Treatment of Volatile Compounds from Municipal Solid Waste

Nov 7, 2017 - The cooking oil scrubber collected a major portion of tar, with 144 mg/m3N remaining in the syngas after scrubbing, and the dew point of...
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Treatment of Volatile Compounds from Municipal Solid Waste Pyrolysis to Obtain High Quality Syngas: Effect of Various Scrubbing Devices Ming Chen,† Dezhen Chen,*,† Umberto Arena,‡ Yuheng Feng,† and Hangqin Yu† †

Thermal and Environmental Engineering Institute, School of Mechanical Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China ‡ Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, University of Campania “Luigi Vanvitelli”, Via A. Vivaldi, 43, 81100 Caserta, Italy S Supporting Information *

ABSTRACT: Volatile compounds produced from pyrolysis of municipal solid waste (MSW) are treated in a scrubbing system, which includes a hot char filter, a condenser cooled with ice water, a cooking oil scrubber, and a Na2CO3 solution scrubber, aimed at obtaining high quality syngas. The performances of each treatment device are evaluated at a laboratory scale. The results indicate that hot char filtration is able to capture 71% of particulates and 32% of the tar, and, thanks to its catalytic activity, syngas yield increased significantly from 0.21 to 0.39 m3N/kgMSW, and the molar percentage of combustible fractions in the syngas increased by 35%. The condenser can capture 90%, 85%, 87%, and 90% of evaporated Na, K, Ca, and Mg, respectively; 50% of particulates and 98% of the tar in the syngas. The cooking oil scrubber collected a major portion of tar, with 144 mg/m3N remaining in the syngas after scrubbing, and the dew point of the tar decreased below 50 °C. The scrubber filled with Na2CO3 solution of 5 wt% running at 70 °C and liquid/gas (L/G) ratio of 6 reduced the concentrations of tar and H2S in syngas to 38.5 mg/m3N and 16 ppm, respectively. Consequently, the tar dew point decreased to a level of about 25−30 °C, then acceptable for downstream applications, such as feeding of a gas engine. The results provide detailed information on the cleaning effects of various syngas cleaning units, their reciprocal influence and contributions to the total removal efficiency, which helps in choosing suitable options for volatile treatment to obtain high quality syngas from MSW pyrolysis. m3N, is of much higher quality than the syngas from gasification processes, and a promising product for feeding gas engines, gas turbines, or just a fuel gas. Syngas from both gasification and pyrolysis processes is usually contaminated with particulates, tars (defined as the organic compounds with molecular weight higher than benzene8), gaseous pollutants such as H2S, HCl, NH3, and alkali, and alkaline earth metals (AAEMs). Common issues with particulate matter and tars are fouling, corrosion, and plugging problems in pipes, heat exchangers, and engines due to condensation and deposition,9 which reduce energy conversion efficiency and cause maintenance and safety concerns if not well addressed. Nitrogen and sulfur in MSW are mainly converted into ammonia (NH3), hydrogen cyanide (HCN), oxides of nitrogen (NO, NO2, N2O, and other NOx), N2, hydrogen sulfide (H2S) and SO2.7 Halogens, such as chlorine, are released as hydrogen halides, mainly HCl.10 AAEMs have the tendency to vaporize in the pyrolysis reactor and condense downstream of power generation systems.11 Consequently, all of these contaminants in the syngas must be cleaned to meet special requirements in practice. Table 1 shows the quality requirements for syngas in some typical applications. Based on the experiences of using syngas from biomass gasification, some gas cleaning technologies have been

