Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities

Biomass and Bioenergy xxx (2017) 1e12

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Research paper

Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities Daniel Schweitzer*, Andreas Gredinger, Max Schmid, Gebhard Waizmann, Marcel Beirow, € rl, Günter Scheffknecht Reinhold Spo IFK (Institute of Combustion and Power Plant Technology), University of Stuttgart, Pfaffenwaldring 23, 70569 Stuttgart, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 September 2016 Received in revised form 31 January 2017 Accepted 2 February 2017 Available online xxx

In the dual fluidised bed steam gasification process a product gas with a relatively high calorific value is produced. For clean biomass such as wood pellets or wood chips this process has been previously demonstrated in pilot-scale. Within this work, the applicability of waste biomass such as sewage sludge or manure as a fuel for this gasification process was investigated experimentally. A special focus was given to the concentration of impurities in the product gas of the gasifier (such as tar, NH3, H2S and Cl). The gasification experiments have shown, that the steam gasification of both biogenic waste materials is possible. For all fuels a high gas yield, close to the gas yield of wood pellets, was achieved and the product gas composition did not vary much between the fuels. Due to the different structures of the fuels and their different nitrogen, sulphur and chlorine contents, the concentrations of the impurities such as tar, NH3 H2S and Cl can vary significantly: tar concentrations of up to 100 g , m
3, NH3 concentrations of up to 0.06 m3 m
3 and H2S concentrations of up to 7000 , 10
6,m3,m
3 were measured. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Syngas Product gas Biomass Waste biomass Pollutant Measurement methods Tar H2S NH3 HCl

1. Introduction It is the target of the German government to increase the share of electricity produced by regenerative sources until 2050 to more than 80% [1]. To achieve this target and to become more independent from fluctuating wind and solar energy, biomass-based processes will be important contributors to the energy-mix [2]. Especially biomass gasification processes may be important technologies due to their high efficiencies and the numerous applications for the produced gases [3]. The relatively high costs and restrictions in the local and seasonal availability make biomass gasification processes economically and logistically challenging, whereas the gasification of biogenic waste materials, such as sewage sludge and manure, shows a great potential due to their high and constant availability over the whole year and their low or even negative costs. Different sources estimate the annual technical potential of

* Corresponding author. E-mail address: [email protected] (D. Schweitzer).

sewage sludge after fermentation in Germany ranging from 1.8 to 2.4 million t , a
1 (on dry basis) (EU27: 11.56 million t , a
1 [4]). That corresponds to an energy potential of 15e35 PJ , a
1 [5e8]. Studies by Kaltschmitt and calculations based on the livestock population and its corresponding manure yield, estimate the total potential of manure in Germany ranging from 15.5 to 23 million t , a
1 (on dry basis) [9e13]. The majority of this biomass potential is used as farm fertiliser, but due to the concentration of the livestock population [14], there are areas with a manure surplus and consequently manure disposal problems [15]. Especially for this fraction, an energetic utilisation is a promising pathway for producing renewable energy and simultaneously for disposing the manure. 1.1. DFB steam gasification process Within this work steam was used as gasification agent. By using steam as gasification agent, a nitrogen-lean product gas with a relatively high LHV can be produced. In comparison to competitive technologies such as fermentation, (steam) gasification processes achieve a complete fuel conversion and unlike fermentation

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Please cite this article in press as: D. Schweitzer, et al., Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities, Biomass and Bioenergy (2017), http://dx.doi.org/10.1016/j.biombioe.2017.02.002

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D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

