Article pubs.acs.org/EF
Soot Concentrations in an Atmospheric Entrained Flow Gasifier with Variations in Fuel and Burner Configuration Studied Using DiodeLaser Extinction Measurements Johan Simonsson,*,† Henrik Bladh,† Marcus Gullberg,‡ Esbjörn Pettersson,‡ Alexey Sepman,‡ Yngve Ö gren,‡ Henrik Wiinikka,‡ and Per-Erik Bengtsson† †
Combustion Physics, Lund University, Box 118, 221 00 Lund, Sweden SP Energy Technology Center AB, Box 726, 941 28 Piteå, Sweden
‡
ABSTRACT: Soot concentration measurements were performed using diode-laser extinction in an atmospheric air-blown entrained flow gasifier at two vertical levels. The gasifier was operated at different air-fuel equivalence ratios and with variations in fuel and burner configurations. Two fuels were investigated: wood powder and peat powder. These were burned using two burner configurations, one giving a rotating flow inside the gasifier (swirl), and one where the fuel and air were injected parallel with the gasifier axis (jet). The diode-laser measurements were performed at the wavelength 808 nm from which the soot concentrations were estimated, and additionally at 450 nm in order to gain insight into the spectral dependence of the extinction to estimate measurement quality. Additional diagnostic techniques were used, such as an electrical low-pressure impactor (ELPI) for soot size distributions and gas chromatography for species concentration measurements. The results show that wood powder produces higher soot concentrations than peat powder, especially at lower air-fuel equivalence ratios. Furthermore, the burner configuration had in general much less impact than the choice of fuel on the soot concentration. Also, measurements in the cold product gas (∼40 °C) have shown that significant amounts of soot particles were produced during pilot scale experiments in a 200 kW, 2 bar(a) pressurized oxygen-blown gasifier using different kinds of biomass as fuels.5,6 Similar to previous investigations1,4 the amount of soot and polycyclic aromatic hydrocarbons (PAHs) were reduced when the process temperature increased from 1250 to 1350 °C due to increased oxygen supply and higher airfuel equivalence ratio.5 Furthermore, the results also indicate that the amount of soot could be reduced by using fuels with lower concentration of volatiles and higher concentration of ash-forming elements since gasification of different bark-based biomass produce less soot compared to gasification of pure stem wood.6 However, the remaining question is still what the soot concentration is inside the hot reactor, since none of the soot samplings techniques in these previous investigations have been performed inside the reactor. Although sampling and analysis of soot using various instruments such as a scanning mobility particle sizer (SMPS) and an electrical low-pressure impactor (ELPI) can give important information about the soot produced in the gasifier, the information from these devices is not measured in situ. Also, the soot particles may have undergone changes in soot properties in the sampling lines, since the measurements often are performed in the syngas pipe after the hot reactor. An alternative way to perform soot measurements is to use optical measurements, which often are laser-based. With such approaches in situ real-time soot concentrations can be
1. INTRODUCTION Entrained flow gasification of biomass is a thermochemical process that in the future significantly can contribute to the production of electricity, liquid fuels, or other types of chemicals. In the gasifier, the feed-stock reacts with a controlled amount of air, oxygen, and/or steam at high temperatures to produce raw syngas consisting mainly of carbon monoxide, hydrogen, carbon dioxide, and water vapor. Unfortunately, undesired products such as tars and soot are also produced in the gasification process.1 Tars consist of large hydrocarbons which condense during gasification during the cooling process to ambient temperature,2 while soot can be defined as the solid carbonaceous product from fuel-rich combustion/gasification.3 These products are undesired since they lower the efficiency of the gasification process and can cause an operational problem due to deposit formation in low temperature regions of the gasifier and on downstream equipment, for example in gas turbine and