1. INTRODUCTION The thermochemical waste-to-energy (WtE) technologies are crucial for sustainable waste management, and the innovative WtE technologies are being constantly pursued by scientists.1−3 Thermochemical processes, which are known as incineration, pyrolysis, and gasification, convert the organic components of municipal solid waste (MSW) into different energy carriers and reduce their negative impacts on the environment. Compared to conventional incineration technologies, pyrolysis and gasification have higher potentials for reducing the emissions of acidic gases (SOx, HCl, HF, NOx, etc.), volatile organic compounds (VOCs), PCDD/Fs, and leachable toxic heavy metals.4 Public concerns about the pollutant emissions that could be related to MSW management support an increasing demand on gasification and pyrolysis technologies, especially in developing countries where the MSW management facilities are not fully developed yet. Some pyrolysis and gasification technologies have been developed at commercial scale in recent years. In particular, pyrolysis is the process of thermal degradation of organic material in an oxygen-free atmosphere, which produces pyrolytic liquid (a mixture of organic chemicals and water), syngas, and charcoal.5 However, the products from MSW pyrolysis should be improved to be of any practical use.5,6 In a recent MSW pyrolysis process involving char reforming of volatiles, energy can be concentrated in the syngas with a little oil of improved quality left.7 The syngas, which usually has a higher heating value (HHV) larger than 15 MJ/ © XXXX American Chemical Society

Received: August 15, 2017 Revised: October 13, 2017 Published: November 7, 2017 A

DOI: 10.1021/acs.energyfuels.7b02388 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

carbonaceous catalyst for tar cracking, whose performance is related to its structure (pore size distribution, specific surface area, etc.) and the content of inherent metallic minerals.16,25,26 At the same time, a hot char bed can also act as a filter able to intercept particulate matter and part of AAEMs, even though the effects of the hot char layer on removing particulate matter and AAEMs have been rarely reported. The potential utilization of a condenser is also different for gasification and pyrolysis processes. In the first case, condensers are adopted in few cases,2 mainly for the interest of saving the syngas sensible heat. On the contrary, in the MSW pyrolysis, the liquid yield is comparable to that of gas, and the condensation step is suitable and necessary to an efficient separation of the liquid products from the gas products. Based on these observations, a condenser cooled with ice water has been used to separate liquid products: it is also able to intercept part of the particulate matter at the same time. In this research, the volatile products from a MSW pyrolysis process have been treated with four steps of scrubbing devices: a hot char filter, a condenser cooled with ice water, a cooking oil scrubber, and a sodium carbonate solution scrubber. The performance of each unit and the overall effects of the system for removing particles, tars, and gaseous pollutants such as H2S, NH3, HCl, and so forth has been also quantified in order to provide a systematic solution to clean all of the pollutants in MSW pyrolysis volatile products.

Table 1. Typical Syngas Applications and Related Quality Requirements2,12,13 upper limitations for different applications

a

contaminants

manufactured gas

IC engine

gas turbine

Particulate Tars H2S NH3 AAEMs

10 mg/m3Na

50 mg/m3N 100 mg/m3N 20 ppm, vbb n/ac 0.1 ppm, mbd

5 mg/m3N 10 mg/m3N 20 ppm, vb 50 ppm, vb 0.1 ppm, mb

20 mg/m3N 50 mg/m3N Not required

m3N - normalized cubic meter. bvb - volume basis. cn/a-not available. mb-mass basis.

d

developed (Table S1). The particulate matter in the syngas is generally removed by means of a two-step system, with cyclone and fabric/ceramic filters; tars can be thermally and/or catalytically cracked, with or without steam supply,14−16 or scrubbed with water or oils.17−19 An oil-based gas washing process, OLGA, has been developed to remove and reuse valuable tar components by means of oily solvents:20 it has been found that cooking oil is the better absorbent for tar removal compared to biodiesel oil, diesel, and other absorbents.19,21 Oil scrubber, especially diesel oil, is also effective for removing NH3.19 Sulfur compounds and HCl can be efficiently removed by alkali solution or metal oxides absorption. A scrubber with Na2CO3 solution as cleaning agent is widely adopted to intercept H 2 S or H 2 S and CO 2 simultaneously.22−24 Finally, condensation, absorbents, and ceramic filter are helpful for AAEMs removal.24 However, most of these research studies are focused on cleaning of syngas from biomass gasification. Only a limited number is instead dedicated to the cleaning of syngas from MSW pyrolysis, which is obtained at moderate temperature, and contains moisture and liquid products. The liquid product is comparable to the syngas in mass yield and is not so refractory as the gasification tar: it also contributes to an important fraction to the recovered energy contained in the volatiles. It could be affirmed that the technology for washing syngas from MSW pyrolysis are different from those so far adopted to clean syngas from biomass gasification, mainly for the necessity to treat volatile products. Most of the research studies focused on the removal of just one or two pollutants, but few reports addressed the treatment of all the pollutants of interest, which is demanded for practical syngas applications. In addition, quantifying the specific removal efficiency and the possible reciprocal influence between the different cleaning processes and devices is important for the holistic solution; for instance, char produced from MSW pyrolysis can be a low-cost