processes, no organic residues (like fermentation residues) are produced that can cause further disposal problems. Furthermore in fermentation processes, high fractions of the nitrogen and sulphur content of the fuel are released as H2S and NH3 into the product gas stream [16]. There are several approaches for realising steam gasification processes [17e20]. This work focuses on the dual fluidised bed steam gasification process. In this process configuration the necessary heat for the endothermic steam gasification is provided by circulating bed material: The bed material (e.g. silica sand, limestone or olivine) and residual char from the gasification reactions is transported from the gasifier into the combustor, where it is heated by combustion of the residual char (and some additional fuel, if required). The heated bed material is then transported back into the gasifier. Thus, the hot material recirculated from the combustor into the gasifier provides the reaction enthalpy for the endothermic steam gasification. Its principle is shown in Fig. 1. In the past years several commercial scale demonstration plants were built in Austria (Güssing, Oberwart), Germany (Senden) and Sweden (Gothenburg) with a capacity between 2 and 20 MW [21e24]. All of these plants use woody biomass as fuel. Mass balances showed, that for woody biomass, the majority of the nitrogen, sulphur and chlorine is released in the gasifier as NH3, HCN, N2, H2S and HCl [25e27]. Due to the low N, S and Cl content of woody biomass, the produced product gas from these biomass shows low concentrations of impurities. Besides the dual fluidised bed steam gasification of woody biomass in demonstration scale, various researchers have investigated the gasification of different biogenic and fossil fuels in smaller scale tests [28e31]. From such tests, it is known that fuels with a high nitrogen, sulphur and chlorine content, generate not only significantly higher concentrations of impurities in the product gas stream of the gasifier, but also in the flue gas stream of the combustor [28]. In previous publications about the steam gasification of biogenic waste materials [30,32e34], mainly the influence of these fuels on the product gas yield and composition is presented, but there is still a lack of knowledge about the influence of these fuels on the concentration of impurities in the product gas stream of the gasifier (mainly tar, NH3, H2S, HCl). Detailed knowledge of the product gas composition and its contained impurities is necessary for designing appropriate gas cleaning facilities, particularly when a subsequent synthesis process is foreseen. For example when using the product gas for a Fischer-Tropsch synthesis, the concentration of impurities have to be not higher than several 10
9,m3,m
3 [35,36]. For the use in a gas engine less rigorous gas qualities are required with H2S concentrations in the range of 100e450 , 10
6 , m3 , m
3 and NH3 concentrations in the range of 50 , 10
6 , m3,m
3 [37]. This paper presents data on product gas composition and the contained impurities for different biogenic waste materials with a high nitrogen, sulphur and chlorine content. Based on these results,

Fig. 1. Principle of the steam gasification process.

the correlation between the fuel composition and the impurities concentration in the product gas is discussed. Furthermore, the effect of limestone addition on the concentrations of tar, H2S and NH3 is studied. 2. Methodology 2.1. Experimental facility and plant operation For the gasification experiments, a dual fluidised bed reactor system with a thermal fuel input of 20 kW was used. The plant consists of a bubbling and a circulating fluidised bed reactor. The schematic of the facility is shown in Fig. 2. The hydrodynamics of this system were previously described by Charitos [38,39]. The gasification reactor has an internal diameter of 150 mm in the fluidised bed zone, a diameter of 200 mm in the freeboard and a total height of 3.5 m. Entrained particles in the gas exiting the gasification reactor are removed by two cyclones and a candle filter. Solid fuels are volumetrically dosed into the reactor. The combustion takes place in a circulating fluidised bed combustion reactor with an internal diameter of 70 mm and a height of 12.4 m. The circulation rate between the reactors is controlled by the opening of a cone valve and the pressure difference between the reactors [40]. Both reactors, the loop seals and the transport legs are electrically heated. The residual char from the gasification and the circulating bed material exits the gasification reactor through an overflow. Via the lower loop seal the material is led into the CFB combustion reactor. In this reactor, the char is combusted and the combustion energy heats the bed material. Due to an electrical heating, the addition of additional fuel into the combustor is not necessary. The entrained bed material of the CFB combustion reactor is separated from the gas stream by a cyclone. Fines are removed by a secondary cyclone and a candle filter. In the upper loop seal of the combustion reactor, the solids flow is distributed to the gasifier and the combustor by the cone valve. In the facility nitrogen is used as a fluidisation agent

Fig. 2. Schematic of experimental facility in DFB configuration.

Please cite this article in press as: D. Schweitzer, et al., Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities, Biomass and Bioenergy (2017), http://dx.doi.org/10.1016/j.biombioe.2017.02.002

