Biomass Gasification Diagnostics with Flame Spectroscopy in an Oxy-fired Pressurized Gasification Facility

Paper from the AFRC 2017 conference titled Biomass Gasification Diagnostics with Flame Spectroscopy in an Oxy-fired Pressurized Gasification Facility

This paper reports and discusses the diagnostic information retrieved from flame spectra acquired in a pilot scale entrained flow gasification facility with biomass as fuel. Typically the operation of a high pressure oxygen fired gasification reactor is monitored by wall mounted thermocouples and on-line gas analyzers. In the current work which was done in the CanmetENERGY gasification facility, using burners designed by Air Liquide, a cooled and purged fibre optic probe coupled to a spectrometer was added to the monitoring system. The flame emission spectroscopy (FES) probe was inserted into an access port of the gasifier reactor to collect the flame radiation. Biomass flames are known to display spectral peaks arising from the alkali metals potassium and sodium. Often either or both of these emissions also display self- absorption. In the measurements reported here, in addition to the above alkali elements, spectral peaks which may be assigned to other metals were also observed. Emission peaks of heavy metals and trace elements, in the spectra of solid fuels, stack gases and ash samples are familiar in laser induced breakdown spectroscopy where the elements are excited to very high temperatures exceeding 10 000°C. However reports on such spontaneous emissions in high pressure biomass gasification flames are not apparent in literature. The acquired biomass flame spectra were analyzed to yield temperatures representing the reaction chamber region enclosing the central axis of the probe. This paper discusses the significance of the observed spectral emissions and spectrally derived temperatures representing the reaction chamber region, in monitoring the performance of the gasification process. The results reported assess the effectiveness of a flame spectroscopy probe as a value added tool for monitoring biomass gasification in an oxy-fired pressurized gasification facility.

AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 Biomass gasification diagnostics with flame spectroscopy in an oxy-fired pressurized gasification facility Thangam Parameswaran, Scott Champagne and Robin Hughes Natural Resources Canada, CanmetENERGY, 1 Haanel Drive, Nepean, Ontario K1A 1M1, Canada Corresponding author: Thangam Parameswaran, [e-mail], 613-992-8929 Abstract This paper reports and discusses the diagnostic information retrieved from flame spectra acquired in a pilot scale entrained flow gasification facility with biomass as fuel. Typically the operation of a high pressure oxygen fired gasification reactor is monitored by wall mounted thermocouples and on-line gas analyzers. In the current work which was done in the CanmetENERGY gasification facility, using burners designed by Air Liquide, a cooled and purged fibre optic probe coupled to a spectrometer was added to the monitoring system. The flame emission spectroscopy (FES) probe was inserted into an access port of the gasifier reactor to collect the flame radiation. Biomass flames are known to display spectral peaks arising from the alkali metals potassium and sodium. Often either or both of these emissions also display self- absorption. In the measurements reported here, in addition to the above alkali elements, spectral peaks which may be assigned to other metals were also observed. Emission peaks of heavy metals and trace elements, in the spectra of solid fuels, stack gases and ash samples are familiar in laser induced breakdown spectroscopy where the elements are excited to very high temperatures exceeding 10 000°C. However reports on such spontaneous emissions in high pressure biomass gasification flames are not apparent in literature. The acquired biomass flame spectra were analyzed to yield temperatures representing the reaction chamber region enclosing the central axis of the probe. This paper discusses the significance of the observed spectral emissions and spectrally derived temperatures representing the reaction chamber region, in monitoring the performance of the gasification process. The results reported assess the effectiveness of a flame spectroscopy probe as a value added tool for monitoring biomass gasification in an oxy-fired pressurized gasification facility. 