2. MATERIALS AND METHODS 2.1. Materials. The MSW samples were collected from a MSW landfill site in Shanghai (China) and dried under the sun for a period of 7 days. After drying, and manual separation of its inorganic fractions including glass, metals, and dust, the samples were shredded into particles of about 10−20 mm in length, and mixed well before carrying out the experiments. The components and elemental analysis results of the MSW sample are presented in Table 2. 2.2. Experimental Methods. Experimental Apparatus. Figure 1 shows a simplified sketch of the experimental apparatus, which mainly consists of a pyrolysis reactor, a nitrogen supply system, and a volatile treatment and cleaning system. The pyrolysis reactor is a cylindershaped, electrically heated fixed-bed reactor, made of stainless steel, having an internal diameter of 100 mm and a length of 150 mm. Nitrogen gas (with purity higher than 99.99%) was introduced into the reactor as the carrier gas to balance the pressure loss with a flow rate of 50 mL/min.. The treatment of volatile compounds includes a hot char filter to remove particulates in the volatiles; two Allihn condensers with ice water as condensing medium, to separate liquid products (here simply denoted as tars) from the syngas; an oil scrubber for removing the tar and particulates; and a sodium carbonate (Na2CO3) solution scrubber to remove H2S and NH3. The hot char from MSW pyrolysis, as

Table 2. Components and Elemental Analysis of MSW Sample components /(wt %) kitchen waste

cloth and fiber

paper

15.38 ± 0.80 5.79 ± 0.16 ultimate analysis/(ad, wt %) C H N O S

43.17 6.03 2.52 48.06 0.22

± ± ± ± ±

1.78 0.47 0.17 0.68c 0.01

27.24 ± 0.69

plastics

wood

21.85 ± 9.57 3.24 ± 0.40 proximate analysis/(ad, wt %)

Moisture Ash Volatile FCc

6.52 26.02 60.65 6.81

± ± ± ±

0.28 0.88 1.11 0.04

residuea

HHVb /(ad, MJ kg−1)

26.47 ± 0.98 17.44 ± 0.18 AAEM contents/(ad, wt %) Na K Ca Mg

0.023 0.053 0.293 0.018

± ± ± ±

0.006 0.008 0.013 0.002

Residue: Components of MSW that are left over after sorting out the previous components. b″ad″ means ″air-dry basis″. cThe contents of fixed carbon (FC) and oxygen were determined by difference. a

B

DOI: 10.1021/acs.energyfuels.7b02388 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of experimental apparatus: 1, Nitrogen cylinder; 2, flow meter; 3, pyrolysis reactor; 4, MSW samples; 5, hot char filter; 6, thermocouples; 7, hot char from pyrolysis; 8, temperature controller; 9, condenser; 10, flask; 11, oil scrubber; 12, Na2CO3 solution scrubber; 13, mist filter; 14, gas analyser; a, b, c, d, e: measurement positions.