D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

for the loop seals and as purge gas for the dosing unit and the pressure transmitters. This leads to a dilution of the product gas by about 20e30% and flue gas by about 10%. Since most of the nitrogen is added above the fluidised bed, the effect of this dilution on the gasification reactions is assumed to be low. For calculating the gas yields and compositions, the purge nitrogen was subtracted from the experimentally measured gas streams. All relevant operating data such as temperatures, pressures, gas concentrations, fuel and steam mass flows were continuously recorded during the tests and mean values were determined for representative experimental periods. Pressures and temperatures were measured at several positions along the reactor height and its periphery. The gas flow is calculated by a N2-dilution by a digital MFM (tracer method). For measuring the concentrations of the gas components, gas samples were taken after the candle filter. The concentration yPG i of the product gas components CO, CO2, CH4, H2 and non-condensable hydrocarbons (C2 - C4) was measured, after humidity condensation, tar removal in isopropanol, gas drying and fine filtration, using the online gas analyser ABB AO 2020 and an Agilent Varian CP4900 Micro-GC (10 m PPU column). In selected experiments, additionally the concentration yGG of the gas comi ponents tar, H2S, NH3 and Cl was measured by discontinuously measurement methods (see section 2.2). Product yields Yi were defined as the ratio of the component flow and the added fuel (on a dry-ash-free basis). The product gas yield Y PG is defined as the total yield of the standard gas components (CO, CO2, CH4, H2, C2 - C4). A documentation of the composition and particle size distribution of all used fuels and bed materials can be found in Tables 1 and 2 and Fig. 4. In all experiments, the gasifier was operated with a steam mass flow of 2.7 kg,h
1 and a fuel mass flow of 3.6e5.4 kg,h
1, resulting in a fluidisation velocity of 0.2 m,s
1 and a molar steam-to-carbon (S/C) ratio of 1.5 mol,mol
1. This S/C ratio was selected since it allows an operation with good hydrodynamics as well as a good gasification performance (at lower S/C ratios, a higher tar formation and a lower fuel-conversion was measured). Due to fluctuations in the steam and fuel dosing rate, an inaccuracy of the steam-to-carbon ratio of about 0.2 mol,mol
1has to be accepted. The solid circulation rate FCR is defined as the mass ratio between the circulating bed material and the fuel entering the gasifier (on a dry-ash-free basis). For all experiments the process was operated with solid circulation rates between 22 and 31 kg,kg
1. This range is caused by a limited accuracy in controlling the circulating bed material mass flow and by fluctuations of the fuel dosing rate. The fluidisation gas velocity of the combustor was kept constant at a value of 4.5 m,s
1 for all experiments. During the tests, bed material samples were taken at the lower loop seal of the reactor system and subsequently the char was separated from the ash and bed material by sieving. Part of the bed inventory was removed periodically from the upper loop seal to maintain the total system inventory at a relatively constant level. All measured values reported in this manuscript, represent values that were measured after reaching stable conditions in

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terms of pressure and temperature profiles, circulation rate, gas composition and yield. For each experiment a stable operation of at least one hour was maintained. Reference experiments (fuel:  sewage sludge and wood pellets, gasifier temperature: 800 C, S/C
1 ratio: 1.5 mol,mol , bed material: silica sand), were repeated in all experimental campaigns over the whole duration of the research project (3 years). They showed a good repetitiveness: The product gas yield Y PG varied only by about ± 0.05 m3 kg
1 and the product 3
3 gas concentrations yPG i varied only by about ± 0.02 m m . 2.2. Description of measurement methods Measuring the concentration of impurities in gasification gases is challenging, because the existing standardised measurement methods were not designed for measuring wet, tar containing gases. Zeisler proposed measurement methods that allow to determine the concentration of NH3 and H2S in such gases [41]. Modified versions of these measurement methods were used within this work. Fig. 3 shows the measurement arrangement for H2S, NH3 and Cl. For each measurement of each compound, a separate set of impinger bottles is needed. The following list shortly describes the used measurement methods. (i) Tar measurements were carried out according to the tar protocol [42]. Within this protocol, tar is defined as a nonspecific term, for the totality of all organic compounds present in the product gas of the gasifier, with the exception of gaseous hydrocarbons (C1 to C6). During the extractive tar sampling, tars are dissolved in isopropanol and then analysed gravimetrically or by GC-MS. Detailed information about the analysis methods can be found elsewhere [43]. (ii) NH3 measurements were carried out in close correlation to the VDI 3838 guideline [44]: the gaseous ammonia is absorbed in a 1 M H2SO4 absorption solution. The ammonia concentration in the absorption solution is subsequently analysed by photometry according to DIN 38406-5 [45]. In the used measurement arrangement a series of 4 impinger absorption bottles is needed. The first impinger bottle is filled with a mixture of isopropanol and NaOH solution for tar and moisture removal. To avoid NH3 absorption in the first impinger bottle, the pH value is increased by NaOH addition. Bottles two and three are filled with sulphuric acid for NH3 capture. The last bottle is left empty to collect discharged solution. (iii) The H2S measurements were carried out in close correlation to the DIN 51855-4 guideline [46]: in this method the gaseous hydrogen sulphide reacts with zinc acetate forming a solid zinc sulphide precipitate which is subsequently quantified by iodometric titration. In this measurement method a series of 4 impinger absorption bottles is needed. The first impinger bottle is filled with a mixture of isopropanol and sulphuric acid for tar and moisture removal. To