1 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 1. Introduction Increasing demands for clean energy has shifted the focus of combustion research to sustainable resources such as biomass as alternatives to conventional fossil fuels. Gasification may be defined as the partial combustion of solid fuels at suitable temperatures and serves as an efficient process for converting biomass into biofuels and useful chemicals. [1]. Gasification of agricultural products is a methodology known for a long time [2]. Biomass gasification results in producer gas containing CO, H2 and traces of CH4 which are combustible. While gasification may be performed at atmospheric pressure, pressurized gasifiers are designed to have more economical syngas production [3]. At CanmetENERGY, high pressure oxygen fired gasification of coal and petroleum coke was demonstrated in a pilot scale reactor. In addition to conventional thermocouples an optical probe based on flame emission spectroscopy (FES) was used in these studies to monitor reactor performance [4]. The present work extends the application of an FES based probe to the monitoring of biomass gasification in the same facility using burners supplied by Air Liquide. The tools traditionally used for monitoring the performance of a gasifier are wall mounted thermocouples and online gas analyzers. Thermocouples selected to suit the operating conditions record reactor wall temperatures while gas analyzers provide the concentrations of the gases produced. These instruments typically log the measured parameters every second. Unlike either of these techniques flame emission spectroscopy directly utilizes the gasifier flame to extract information about the gasifier performance. In this method the radiation from the flame is collected with a fiber optic probe coupled to a small spectrometer. The FES probe, installed in an access port, collects flame spectra in milliseconds and follows the changes in the viewed region of the gasifier chamber occurring on a shorter time scale. Chemiluminescence observed in the visible region 300-600 nm of nonluminous hydrocarbon flames originate from the OH, CH and C2 species and are known to be useful to estimate air/fuel ratios and temperatures [4,5]. Hydrocarbon peaks are not always observed in luminous flames from coal, petroleum coke and biomass but emission peaks from the alkali metals sodium and potassium are visible. Alkali metals are known to cause damage to a reactor due to slagging and fouling [6]. Therefore it is of interest to examine spectral emissions from alkali and other metals occurring in a gasifier flame. The spectral intensities also have contribution from the continuous emission from the soot particles and this property is useful to estimate flame temperatures by applying the blackbody approximation to selected regions of the spectral profile. The theory of FES is reviewed in detail by Ziziak [5]. 2 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 There are numerous optical methods which have been applied for combustion research [7]. Many of these techniques are laser-based and relatively complex but useful for temperature, gas concentration, hydrocarbon species and soot measurement. Optical methods applicable for online metal species are reviewed by Monkhouse [8]. Laser induced breakdown spectroscopy (LIBS) [9], for example is a technique which records emission spectra from elemental species by generating a high temperature ( > 10000 C) plasma in the test sample but requires a high powered laser and not always preferred for industrial online measurements. FES uses the flame radiation as the light source and when coupled with a miniature spectrometer becomes attractive for industrial applications. The current work examines the usefulness of FES to monitor high pressure biomass gasification with a simple, robust and economical approach. 2. Materials and Methods 2.1 Gasification facility Gasification tests described in this report were conducted with CanmetENERGY's gasifier, previously detailed in Ref. 10. The refractory lined gasifier has an inner diameter of 0.20 m and a length of 2.1 m. A burner supplied by Air Liquide was integrated with the reactor. The existing dense phase conveying fuel system was modified to convey the biomass fuel to the burner with carbon dioxide (~38.5 kg/hr) as the transporting gas. A detailed description of the fuel conveying system is provided in Ref. 11. A schematic diagram, of the cross section of the gasifier as operated, is in Figure 1a. Type B ceramic sheathed thermocouples measured the temperatures at four locations on the refractory wall. These are labelled TE411, TE412, TE413 and TE414 from top to bottom at the nozzles shown in Figure 1a. Prior to gasification, natural gas fired with a small flow of oxygen combined with air was used to preheat the gasifier. Critically located thermocouples served to detect and prevent unwanted overheating of the reactor. High pressure carbon dioxide was used to carry the fuel from the source to the burner. Once the preheat temperature was deemed sufficient, natural gas was turned off. Following this, the FES probe was inserted in the access port (Fig.1a), the gasifier was pressurized to 15 bar(g) and biomass was introduced. During this testing campaign gasification tests were conducted for several hours on a number of days and for one set of tests the pressure was reduced to 8 bar(g). 