Table 3. Values of the Operating Parameters of the Sodium Carbonate Solution Scrubber, in the Different Experimental Scenarios Scenario

1

2

3

4

5

6

Scrubbing solution L/G ratio Temperature

Water 3 25 °C

0.1% Na2CO3 3 25 °C

1% Na2CO3 3 25 °C

5% Na2CO3 3 25 °C

5% Na2CO3 6 25 °C

5% Na2CO3 6 70 °C

reported by Wang et al,7 was adopted to form a filter of 74 mm in height and 40 mm in diameter in a stainless steel tube, operated at 550 °C, which corresponds to an average superficial velocity of 280 h−1. The Allihn condensers have a heat exchanging surface of 680 cm2 in total. The oil scrubber uses 100 mL of cooking oil with a viscosity of 120 cP (Kerry Oils & Grains Co., Ltd., China) as a solvent for trapping liquid product in the volatiles, the amount of oil adopted is comparable to that of a previous research.21 For the Na2CO3 solution scrubbing step, pH and L/G ratio are the most important factors affecting the efficiency;22 therefore, different concentrations (affecting pH) and amounts of Na2CO3 solutions are utilized to find the proper Na2CO3 concentration and L/G ratio to optimize the scrubbing effect. Table 3 provides the operating parameters of the experimental tests, which also include the temperature of Na2CO3 solution. Both the scrubbers use glass beads to form a packing layer, which improves the contact between syngas and liquids. After the scrubbers, a glass fiber filter captures mist with particles larger than 0.5 μm, with high efficiency27 in order to collect residual tar before check the tar scrubbing efficiency. The experiments were carried out in batches. Each run was performed with the same quantity of MSW sample and char, 100 and 42 g, respectively. Then, the temperature of hot char filter was increased by an electrical heater at a heating rate of 10 K min−1 up to the preset temperature of 550 °C. The pyrolysis temperature was set to 550 °C too, which is a commonly used temperature for pyrolysis proceses; the hot char filter remained at the same temperature as the

pyrolysis reactor therefore avoided its reheating and saved the energy consumption in practice. As the temperature of the hot char filter reached 550 °C, the pyrolysis reactor was heated with the same heating rate to 550 °C and then kept at this temperature for 30 min. Simultaneously, the volatile products were forced to pass through the hot char filter, driven by nitrogen gas, and then went to the ice water condensers and scrubbers. Analysis and Measurements. Ultimate analysis of the MSW samples was performed using a Vario EL III type elemental analyzer (Elementar Analysensysteme GmbH, Germany). The proximate analysis was carried out based on the Chinese national standards GB/T476−2008.28 The pH of the Na2CO3 solution was measured by means of a PHS-25 type portable pH meter (Shanghai REX Instrument Factory, China). An AGT2660 type glass-fiber filter (Munktell Filter AB, Sweden), with its mass measured beforehand, was installed behind the condenser to capture and measure the slip of the tar and the particulates from the condenser. After the experiment, the glass-fiber filter was put into a vacuum dryer at ambient temperature for more than 48 h to remove moisture and determine the intercepted tar and particulates, and then it was cut into small pieces and washed with dichloromethane in a flask to let the tar dissolve. Then, the dichloromethane solution was filtered through carrier-slow quantitative filter paper (with mass already known) to separate the particulates from the tar. After filtration, the residue on the filter paper was washed with clean dichloromethane three times to remove the residual tar. C