catalysts. Hence, it is an overall goal to minimize the formation of soot and tars already in the gasification process. Previous investigations of biomass gasification1 and pyrolysis of biomass in atmospheric drop tube furnaces4 have shown that the yield of produced soot and tars is significant and also that the process temperature significantly influences the yield of produced soot and tars. At low process temperatures (∼1000 °C), the produced gas contains significant amounts of tars with low amounts of soot. When the process temperature increases to above 1000 °C, the tar yield in the product gas decreases at the same time as the soot yield increases significantly. At the highest process temperatures (1350−1400 °C), the product gas contains significant soot concentrations but only minor concentrations of tars.1,4 © 2016 American Chemical Society
Received: October 30, 2015 Revised: February 4, 2016 Published: February 19, 2016 2174
DOI: 10.1021/acs.energyfuels.5b02561 Energy Fuels 2016, 30, 2174−2186
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Energy & Fuels
according to Beer−Lambert law if the length, L, of the attenuating medium is known:
measured inside the hot reactor. Both techniques (sampling and optical) may however be limited to only a few spatial positions because of operational costs and complexity, as sampling lines and advanced instrumentation are needed for gas analysis and optical ports must be installed for the optical measurements to obtain access to the intended probe volumes. Laser diagnostics of soot has successfully been applied both for fundamental research in laboratory flames as well as in applied research, for example in furnaces and internal combustion engines with optical access.7−9 Laser-induced incandescence (LII),10 elastic light scattering (ELS),11,12 and extinction are three commonly used laser diagnostics techniques for soot measurements. These techniques can be used separately or in combination to give particle sizes, temperatures, and soot volume fractions to mention some.13,14 There is a limited number of papers on laser diagnostics for biomass gasification applications in the literature, and they have mainly focused on molecular species concentrations using tunable diode laser absorption spectroscopy (TDLAS).15−17 In the work by Sur et al.16 TDLAS was used to examine the concentrations of CO, CO2, CH4, and H2O in the syngas downstream from an oxygen-blown entrained flow coal gasifier. The results show concentrations matching those measured using a gas chromatograph (GC) with good accuracy. Additionally, the laser techniques provided much better temporal resolution in the measurements compared to more conventional techniques like the GC. In the work by Chhiti et al.,18 laser extinction measurements were used for soot concentration measurements in gasification. A model describing soot formation and oxidation during bio-oil gasification was presented, and validation of the model was made by laser extinction at the output of a laboratory scale entrained flow reactor, with a fuel flow of 0.3 g/min oil. However, no information was given about experimental details and description of the evaluation procedure. The focus in the present work has been on evaluation of soot concentrations based on light extinction measurements in an atmospheric air-blown entrained flow gasifier located at SP Energy Technology Center AB, Piteå, Sweden, at different operating conditions. In addition to variation in the overall airfuel equivalence ratio, λeq, the gasifier was operated using two different fuels: wood powder from stem wood and peat powder. Two different burners were used, one creating a rotating flow in the gasifier (swirl burner) and one giving an initial flow parallel to the burner axis (jet burner). The extinction measurements were performed with two spatially overlapped laser beams at wavelengths of 450 and 808 nm, respectively, and simultaneously at two different vertical levels of the gasifier. By using two laser wavelengths, an indication of the spectral dependence on the extinction could be obtained, and additional contributions to the total extinction from processes such as soot scattering could be estimated. The laser extinction measurements for soot were complemented by ELPI and gas chromatography as well as TDLAS measurements of CO and H2O.