3 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 2.2 Setup of the fibre optic probe in the gasifier A fibre optic probe was inserted through an access port in the gasifier such that the probe tip was close to the inner surface of the refractory wall. The access port selected provides line of sight close to the root of the flame. The probe consists of a metal coated fiber which can withstand temperatures as high as 700˚C, enclosed in a steel jacket which provides water cooling and nitrogen purge. The inserted location the FES probe was selected to collect the radiation from a portion of the flame and the wall facing the probe as well as the gasifier interior lying within an angle of 8.8º enclosing the central axis of the probe. With the current burner it is estimated that the probe views the hot gasifier wall close to the region at the same elevation as the thermocouples TE411 and TE412 when there is no flame. When a gasification flame is present it is likely that the probe sees more of the edge of the flame and less of the other regions in the chamber (Fig.1b). More details about the probe are given in Ref. 4. The free end of the high temperature fibre is connected to a 30 m flexible fused silica (uncoated armored) fibre which is coupled to the data acquiring spectrometer located in the control room as shown in Figure 1a. FES data from the gasifier was acquired by means of an Ocean Optics spectrometer system equipped with a wide spectral range (200800nm) and the Spectrasuite software. Fig. 1a Sketch of the FES probe in the CanmetENERGY pilot scale biomass gasifier 4 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 F E S f i e l d o f v i e w Fig. 1b FES probe - field of view Flame spectra were acquired during most of the time period of gasification. The integration (exposure) time of the probe was adjusted) to ensure that the signal was strong but not saturated. These optical measurements were conducted to evaluate the performance of the FES probe with an Air Liquide burner and biomass fuel. All the flame spectral data reported here were collected with the same burner. These tests were carried out under a contract for Air Liquide to evaluate their burner designs. Due to the proprietary nature of those designs, additional details of the burner configuration cannot be disclosed. 3. Fuel type and operating conditions The term biomass may refer to a variety of agricultural material or even municipal waste which contains organic matter. Thus the composition of biomass fuel can vary considerably [12]. In addition to fixed carbon, volatile matter and moisture, biomass can contain elements such as Si, Al, Fe, Ca, S, Mg, K, Ti, Na and P. Ontario sawdust (wood flour) was the biomass solid fuel used in the current work. The composition of the ash samples of this saw dust measured with X-ray fluorescence is given in Table 1. Although the values are not directly used in the current report the results of the proximate and ultimate analysis of the Ontario saw dust biomass are given in Tables 2 and 3 respectively. 5 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 The maximum fuel flow rate was in the range 70- 80 kg/hr. Oxygen flow was controlled to maintain the prerequisite wall temperature of 1300°C as measured by the bottom most thermocouple TE414. During gasification a gas chromatograph continually recorded the exhaust gas concentrations. FES data was collected during selected periods. Table 1. Composition of Ontario Sawdust ash measured with x-ray fluorescence Component SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO SO3 Na2O K 2O Barium Strontium Vanadium Nickel Manganese Chromium Copper Zinc Composition 11.05 wt% 2.30 wt% 4.63 wt% 0.23 wt% 1.34 wt% 25.75 wt% 4.32 wt% 2.75 wt% 12.39 wt% 10.37 wt% 1951 ppm 892 ppm <50 ppm <50 ppm 4669 ppm <50 ppm 339 ppm 1099 ppm Loss on Fusion Sum 23.96 99.99 wt% wt% Table 2. Proximate analysis of Ontario Sawdust, as received Parameter Moisture TGA Secondary Moisture Ash TGA Volatile Fixed Carbon As Analysed 6.84 Dry @ 105°C 0.68 76.89 15.49 0.73 82.62 16.65 Dry Ash Free 83.23 16.77 wt% wt% wt% wt% wt% Table 3. Ultimate analysis of Ontario Sawdust, as received Parameter Carbon Hydrogen Nitrogen Total Sulfur Oxygen by Difference As Analysed 46.4 5.50 0.44 <0.05 40.03 6 Dry @ 105°C 49.9 5.91 0.47 Dry Ash Free 50.3 5.95 0.47 43.02 43.36 wt% wt% wt% wt% wt% AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 4. Results and discussion 4.1 High pressure gasification flame spectra of biomass A flame spectrum is a plot of the flame radiation intensity as a function of the wavelength. In Figures 2-3 gasification flame spectra (raw counts) collected at 15 bar (g) on different days are displayed. Fig. 2 Biomass gasification spectra - example 1 Fig. 3 Biomass gasification spectra - example 2 7 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 The mass flow rate of biomass (BM) measured by the fuel feed system and the mass flow rate