DOI: 10.1021/acs.energyfuels.7b02388 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Role of hot char filtration on separating particulates, AAEMs, and tar from volatiles: (a) Contaminants in volatile before and after hot char filtration. (b) AAEMs in volatile before and after hot char filtration. The precipitate on the filter paper was dried in an oven at 80 °C to evaporate the dichloromethane and measure the mass of the particulates, the mass of the tar was then obtained by difference. Dichloromethane was used to extract the oil phase in the liquid products collected in the condenser. The bottom and wall of the condenser was also washed with dichloromethane and the washings lotion was mixed with condensate. The mixture of condensate and dichloromethane was filtered with the carrier-slow quantitative filter paper to determine particulates collected in the condenser in a similar way as above. Then, the solution passing through the filter paper was put in a static precipitation bottle to be separated into an oily layer on the top and an aqueous layer on the bottom. The aqueous layer was removed with a separator funnel, and the oily layer was transferred into a dark brown bottle for further analysis, their mass was measured at the same time. The pyrolysis and scrubbing experiments were repeated for all cases with deviations less than ±5%, averaged results with their deviations were reported. In addition to the mass measurement of tar and particulates, concentrations of AAEMs in particulates collected in the condenser (denoted as the precipitate in the condenser), in both the oil phase and the aqueous phase, and those slipped into the syngas were measured. The concentrations of AAEMs in the MSW sample, in particulate matter, and in oil/aqueous phases were measured through microwave-assisted acid digestion (MW-AD) using an ETHOS 1-type microwave oven (Milestone Inc., Italy). To carry out MW-AD, first 0.3 g of the testing sample was put in a pressure-resistant PTFE kettle. Then, 5 mL of HNO3, 4 mL of HCl, and 3 mL of HF were added into the kettle. The mixture of sample and acids was kept at room temperature for 0.5 h, and then moved into the microwave oven to be digested following a preset temperature-programmed method. After digestion, the mixture was water-cooled below 80 °C and poured into a PTFE crucible. Then, 2 mL of HClO4 was added to the crucible to kick off unreacted HF. The mixture was then electrically heated to 130 °C, and another 5 mL of HNO3:H2O (1:1) solution was added and kept at this temperature to fully digest all of the organics. The obtained liquid sample was then diluted to exactly 100 mL with deionized water, and then the obtained solution was analyzed with an Agilent 720-ES type inductively coupled plasma optical emission spectrometer (ICPOES) (Agilent Technologies, Inc. USA). AAEMs in the syngas was determined by difference. The oil phase was filtered with a needle tube type polyether sulfone (PES) membrane filter (0.22 μm) to identify the compounds in the oily layer separated from the condensate; then the oil sample was diluted with dichloromethane and analyzed with a DSQ-type GC-MS analyzer (Finnigan, USA). The GC-MS was equipped with an electronic impact (EI) ionization source. The gas chromatography (GC) analyses were performed on a 30 m × 0.25 mm HP-5MS-type capillary column coated with a 0.25-mm-thick film. The GC injector port was kept at 300 °C. The GC analytical program was set as follows: the sample was maintained at 35 °C for 5 min, then increased

from 35 to 120 °C at 10 °C/min, 120−250 °C at 10 °C/min, 250− 300 °C at 10 °C/min, and kept at 300 °C for 10 min. The percentages of the compounds were calculated from the total ion chromatogram (TIC) peak area. The mass spectra were obtained in a scan mode in the range from 40 to 500 amu (atomic mass unit) to preliminarily identify the different oil molecules present in the samples, allowing for the quantification of low polarity or nonpolar components. A GC 9160 gas chromatograph (Shanghai Ou Hua Analytical Instrument, China) with two packed columns and two thermal conductivity detectors (GC/TCD) was used for the analysis of the permanent gases using argon as the carrier gas. CO, CO2, H2, and CH4 were analyzed with a column of 3 m in length and 2 mm in diameter, packed with zeolite of 60−80 mesh. Other hydrocarbon gases were analyzed with a column of 3 m in length and 2 mm in diameter, packed with zeolite of 80−100 mesh. The GC 9160 gas analyzer was calibrated with a standard gas before each measurement. The volume production of the syngas was measured with a BK-G25 M type gas flowmeter (Shanghai Krom Instrumentation Co, Ltd., China) installed after the sodium carbonate solution scrubber. All of the measurements were performed in parallel triplicate. The higher heating value of the MSW sample was determined by means of an XRY-1B type oxygen bomb calorimeter (Shanghai Changji Instrument, China). A Gasmet DX4000 type Fourier transform infrared spectrometer (Gasmet Technologies OY, Finland) was used to analyze the gaseous contaminants, including N2O, SO2, HNCO, NH3, and HCN online. Emissions of the VOC were detected online using a PGM-7340 type VOC analyzer with PID sensor (RAE, USA). A PGM-7800 type multigas monitor (RAE, USA) was used to analyze H2S, NH3, and NO2 after the final scrubber, the contents of which were rather low already. The emission characteristics of HCl were monitored with a Portasens II (C16) portal gas detector that was installed with double-range electrochemical sensors. All of these instruments are able to provide immediate and continuous readings. 2.3. Data Processing. Char and liquid yields were accurately determined with mass measurements, defined as the ratio of mass of char or liquid to the initial mass of MSW sample; and the corresponding gas yield was calculated by mass balance. The N2 produced in the pyrolysis process was determined by calculating the difference between the N2 fraction measured by GC and the carrier gas fraction. The HHV of the syngas gas was calculated with