I = I0·e−KextL
(1)
The extinction coefficient can be written as a combination of the absorption coefficient, Kabs, and the scattering coefficient, Kscat, i.e. Kext = Kabs + Kscat. When the particle size is much smaller than the laser wavelength, the contribution of scattering to extinction is very small, and it is common practice to neglect the scattering contribution.11,19 This is often an adequate approximation for soot in laboratory flames, such as premixed flat flames,20−22 where primary particle sizes are on the order of a few tens of nanometers and also the aggregates are small open structures with high fractal dimension.23−25 The validity for this assumption during measurements in a gasification environment will be discussed later. If the scattering contribution to the extinction is negligible, i.e. Kext ≈ Kabs, one can hence assume that K abs =
π2 E(m)Nd3 λ
(2)
where λ is the laser wavelength, E(m) is the absorption function, m = n − ik is the complex refractive index of the soot particles, N is the number density of soot primary particles, and d is the soot primary particle diameter. The absorption function is defined as E(m) = −Im
(
m2 − 1 m2 + 2
). If the above-mentioned
assumption applies it is possible to calculate the soot volume fraction according to fv =
K absλ 6πE(m)
(3)
The soot volume fraction, f v, is a way to express soot concentration as the volume that soot constitute per gas volume, thus f v is dimensionless. The uncertainty in the evaluation of the soot volume fraction is mainly related to the absorption function, E(m), which can have uncertainties up to 30−40%.10 The value of E(m) is related to the soot optical properties, which depends on soot composition and formation conditions. Recent investigations have shown that newly formed soot have a considerably lower E(m) than mature soot and also that the newly formed soot have values of E(m) which are higher toward the UV part of the spectrum than toward the red (i.e., the E(m) is dependent on laser wavelength).20,21,26 Mature soot do, however, not show this wavelength dependence, and recent results suggest that an E(m) of 0.35 should be used based on measurements on mature soot in laboratory flames.22,25,27 We assume that the soot inside the gasifier is mature, which is a reasonable assumption since soot maturity has been reached on time scales on the order of 10−100 ms in laboratory flames,21 and the time scales in the present measurements are on average orders of magnitude longer. Optical soot measurements in gasification are challenging, as there are potential interferences from other species which, if uncompensated for, will introduce systematic errors in the evaluated concentrations. As previously mentioned, when using visible wavelengths, the soot refractive index, and hence the E(m), is relatively independent of wavelength, and one should expect Kext to show a pure inverse relationship with laser wavelength. Any deviation from the inverse relationship would indicate additional contributions from either molecular
2. THEORETICAL CONSIDERATIONS Extinction is a line-of-sight technique where the initial intensity, I0, and the transmitted intensity, I, of for example a laser beam are measured before and after an attenuating medium, which in the case of the present work are soot particles inside the gasifier. The extinction coefficient, Kext, can be calculated 2175
DOI: 10.1021/acs.energyfuels.5b02561 Energy Fuels 2016, 30, 2174−2186
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Figure 1. Schematic sketch (left plate) and a photo of the gasifier (right plate). T1−T8 are thermocouples located in the gas stream. T9−T16 are thermocouples located in the refractory insulation.
If we assume no influence of molecular absorption on 808 nm, we can write the experimentally determined extinction coefficients as
absorption or light scattering from soot. For soot measurements, it is well-known that PAHs interfere with the measurements by absorbing some of the light when using visible and ultraviolet wavelengths. It has been shown in previous work that this interference decreases with increased wavelength, and for wavelengths above about 700 nm the interference are negligible.20,21,28 As PAHs is one important group of species in tars,2 it is likely that the presence of significant concentrations of tars in vapor phase will contribute to the extinction at 450 nm but not so at 808 nm. Additionally, molecular absorption from other species such as H2O and OH needs to be considered. The wavelengths used have been selected to not overlap any strong absorption lines of H2O or OH. For further discussion regarding this, see previous work by us.21 Finally, the light scattering from soot must be considered. Assuming the majority of the particles to be small enough to be treated within the Rayleigh-Debye-Gans (RDG) limit, the scattering will be approximately proportional to λ−4, which means that for a case with significant level of scattering, the extinction at 450 nm is expected to increase when compared to the extinction at 808 nm.11 Taken together, this means that both molecular absorption of tars species in vapor phase and light scattering from soot is expected to increase the ratio between the extinction at 450 nm and the extinction at 808 nm. The dual-wavelength approach provides a means to check the validity of the assumptions used for the derivation of soot volume fractions, using eq 3, as well as estimating the uncertainties due to the aforementioned interferences. However, it is not possible to fully separate the contribution from scattering and the contribution from molecular absorption as will be briefly outlined below.