Table 1 Bulk density, composition and heating value of used fuels. Bulk density kg,m
3

wood pellets sewage sludge cattle manure pig manure

650 980 203 300

Ultimate analysis

Proximate analysis

Water (a.r. basis) kg, kg
1

Ash (dry basis) kg, kg
1

Volatiles (daf basis) kg, kg
1

C

0.079 0.078 0.081 0.121

0.009 0.475 0.202 0.199

0.805 0.478 0.630 0.628

0.507 0.488 0.512 0.489

H

O

N

S

Cl

<0.003 0.023 0.011 0.011

<0.003 0.002 0.008 0.005

LHV (a.r. basis) MJ,kg
1

(daf basis) kg, kg
1 0.067 0.074 0.060 0.063

0.424 0.342 0.384 0.401

<0.003 0.071 0.026 0.031

17.0 9.2 14.3 12.9

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D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

Table 2 Composition and particle size of used bed materials. Al2O3

CaO

Fe2O3

K2O

MgO

SiO2

TiO2

dp,10

dp,50

g, kg
1 silica sand calcined limestone kaolin

13 9 424

1 918 1

4 3 5

5 1 13

e 9 1

dp,90 10
6,m

979 60 550

e e 3

210 165 140

350 370 600

900 640 1600

are filled with a zinc acetate solution for H2S capture. The last bottle is left empty to collect discharged solution. (iv) The Cl measurements were carried out in close correlation to the DIN EN 1911 [47]. Demineralised water was used as capture solvent. In this measurement method a series of 3 impinger bottles is needed. The first two impinger bottles are filled with demineralised water. The last bottle is empty to collect discharged solution. Due to the good solubility of Cl containing compounds, the use of an additional tar and moisture removal bottle is not possible. Therefore the majority of tars and other organic components were removed from the sample solution after sampling and before analysis using a Metrohm IC-RP C18 filter for organics. The chlorine concentration in the absorption solution is analysed by Coulometry. In Coulometry, Cl is not detected directly, but the total dissolved Cl content in the solvent is measured. Therefore besides chlorine, also other electrochemical reactive compound like bromine, iodine, oxalate, thiosulphate or thiocyanate compounds are detected. Consequently the Cl measurement results overestimate the actual Cl concentrations in the sampled gas slightly. Fig. 3. Measurement arrangement.

To test the accuracy of these methods, test measurements using pure and tar containing gases were conducted. In these experiments, gases with known impurity concentrations (1040 , 10
6,m3,m
3 for H2S; 5110 , 10
6,m3,m
3 for NH3; 502 , 10
6,m3,m
3 for HCl) were used. The results of the H2S test measurements showed only a minor variation of ± 5% in respect to the input gas. The NH3 measurements showed a wider variation of about ± 20% in respect to the input gas. For all HCl measurements, the results were about 20% below the test gas. More test measurements are currently ongoing to further validate these methods and to improve their accuracies. The measurement of HCl in product gases is not possible, since HCl reacts with NH3 forming ammonia chloride when the tem perature drops below 300 C (equation (1)). This ammonia chloride is dissolved together with other Cl containing compounds in the impinger bottles where the totality of all Cl compounds is analysed. This has an effect on the NH3 measurement too, but since the Cl content in the product gas is rather low in comparison to the NH3 content (see Fig. 10), the effect on the NH3 content is rather low. Furthermore a fraction of this ammonia chloride can be captured by the sample gas filters and consequently a lower Cl content is found in the sample gas.

HClðsÞ þ NH3ðgÞ

Fig. 4. PSD of the used materials.

avoid H2S absorption in the first impinger bottle, the pH value is lowered using sulphuric acid. Bottles two and three

T < 300  C

ƒƒƒ! NH4 ClðsÞ

(1)

2.3. Fuels and bed materials Within this work wood pellets (PELLOX® pellets provided by €rme), sewage sludge, pig manure and cattle manure were ScharrWa used as fuel. The fermented sewage sludge was provided from several small wastewater treatment plants close to Marbach,

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D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