HHV = C1 × HHV1 + C2 × HHV2 + ··· + Cn × HHVn where C1, C2, ···, Cn are the molar percentages of the gas compounds 1, 2, ···, n and HHV1, HHV2, ···, HHVn are the HHVs of the gas compounds 1, 2, ···, n. The dry volumetric gas yield Y can be calculated with

Y= D

V M DOI: 10.1021/acs.energyfuels.7b02388 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels where V is the total syngas volume and M is the mass of MSW sample fed to the pyrolysis reactor. To obtain the concentration of particulate matter in the volatiles, the weights of particulate matter collected in the condenser and by the glass fiber behind the condenser are summed up. The AAEMs contained in the volatiles are the sum of AAEMs collected in the condenser (distributing among the particulate matter, oil phase, and aqueous phase) and intercepted by the glass fiber behind the condenser.

accounted for 98.42% of the total gas. In contrast, before hot char filtration, those small molecular gases and light molecular hydrocarbons accounted for 91.71% of the total gas, while the others were heavy molecular species for which the GC was not calibrated. H2 and CH4 increased drastically, while the CO2 content showed the opposite tendency, making the molar percentage of the combustible fractions in the syngas higher than about 71 vol % after hot char filtration. The increased yields of H2, CO, and CH4 are the result of tar cracking reactions. At the same time, the molar percentage of H2+CO increased, with the ratio of H2/CO moving from 1.20 to 1.46. This proves that hot char bed favors the cracking and reforming of hydrocarbons and tar in the vapor phase to produce H2 and CO; therefore, more valuable syngas is obtained, as suggested by previous studies.7,29 The hot char filtration can also capture the evaporated AAEMs, as shown in Figure 2b. This positive effect has been not reported previously. Some studies have examined AAEMs released from biomass to determine whether they exist as solid particles, or as vapor, or are combined with tar. Kurkela et al.30 and Dayton et al.31 studied the release behaviors of AAEMs during the oxidation of biomass and reported that they primarily volatilize as chlorides into the gas phase. This finding implies that released AAEMs exist as solid particles because the boiling points of AAEM chlorides are above 1600 K. Additionally, Okuno et al.11 measured the retention of AAEMs in char to investigate their release behavior during pyrolysis of pulverized pine and sugar cane bagasse and found that AAEMs were primarily released as elemental species other than chlorides. However, Hirohata et al.32 studied released behavior of AAEMs during biomass steam gasification and found that most of the released AAEMs were combined with tar. In this research, the characteristics of release of AAEM species can be reasonably explained by considering that most AAEMs are released as solid particles but a portion is combined with tar; therefore, 51.9% of Ca, 57.0% of K, 43.2% of Mg, and 37.5% Na can be captured in hot char. However, the residual concentrations of tar, particulates, and AAEMs in the volatile after hot char filter are still very high and need further treatment. 3.2. Effect of Condensation on Separating the Liquid Products and Contaminants. Figure 3a illustrates the removal efficiencies of condenser for tar and particulates, and

