zachariah and Tsang49 computed the thermochemistry and kinetics of the high-temperature reactions of silicon oxy-hydride species (SixHyOz). A SiH4 combustion model using such 12994
dx.doi.org/10.1021/ie502039x | Ind. Eng. Chem. Res. 2014, 53, 12993−13005
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more spherical in shape. Zachariah and Semerjian64 reported that in diffusion flames, L2 forms larger particles than SiH4, which they attributed to its higher flame temperatures, and correspondingly faster sintering rates. Chung et al.65 investigated SiH4 diffusion flames and concluded that at low SiH4 concentrations and/or when a high-temperature flame is used, SiO2 is the nucleating species. At high SiH4 concentrations and/or for low-temperature flames, nucleation of SiOx (x = 0 and/or 1), in addition to SiO2, also takes place. Parameters like flame stoichiometry, diluent concentration, and gas flow rates all influence particle formation rates and morphology because they affect the temperature and the particles’ residence time in the flame. More recently, Dultsev66 found that both the gasphase and substrate temperatures impact the porosity of the silica layer. For example, to obtain dense layers, the substrate temperature must be higher than the gas temperature. Different additives in the feed gas (e.g., NH3, C3H6) also affect the silica porosity. As noted above, due to lack of a reliable mechanism to model particle formation from siloxanes during biogas combustion, SiH4 was chosen in this study as a model Si-containing contaminant. It was immediately recognized though that, despite the much greater attention SiH4 decomposition has received, still contradictions remained regarding the exact pyrolysis mechanism (e.g., whether the first step is SiH2 or SiH3 production) or the silica generating channels. Thus, the first objective of the study became to assess these differences via RNG combustion experiments and appropriate simulation tools. As part of the study, experiments were therefore carried out to study the fate of SiH4 during combustion under wellcontrolled conditions and at concentrations typical of Sicontaining RNG contaminants, and to characterize the solid products formed. In addition, based on the data, the performance of the two main SiH4 decomposition models available36,50 were evaluated. In what follows, first the experimental and modeling approaches are described, and results are presented and discussed next.
premixed RNG/air stream to generate the desired range of SiH4 concentrations (2−30 ppmv). The ability to generate reliably the required concentrations was verified via a GC/MSD system (Agilent Technologies, 7890A GC System; 5975C Inert XL MSD), calibrated using standard gas samples of SiH4 in N2. The final RNG/air/SiH4 feed stream is introduced into the bottom burner of the experimental system, a schematic of which is shown in Figure 2.68−70 The burner is a straight tube
Figure 2. Schematic of the opposed-jet flow configuration.
surrounded by a N2 coflow channel to ensure the isolation of the main jet from the ambient environment. Flat flames are generated and stabilized by counterflowing a N2 stream from the top burner, against a premixed RNG/air mixture from the bottom burner. The nozzle diameter for both burners and the separation distance between them was 14 mm. The air/fuel equivalence ratio (φ) of the stream exiting the lower burner was kept at 0.8, and the global strain rate K (defined as twice the nozzle exit velocity divided by the burner separation distance) was kept constant at 131.6 s−1. Thermocouples used to measure the flame temperatures are known to perturb the flow fields, and to interfere with heat transfer mechanisms, like radiation, thus their presence needs to be accounted for.71 Here, an R-type thermocouple (Omega P13R-003) was utilized, coated with a Y/BeO ceramic film to minimize its surface reactivity.72,73 To avoid bending of the thin thermocouple wire when placed in the hot flame, which makes it difficult to determine the measurement position, a special device was built74,75 The thermocouple assembly was mounted onto a multistage translational stand capable of moving in micrometer increments. The cold junction temperature was adjusted by calibrating it at room temperature. The radiation corrections were made76,77 using an emissivity value of 0.3.78 Gas samples were taken from the flat flame using a quartz microprobe with an end-opening of ∼150 μm positioned on a stand having three-dimensional (3-D) (XYZ) mobility (Thorlabs, PT3/M, 25 mm XYZ Translation Stage) to measure the variations of SiH4 concentration in the flame. To begin the sampling, the tip of the probe was positioned at the exit of the lower burner (Z = 0), and then moved vertically in steps to collect gas samples in the preflame, flame, and postflame regions. The positioning system, which includes a cathetometer, was capable of locating accurately the initial probe position (and the burners themselves) within 25 μm, thus minimizing
2. EXPERIMENTAL SECTION The SiH4 decomposition was studied in the opposed-jet flame configuration shown in Figure 1. Temperatures were measured with thermocouples, while the laser extinction technique was used for the in situ measurement of the solid particles’ volume fraction, f v. Gas samples were withdrawn from the flame and analyzed via a gas chromatograph equipped with a mass selective detector (GC/MSD) to determine the SiH 4 concentrations. To probe the nature of deposits formed on surfaces in proximity to the RNG flames, flat Ni/Cr metal strips were placed downstream of a Bunsen flame and the chemical composition and structure of the solid particles were investigated by scanning electron microscopy (SEM) and energy dispersive analysis by X-ray (EDAX). The simulated RNG utilized in this study consists of 98% CH4 and 2% CO2, both 99.97% pure, from the Gilmore Liquid Air Company (South El Monte, CA, USA). The SiH4 gas (99.999%, CAS: 7803-62-5) used was purchased from Air Liquide America Specialty Gases LLC (Morrisville, PA, USA). To generate ppmv-level SiH4 concentrations in the main feed, a mass flow controller (MFC, Teledyne Hastings HFC-202) was utilized to deliver 0.2 sccs of SiH4 gas into a heated air stream, followed by further dilution to reach a concentration of ∼1000 ppmv. Another mass flow controller (MFC, Teledyne Hastings HFC-202) was then utilized to deliver ∼1% of this mixture to a 12995
dx.doi.org/10.1021/ie502039x | Ind. Eng. Chem. Res. 2014, 53, 12993−13005
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Table 1. Chemical composition (wt %) of the Ni/Cr Alloya
a
Cr
Fe
Mo
Cu
Co
W
Ni
Ti
C
Mn
Si
Al
other
20.5− 23.0
17.0−20.0
8.0−10.0