Germany. The raw sludge was dried by the off-heat of a biogas plant in Grobbottwar, Germany. The dried sewage sludge in its original form, appeared as dense particles with a particle size of several centimetre and a high bulk density. It was crushed into the desired particle size using a beater mill. The raw cattle and pig manure were dried in a rotary dryer and appeared as fibrous fuels with a low bulk density. The composition (oxygen content obtained by difference) of the fuels used within this work is shown in Table 1 (analyses according to [48e54]). Within this work, silica sand, limestone and kaolin (an aluminasilicate) were selected as bed material or additive. Limestone was selected for its good tar reforming efficiency [28,30,55] and its positive effect on reducing the concentration of NH3 and HCN in the product gas of the gasifier [56] and the SO2 concentration in the flue gas of the combustor [57]. When using pure limestone as bed material the high attrition rate [58] of limestone causes a high Ca content in the fly-ash. That complicates a recovery of phosphorus and additionally increases the costs for fresh bed material and for disposal of the fly-ash. As an alternative, limestone can be used as an additive. This reduces the total limestone demand and therefore its purchase and disposal costs [59]. Kaolin (calcined kaolin AS45 provided by Amberger Kaolinwerke) was selected for its positive effect in suppressing ash melting (especially for sodium-containing fuels) [60], while silica sand serves as a reference bed material. The composition of the bed materials is shown in Table 2 (analysis according to [61]). The particle size analysis of the bed materials was done by a laser diffraction using a Malvern Mastersizer 3000. The particle size distribution of the sewage sludge was analysed by sieving following DIN 66165 and DIN ISO 9276-1 [62,63]. The cumulative particle sizes distribution [64] of the materials in shown in Fig. 4. 3. Results and discussion 3.1. Product gas yield and composition To study the usability of the proposed fuels for DFB steam gasification processes, the different fuels were gasified under similar conditions. At reference conditions, (gasification tempera ture: 800 C, S/C ratio: 1.5 mol,mol
1, bed material: silica sand) a good gasification behaviour for all fuels was observed. High gas yields were achieved and no bed agglomerations were detected. PG for the Fig. 5 shows the product gas compositions yPG i and yields Y different fuels: the product gas composition does not vary significantly between the different fuels at reference conditions. Only for sewage sludge the dry product gas composition shows slightly higher H2 concentrations (0.46 vs. 0.42 m3 m
3) and slightly lower CO concentration (0.12 vs. 0.20 m3 m
3) compared to the product gas of similar experiments performed with wood pellets as fuel. The variations in the product gas compositions can be caused by the different fuel composition and the different fuel conversion behaviour. This trend corresponds to investigations of Pfeifer [30]. The gas yield of wood pellets (on a dry-ash-free basis) is slightly higher than the gas yield of sewage sludge and manure. This higher gas yield can be explained by the higher carbon conversion of wood pellets. Elemental balances of the gasifier show, that for sewage sludge, about 51 mol.% of the carbon contained in the fuel was is found in the product gas, whereas for wood pellets, 63 mol.% of the carbon contained in the fuel was found in the product gas. The rest of the carbon was found in tars and the flue gas of the combustor. Another aspect that can influence fuel conversion and therefore product gas yield is the fuel preparation: Especially, the cattle and pig manure used within this work appear as fibrous material with a low bulk density. Caused by this low bulk density and the fibrous consistence, the hydrodynamic behaviour of those fuels in the

5

Fig. 5. Product gas yield and composition for the different fuels (gasifier temperature:  800 C, S/C ratio:1.5 mol,mol
1, bed material: silica sand).

fluidised bed is different to wood and higher amounts were discharged out of the gasifier, resulting in a lower fuel conversion and therefore a lower gas yield. Energy balances have shown, that when using sewage sludge as fuel instead of wood pellets, no additional fuel is necessary in the combustor. Caused by the lower fuel conversion in the gasifier, a higher amount of char is transported into the combustor. There the combustion of char releases enough energy for heating the circulating bed material. 3.2. Influence of temperature on gasification process To study the effect of temperature on gas composition and gas yield, different gasification temperatures were studied, while all other operational parameters were kept constant. Figs. 6 and 7 show the influence of temperature on product gas yield and product gas composition. The product gas yield of sewage sludge rises with increasing  temperature from 0.4 m3 kg
1 at a gasifier temperature of 710 C to  3
1 0.85 m kg at a gasifier temperature of 800 C (Fig. 6). Correspondingly, the cold gas efficiency rises from 45 to 60%. Especially at temperatures of about 800 +C the gas yield for sewage sludge was only slightly lower than the gas yield for wood pellets at similar conditions. With decreasing temperatures, the relative difference in gas yields between the two fuels increased. In respect to the gas composition during steam gasification of sewage sludge, it was observed, that with increasing temperature, especially the H2 concentration increased and the concentration of methane and non-condensable hydrocarbons (C2 - C4) decreased (Fig. 7). This is in agreement with the composition of char samples.  At a gasification temperature of 750 C, char samples show a carbon content of 0.143 kg,kg
1, whereas at a gasification temperature of  800 C, the carbon content decreases to 0.126 kg,kg
1.