of oxygen from the CanmetENERGY oxygen supply system, were recorded continuously during the gasifier operation. In figures 2-3 flame spectra collected at three BM/O2 ratios at a pressure of 15 bar(g), are shown. A plot of the hot walls acquired before biomass addition is included for comparison and to identify the spectral features that originate purely from the gasification flame. Fig. 4 is a plot of similar spectra collected at the lower pressure of 8 bar (g). Fig. 4 Biomass gasification spectra 8 bar (g) In all the three figures biomass gasification spectra reveal clearly identifiable features at 589 nm and at 766-769 nm. These features are associated with the known spectral lines of Na and K respectively, appearing as intensity dips rather than peaks due to self-absorption. Na and K peaks/dips are often observed not only in biomass flames [13], but also in coal and sometimes in natural gas flames when these metals may be present in the reactor refractory. The interesting observation in figures 2-4 is that pressure broadened peaks are also observed near 624 nm and 554 nm. All these features are present in the BM spectra, but not in the spectra recorded before addition of biomass suggesting that they originate from the biomass gasification flame. All these spectral features are observed even after the flame spectral profiles are corrected (as described in the next section) for instrument response, giving further reason to associate them with probable elemental emissions. The prominent dip near 725 nm present in the BM and the wall spectra is an artifact of the spectrometer response. 8 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 The biomass composition data, in Table 1 shows that the elements Al, Ba and Ca are present in significant amounts. Comparing this information with the strong spectral peak locations in Table 4 we are inclined to relate the 554 nm peak to Ba and the 624 nm peak to Al. Table 4. Spectral lines near the peaks observed in biomass gasification spectra [14] Excitation Wavelength (nm) Line strength Aki (108s-1) Relative intensity (a.u.) Al II 624.336 1.1 400 Ba I 553.5481 1.19 1000 Ba II 614.1713 0.412 300 Ca I 610.2722 0.096 500 Ca I 612.2219 0.287 600 Ca I 616.2172 0.354 600 Ca I 671.768 0.12 60 Fe 624.756 unknown 50 KI 766.4899 0.379 1000 KI 769.8964 0.374 1000 Na I 588.995 0.616 1000 Na I 588.5924 0.614 500 VI 609.0208 0.26 110 VI 624.311 unknown 60 The weaker peak at 671 nm (not shown with arrows) is probably due to Ca, although it is not clear why the other stronger peaks of Ca are absent. It may also arise from Li although the amount of this element in the biomass is not known, but is expected to be very low. The weak feature near 819 nm in Fig. 4 may be associated with Na. The broad feature observed near 648 nm has no clear origin. Similar observations were made in petroleum coke gasification spectra as well [4]. The results presented demonstrate that FES is capable of detecting the presence of alkali and other metals in high pressure gasification flame spectra. 9 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 Such emissions can contribute to slagging and fouling and it is useful to monitor their presence in the gasifier. Similar tests with biomass of varying but known composition will confirm the origin of the peaks and establish correlation between the observed peak intensities and the metals present but such tests were outside the scope of the work reported in this document. Given the knowledge of the ash constituents present in the gasifier, the risk of fouling convective heat exchange surfaces can be estimated [15,16] and it may be possible to adjust parameters to reduce this risk by, for example, adjusting the rate of introducing additives into the fuel or by adjusting process temperatures [17,18]. Fig. 5 Biomass gasification spectra at 15 bar (g) and 8 bar (g) Thus FES may provide means of predicting the ratio of ash constituents on a continuous basis and thus yield the information required to make process control adjustments. It is known that atomic/molecular spectral lines tend to broaden with increase in pressure and the line widths increase. Figure 5 compares two selected spectra from the data collected at 15 bar (g) and at 8 bar (g). These were selected because their BM/O2 ratios were nearly equal with a value of ~1.7. The spectra were collected with the same exposure time and were normalized. Both spectra show the same emission peaks but the peaks at 554 nm and 624 nm are broader at the higher pressure as one would expect. The self-absorption dips at 589 nm and 766-769 nm also display the same trend. 