3. RESULTS AND DISCUSSION 3.1. Role of Hot Char Filter. Figure 2a shows that hot char filtration is effective for removing particulate matter, with a capture efficiency of 71%. Similar to previous research,7 32% of oily compounds (simply notated as tar) disappeared after hot char filtration; there is then a remarkable increase in syngas yield from 27.23 wt% to 39.29 wt%, as shown in Table 4. The Table 4. Influence of Hot Char Bed (550 °C) Filtration on the Yields of Three Products products

before filtration

product yield (wt%) Gas 27.23 ± 1.12 Liquid products 30.44 ± 2.02 Char 42.33 ± 2.56 gas composition (mol%, dry basis) H2 15.97 ± 1.21 CO 13.36 ± 0.70 CO2 38.54 ± 3.30 CH4 19.84 ± 1.84 C2H4 1.86 ± 0.22 C2H6 1.72 ± 0.18 N2 0.42 ± 0.12 Summary of the detected gas 91.71 gas characterization Syngas (H2 + CO) (%, dry basis) 29.33 H2/CO (mol/mol) 1.20 HHV (MJ/m3N) 14.01a 3 Dry gas yield (m N/kg) 0.21 a

after filtration 39.29 ± 1.43 17.86 ± 1.12 42.85 ± 0.19 24.13 ± 2.04 16.84 ± 1.13 26.80 ± 2.96 26.65 ± 2.74 1.69 ± 0.26 1.68 ± 0.33 0.63 ± 0.09 98.42 40.67 1.46 18.07a 0.39

HHV of the detected light gas species.

resulting gas compounds were mainly H2, CO, CO2, and light molecular hydrocarbon species (CH4, C2H4, C2H6), which

Figure 3. Effect of condenser on interception of contaminants and AAEMs. (a) Contaminant removal efficiency in the condenser. (b) Distribution of AAEMs among the condenser precipitate, oily phase, aqueous phase, and syngas. E

DOI: 10.1021/acs.energyfuels.7b02388 Energy Fuels XXXX, XXX, XXX−XXX

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after condensation, the particulates collected by the glass-fiber filter installed behind the condenser present an oil stain when pressed with a filter paper, showing oil-polarity and being easily captured by the cooking oil, may explain this behavior. For the gaseous pollutants, VOC consists mainly of organic compounds; therefore, it has a good solubility in cooking oil, corresponding to a high removal efficiency of 77%. For NH3 and H2S, which are polar and hydrophilic, their removal efficiency was as low as shown in Figure 4. Although eliminating all of the tar is desirable, a more practical strategy is to simply remove sufficient tar with dew points lower than the lowest temperature experienced by the syngas in the desired applications. The energy research center in The Netherlands (ECN) stated a well-accepted tar definition and provided a typical tar classification system. Figure 5

Figure 3b shows the distribution of escaped AAEMs in the volatiles. These results indicate that the condensation step can capture up to 98% of the tar in the volatile and 50% of particulates, at the same time. The captured tars can be returned to the pyrolysis reactor or just burned to provide the energy for the whole system, as their quality is greatly improved.7 Approximately 90%, 85%, 87%, and 90% of the evaporated Na, K, Ca, and Mg in the volatiles passed through hot char filtration and can be captured during this step. Ca and Mg existed largely in the oil and aqueous phases, while Na and K were mainly dissolved in the aqueous phase. Ca was also present in the solid phase that precipitated in the condenser. K had the highest concentration in the syngas. AAEMs in the syngas mainly existed as fine particles, and they are one of the important contaminant species for syngas utilization. It was reported32,33 that AAEMs could be combined with oil products through chemical bonds, such as -OM and -COOM (M = Na, K, Ca, Mg), and therefore be intercepted during the condensation step. However, the reported results indicate that the four AAEMs would still exist in the syngas after condensation and a further cleaning step is necessary; at the same time, it can be seen that oil products can also be contaminated with AAEMs. 3.3. Performance of Oil Scrubber. After condensation, tar, particulates and AAEMs, in the syngas are still present at concentrations higher than the required levels of Table 1. Thence, further cleaning steps are necessary. Tar is a mixture of several hundreds of organics showing hydrophobic and nonpolar characteristics that have very low or no solubility in water. Therefore, hydrophobic absorbents, especially oil solvents, have been extensively investigated according the principle that “like dissolves like”. Studies showed that cooking oil had the highest tar absorption capacity.18,19,21 Therefore, it was adopted here as the scrubbing solvent for removing the oil phase (tar) from the MSW pyrolysis syngas. The results shown in Figure 4 indicate that the tar contained in the syngas could