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D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

Fig. 6. Influence of gasifier temperature on gas yield for sewage sludge and wood pellets (S/C ratio: 1.5 mol,mol
1, bed material: silica sand).

Fig. 8. Influence of gasifier temperature on component gas yield for sewage sludge (S/ C ratio: 1.5 mol,mol
1, bed material: silica sand).

different gas components Yi (Fig. 8), especially the production of H2 and CO2 increases with increasing temperature. In contrast the production of methane and hydrocarbons remained relatively constant with increasing temperature. The trend corresponds to the gasification fundamentals: CH4 and hydrocarbons are formed during the initial pyrolysis when the fuel enters the gasifier. This is relatively independent from the gasification temperature. Only smaller amount of these gases are created by cracking reactions of hydrocarbons [29] and only smaller amounts are reduced by reforming reactions (due to their low reactivity at a temperature  range of 700e850 C) [66]. The increasing CO, H2 and CO2 yields with increasing temperature are mainly caused by an increasing fuel conversion and an increasing reforming of pyrolysis products.

3.3. Impurities in gasifier product gases

Fig. 7. Influence of gasifier temperature on product gas composition for sewage sludge (S/C ratio: 1.5 mol,mol
1, bed material: silica sand).

Investigations by other authors showed a higher CO and lower H2 and CO2 concentration in the product gas at similar conditions [30,65]. Possibly, in the experiments presented here, less CO2 and H2 is shifted into CO (equation (2)).

CO þ H2 O%CO2 þ H2

DH0273 K ¼
40

kJ mol

(2)

When looking at the influence of temperature on the yields of

In addition to the product gas yields and compositions, knowledge about the concentration of tars and other impurities is important for designing the product gas cleaning system. Especially when looking at the product gas utilisation pathways, often strict concentration limits have to be fulfilled (see chapter 1.1). Figs. 9 and 10 present the concentration of the most important impurities (tar, NH3, H2S, Cl) in the product gas of the gasifier for the different fuels.  Especially for sewage sludge gasification at 800 C and silica sand as bed material, a high gravimetric tar yield Ygrav: tar of about 80 g,kg
1 was detected. At lower gasification temperatures, even higher values were measured. However, such high gravimetric tar yields for sewage sludge cannot directly be compared to the tar yield of similar experiments performed with wood pellets as fuel. When looking at the composition of the sampled tar, there are considerable differences: The gravimetric tar of wood pellets consists mainly of carbon (0.86 kg,kg
1) and only smaller amounts of hydrogen (0.078 kg,kg
1) and nitrogen (0.016 kg,kg
1), whereas the gravimetric tar of sewage sludge consists of considerably lower concentrations of carbon (0.57 kg,kg
1) and much higher

Please cite this article in press as: D. Schweitzer, et al., Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities, Biomass and Bioenergy (2017), http://dx.doi.org/10.1016/j.biombioe.2017.02.002

D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

Fig. 9. Tar yield for the different fuels (temperature: 800 C, S/C ratio: 1.5 mol, mol
1, bed material: silica sand). 

7

concentrations of hydrogen (0.114 kg,kg
1), nitrogen (0.122 kg,kg
1) and sulphur (0.026 kg,kg
1). The high gravimetric tar yield for sewage sludge can be explained by studies of Yu and Amir [67,68]. They analysed the fuel structure of sewage sludge in detail and detected that sewage sludge contains heavy aromatic compounds. These aromatic compounds are volatilised during the gasification and due to their low residence time in the fluidised bed, only a small fraction is cracked into gases or lighter tars [69]. Caused by the high molar weight of these aromatic organic compounds, they are detected as gravimetric tar, while they cannot be detected by gas chromatography. Such heavy tars can still contain nitrogen, sulphur and chlorine containing organic compounds, such as methyl or indole species [67] as well as some inorganic compounds such as ammonium chloride. From Fig. 9 another unexpected trend can be observed: the tar yield for sewage sludge YGC
MS tar measured by GC-MS is slightly lower compared to the tar yield of similar experiments performed with wood pellets as fuel. In general, high gravimetric tar concentrations results in high GC-MS tar concentrations [70]. Again the fuel structure may be an explanation for this unusual trend. The structure of wood pellets [29] favours the formation of lighter tars (which are detectable by gas chromatography) [43], while the different fuel structure of sewage sludge favours the production of more heavy (and therefore not GC-MS detectable) tars, which are only to a small extent cracked into lighter compounds. Hence, for different fuels, variations in the ratios between the ligher (GC-MS detectable tars) and more heavy (gravimetric tars) tars are possible. For future experimental investigations, a detailed



Fig. 10. NH3, H2S and Cl concentrations for sewage sludge, cattle and pig manure (temperature: 800 C, S/C ratio: 1.5 mol,mol
1, bed material: silica sand).