10 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 4.2 Temperatures of biomass gasification flames The distribution and variation of temperatures in a gasifier are usually measured with wall mounted thermocouples. FES can be used to estimate temperature variations inside the gasifier. Fig. 6 Temperatures in biomass gasification flames at 15 bar(g) Fig. 7 Temperatures in biomass gasification flames at 8 bar(g) This is achieved by applying the blackbody approximation to the flame spectra. In practice a reference spectrum acquired from a blackbody standard source is used to correct the observed flame spectra for instrument response. A selected wavelength region of the corrected spectrum is then compared with a library of theoretical spectra calculated at different temperatures. The details of this standard method are described in our earlier work on petroleum coke gasification flames [4] and the same approach is applied 11 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 to the biomass spectra in the present work. The spectral region 675 nm - 710 nm which is free from strong emissions was used for the analysis. These spectrally derived temperatures represent the region viewed by the FES probe. The variation of FES temperatures and the thermocouple temperatures at different BM/O2 ratios and pressures are illustrated in figures 6 - 7. At both pressures temperatures recorded by TE411, which is at a higher location, are higher than TE414 which was maintained at 1300°C. The FES temperatures do not exceed 1300°C at 15 bar(g) and are lower at 8 bar(g). As the ratio BM/O2 changes FES temperatures show more similar fluctuations than TE411 suggesting that FES responds better to the unstable conditions inside the reaction chamber. In general, both TE411 and FES temperatures vary inversely with BM/O2 ratios. Pressure does not appear to have a significant impact on the temperature variation with BM/O2. Seo et al. [19] have also reported that pressure has little effect on their coal gasifier outlet temperature. 4.3 Spectral peak intensities in biomass gasification flames The corrected flame spectra were used to calculate the integrated intensities of the strong peaks at 554 nm and 624 nm and examine the effect of BM/O2 on these peaks. These graphs are shown in Figures 8 and 9 for the two gasification pressures used in the tests. In these figures the intensities of both peaks respond similarly to BM/O2 fluctuations. In both cases the peaks appear to get stronger at lower BM/O2 and lower at higher BM/O2. This trend is consistent with the observation that gasification temperatures are higher at lower BM/O2 ratios. At lower fuel to oxidizer ratios more of the fuel reacts with the oxidizer and hence the higher temperatures and higher excitation of the trace metals in the fuel. Fig.8 Spectral peak intensities in biomass gasification flames at 15 bar (g) 12 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 Fig.9 Spectral peak intensities in biomass gasification flames at 8 bar(g) 5.0 Conclusions A flame emission spectroscopy probe was installed in a pilot scale gasifier to monitor the gasification process when fired with biomass and oxygen at pressures of 15 bar(g) and 8 bar(g). The cooled and purged probe operated satisfactorily during several days of testing. The flame spectra collected were analysed to estimate gasifier temperatures which were lower than the thermocouple measured wall temperatures since the probe was able to measure temperatures closer to the root of the flame where the temperature is expected to be lower based on computational fluid dynamics models of similar systems [18]. The spectra revealed spectral peaks which appear to be related to the alkali and other metals present in the biomass. This observation has significance because some metal emissions are known to have a negative impact on the gasifier operation. Na and K emissions are not unusual in biomass flames. Other metal emission peaks are observed in a LIBS (Laser induced breakdown spectroscopy) high temperature plasma [9]. In the case of LIBS the very high temperature excites and causes any atom to emit its characteristic radiation. Sodium and potassium have very low ionization potentials and hence emit even at flame temperatures. More work is required to understand the origin of the broadened spectral peaks at 624 and 554 nm and weaker peaks at other wavelengths observed in the biomass gasification spectra. To the best of our knowledge such peaks in high pressure biomass gasification flames, are not reported in literature. Further tests with biomass of varying composition will determine if an FES probe can be used as a single tool to monitor the reactor temperature as well as metal emissions during high pressure oxyfired gasification. 13 AFRC 2017 Industrial Combustion Symposium Houston Texas USA Sep 18-20,, 2017 6.0 Acknowledgement This work was supported by the Government of Canada's Program of Energy Research and Development, and Air Liquide. Richard Lacelle, Jeffery Slater, Christopher Mallon and Alex McCready assisted in the maintenance and operation of the gasification facility and the flame emission spectroscopy probe. 7.0 References 1. 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