⎧ Kext,450 = K abs,450 + K scat,450 + K mol,450 ⎪ ⎨ ⎪ ⎩ Kext,808 = K abs,808 + K scat,808
(4)
where Kmol,450 is the contribution from molecular absorption to the total extinction at 450 nm. Neglecting multiple scattering, assuming the particles to be within the RDG limit (i.e., particle diameters much smaller than the laser wavelength) and a refractive index, m, which is the same at 450 and 808 nm, we may express eq 4 as ⎧ 1 1 x+ y = Kext,450 − K mol,450 ⎪ ⎪ 450 4504 ⎨ 1 1 ⎪ x+ y = Kext,808 ⎪ ⎩ 808 8084
(5)
where x and y are related to the absorption and scattering coefficients as Kabs = xλ−1, and Kscat = yλ−4 with λ in nm. A unique solution to the equation system (5) is not available since Kmol,450 is unknown, and the soot volume fractions presented in this paper can therefore not be compensated for these effects. However, eq 5 may be used to infer the upper limit of the overestimation of the soot volume fraction levels that will result from neglecting the scattering. This maximum occurs when Kmol,450 is assumed to be zero and hence where the deviation from 1/λ-dependence of the extinction is solely explained by the influence of scattering. For any nonzero contribution from molecular scattering at 450 nm, the 2176
DOI: 10.1021/acs.energyfuels.5b02561 Energy Fuels 2016, 30, 2174−2186
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3.2. Fuels. Two different raw materials have been used as fuels, stem wood (70−80 wt % pine, 20−30 wt % spruce) and peat. Both fuels were received pelletized and later milled to powder by a hammer mill until all raw material has passed through a screen with a size of 0.75 mm. The resulting powder particle size is depending on material properties and the history of the fuel (i.e., the fuel size before the pelletizing), and therefore the particle size distribution of the powder for stem wood and peat were not the same after milling (see Table 1). The chemical compositions of the fuels are also presented in Table 1. The stem wood has significantly lower fixed carbon and ash content compared to the peat but instead higher volatile content. This increased amount of volatiles is hence expected to increase the soot concentration.
estimated overestimation of f v due to scattering will be lower than this maximum level. For this complex measurement situation in a gasifier there are several potential uncertainties: the assumed constant optical properties of all soot, the assumed extinction path length, the estimated scattering contribution to the extinction, the potential additional absorption by nonsoot species such as tars, PAHs, and biomass residuals, and beam steering due to temperature gradients inside the gasifier.
3. EXPERIMENTAL SECTION 3.1. Gasifier. The top-fired down-draft entrained-flow gasifier, see Figure 1, was constructed to run at atmospheric pressure. The gasifier has the shape of a standing cylinder with flat top and conical bottom. The gasifier height is ∼3.9 m, and it has an inner diameter of 50 cm. The gasification chamber was enclosed by 200 mm of refractory lining inside the outer steel shell. Optical access was enabled through sight windows where the glass was kept clean by a purge flow of nitrogen set to ∼2 L/min for each window. The fuel feeding rate was controlled by the rotational speed of feed augers in the bottom of the fuel hopper. The fuel was pneumatically transported in the fuel line to the gasifier and enters the burner centrally from the top. The average fuel consumption during operation was ∼20.2 kg/hour for wood powder and 22.9 kg/h for peat powder. Downstream the gasifier of the produced gases was combusted by the supply of preheated air (800 °C) inside a boiler. The gasifier was operated with two different burners, both with an outer diameter of 89 mm and with a central fuel pipe (inner diameter 50 mm) where the powder is injected into the gasifier with the transport air. The first burner, see Figure 2 (a), has in total three air
Table 1. Fuel Analysis powder size distribution (wt %)