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D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

analysis of the composition (C, H, O, N, S, Cl, ash) of the tars may be interesting to allow more detailed conclusions about the origin of the different tar species. For cattle and pig manure, only low gravimetric tar concentrations of about 35 g,kg
1 were measured. Currently, there is no clear explanation for this unexpected trend. More research is necessary, with a special focus on studying the influence of fuel structure and composition on tar formation. In Fig. 10 it can be seen, that besides the tar yield also the concentration of other impurities varies significantly for the various fuels. Especially the NH3 content in the product gas can reach very high concentrations, ranging from 0.02 to 0.06 m3 m
3, corresponding to a conversion rate of fuel nitrogen to NH3 of 35e50%. Figs. 11e13 show the yields of H2S, NH3 and Cl in relation to the sulphur, nitrogen and chlorine content of each fuel. The high NH3 concentrations in the product gas can be explained by the high nitrogen content in the fuels (see Table 1). In accordance to previous publications of Kitzler and Wilk [28,71], there is a clear correlation between the nitrogen content in the fuel and the NH3 content in the product gas. Mass balances showed, that about 95% of the nitrogen contained in the fuel is released in the gasifier. This correspond with previous studies by Tian [26]. In contrast to NH3, there is a trend but no clear relation between the concentration H2S and Cl in the product gas and the content of S and Cl in the fuel. One explanation for this can be the more complex distribution of sulphur and chlorine. In the fuel, sulphur and chlorine are not only present in organic compounds, but also in inorganic components like minerals and salts [25,72]. In the gasifier, mainly the organic sulphur and chlorine can be volatilised into gaseous compounds and the rest remains as solid compounds in the gasification char. Additionally, a fraction of the gaseous sulphur and chlorine compounds can be captured by minerals such as CaO (equation (3)). Furthermore when the gas temperature  drops to temperatures below 300 C, HCl reacts with ammonia into ammonia chloride (equation (1)), which reduces the content of these species in the product gas.



Fig. 12. H2S yield dependent on S content of the fuel (temperature: 800 C, S/C ratio: 1.5 mol, mol
1, bed material: silica sand).



Fig. 13. Cl yield dependent on Cl content of the fuel (temperature: 800 C, S/C ratio: 1.5 mol, mol
1, bed material: silica sand).

3.4. Effect of different bed materials on gasification process



Fig. 11. NH3 yield dependent on N content of the fuel (temperature: 800 C, S/C ratio: 1.5 mol, mol
1, bed material: silica sand).

To enable an efficient and economic downstream product gas utilisation, in-situ minimisation of the concentration of impurities in the product gas is desirable. Within this work the effect of bed materials and additives on the impurity concentration is studied. Fig. 14 gives an overview of the gravimetric tar, H2S and NH3 concentrations obtained with different bed materials and additives. As expected, different bed materials can have a substantial effect on the concentration of impurities. The use of kaolin as bed material caused a reduction of the gravimetric tar concentration by about 30%. Even better results were achieved with limestone. In these

Please cite this article in press as: D. Schweitzer, et al., Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities, Biomass and Bioenergy (2017), http://dx.doi.org/10.1016/j.biombioe.2017.02.002

D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

9

Fig. 14. Gravimetric tar and H2S concentration for different bed materials for gasification of sewage sludge and wood pellets (temperature: 800 C, S/C ratio: of 1.5 mol,mol
1). 

experiments, silica sand was used as bed material and limestone was used as additive. Even low CaO contents in the inventory of only 0.1e0.2 kg,kg
1 were sufficient to reduce the gravimetric tar concentrations significantly by about 70e90%. In addition to the tar reduction, limestone has a positive effect on reducing the H2S and NH3 concentrations in the product gas. The H2S concentration reduction corresponds to results of various other authors [73e75]. In the presence of CaO, H2S is converted into calcium sulphide [76], which is then further oxidised in the combustor into calcium sulphate. But due to the chemical equilibrium and the low kinetics, this H2S removal cannot reach as high desulfurisation efficiencies, as the SO2 removal in combustion processes [57,77].

3.5. Outlook More detailed discussion about the advantages and disadvantages of this sewage sludge and manure utilisation pathway, the economics and especially a comparison with other utilisation pathways (e.g. fermentation) should be part of future research work. Future research studies should also focus on the fate of sulphur, nitrogen and chlorine to improve the understanding of the formation pathways of relevant impurities in steam gasification processes and to identify possibilities for reducing them. The same is true for the tar formation. Another important aspect for future studies is the recovery of nutrients from gasification ashes. 4. Conclusion

CaOðsÞ þH2 SðgÞ #CaSðsÞ þH2 OðgÞ

(3)

CaSðsÞ þ2O2ðgÞ /CaSO4ðsÞ

(4)

Furthermore CaO as catalytic material, reduces NH3 into N2 and H2. A CaO content in the bed material of about 0.2 kg,kg
1 reduced the NH3 concentrations in the product gas by about 50%. Hence the addition of limestone is a cheap and efficient way for reducing the concentration of impurities in the product gas. For kaolin the H2S and NH3 concentrations were not measured.

Within this article the use of waste biomass such as sewage sludge and manure was demonstrated as fuel for the dual fluidised bed steam gasification process in lab-scale. These fuels with their high and constant availability and their low costs, can decrease the operating costs considerably. Furthermore the use of these fuels enables new applications, such as disposal applications. Investigations at a dual fluidised bed reactor system with a thermal fuel input of 20 kW have shown that the use of these fuels is possible: no bed agglomerations were detected and the product gas yield was close to the product gas yield of wood pellets.

Please cite this article in press as: D. Schweitzer, et al., Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities, Biomass and Bioenergy (2017), http://dx.doi.org/10.1016/j.biombioe.2017.02.002

10

D. Schweitzer et al. / Biomass and Bioenergy xxx (2017) 1e12

Challenging can be the higher concentration of impurities (tar, NH3, H2S and Cl) in the gas stream of the gasifier. The experiments have shown, that for all tested bed materials and additives (silica sand, kaolin, limestone), no bed agglomera tions up to a gasification temperature of 820 C and up to a com bustion temperature of 950 C were detected. DFB gasification experiments showed, that the product gas yield of the different fuels was 10e20% lower than the product gas yield of similar experiments performed with wood pellets. The product gas composition of the main components (H2, CO, CH4, C2 - C4, CO2) did not vary much between the different fuels. This was different in respect to impurities in the product gas. In case of sewage sludge, high gravimetric tar concentrations of up to 100 g,m
3 were measured. The high nitrogen, sulphur and chlorine content in the fuels causes high NH3, H2S and Cl concentration in the product gas of the gasifier. NH3 concentrations of up to 0.06 m3 m
3, H2S concentrations of up to 7000 , 10
6,m3,m
3 and high Cl concentrations of up to 1300 , 10
6,m3,m
3 were measured. This highlights the need for an additional product gas cleaning equipment in order to avoid problems in the downstream equipment. In case of NH3, a good correlation between the NH3 concentration in the product gas and the nitrogen content in the fuel was observed. In case of H2S and Cl, such a dependence between the fuel composition and the concentration in the product gas was less clear. Furthermore the experiments have shown that the addition of limestone can reduce the concentrations of impurities such as tar, NH3 and H2S considerably.

References

5. Acknowledgement

[10]

This work was carried out within the R&D program Demonstration of efficient Biomass Use for Generation of Green Energy and Recovery of Nutrients (DeBugger). The authors gratefully acknowledges the financial support from the EIT and the KIC InnoEnergy funding organisations and the support of the project partner Outotec GmbH.

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6. Abbreviations [15]

[16] symbol a.r. CFB CV daf DFB e.g. et al. GC-MS GG

unit

LHV m_ MFM PG

MJ, kg
1 kg, h
1

PSD S/C V_

3

m ,h


1

[17]

[18]

[19]

[20]

dry gas volume fraction of component i

yGG i yPG i Yi

description as received circulating fluidised bed cone valve dry-ash-free dual fluidised bed exempli gratia (for example) et alia (and others) gas chromatography-mass spectrometry gasifier gas (contains components CO, CO2, CH4, H2, C2 - C4, tar, H2S, NH3, Cl) lower heating value mass flow mass flow meter product gas of gasifier (contains components CO, CO2, CH4, H2, C2 - C4) particle size distribution molar steam-to-carbon ratio volume flow at stp dry product gas volume fraction of component i

Y PG

m3,kg
1 m3,kg
1

yield of component i product gas yield

FCR

kg , kg
1

solid circulation rate

[21]

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Please cite this article in press as: D. Schweitzer, et al., Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities, Biomass and Bioenergy (2017), http://dx.doi.org/10.1016/j.biombioe.2017.02.002