Figure 5. Dew point of different types of tar changing with their concentrations.34

provides an illustration of the relationship between the tar dew point and tar concentration.34 According to ECN, tars of Class 1 refers to GC undetectable heavy tars such as 7- and higher ring compounds. Since these are not present in the syngas after condensation, Figure 5 only provides the information on tar dew points vs tar concentrations of Class 2−5. Tars of Class 5 dominate the high dew points of the total tar even for a very low concentration; tars of Classes 2 and 4 should be removed to ensure concentrations lower than 1−10 mg/m3N related to dew points lower than 30−50 °C, which can meet the requirements of a gas engine compressor/intercooler.35 Tars of Class 3 have a very low dew point and can be neglected. Figure 6 shows that, after the oil scrubber, the outlet syngas was free of tars of Classes 5 and 1. Tars of Classes 2 and 4 showed removal efficiency values of 98% and 99%, respectively. Nearly all of the tars present in syngas afterward were in Class 3, which corresponds to the lowest dew points. The dew point of the total tars was estimated to be below 50 °C according to Figure 5, as the concentrations of tars of Classes 2 and 4 are below 10 mg/m3N. 3.4. Performance of Na2CO3 Solution Scrubber. Figure 7 shows the real-time emissions of the important contaminants in the syngas, including H2S, NH3, and VOC, when the syngas, already washed by the oil scrubber, passed through the Na2CO3 solution scrubber. This solution was selected as the scrubbing

Figure 4. Concentrations of contaminants in syngas before and after oil scrubbing.

be reduced from 3.64 to 0.14 g/m3N after oil scrubbing, corresponding to a tar removal efficiency of 96%. Nevertheless, the tar concentration in the syngas afterward is still not acceptable for downstream applications when compared to the requirements shown in Table 1. Figure 4 also shows that oil scrubbing has a fairly good effect for particulates interception: all of the particulates carried in the syngas were captured in the oil scrubber. The evidence that, F

DOI: 10.1021/acs.energyfuels.7b02388 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 6. Removal efficiency of different types of tar by oil scrubber.

Figure 8. pH change of different Na2CO3 solutions.

agent due to its low cost and nontoxicity, as well as its potential for removing H2S and NH3 simultaneously. The results shown in Figure 7 show that, as expected, Na2CO3 solution was very effective for the adsorption of H2S, NH3, and VOC when compared to Scenario 1, which adopts just pure water in the scrubbing flask (Table 3). The emission peaks of H2S, NH3, and VOC appeared at approximately 335, 400, and 400 °C, respectively, during the pyrolysis step. Those emission peaks shown in Figure 7 correspond to the emission peaks of contaminants during pyrolysis. It can be seen that with the Na2CO3 concentration increasing, the removal efficiency values of the three pollutants increased accordingly. However, as the concentration increased from 1% to 5% (Scenario 4), the concentrations of H2S, NH3, and VOC were comparable to those of Scenario 3 in Figure 7. Previous studies reported that the pH of the scrubbing solution can have a strong influence on the removal efficiency of H2S, and its removal with carbonate solution is unfavorable under mild alkaline conditions.36 As illustrated in Figure 8, the pH of the Na2CO3 solution with a concentration of 1 wt% decreased dramatically after scrubbing the syngas, while a higher concentration such as 5 wt% can maintain a high pH afterward. This is important for H2S, NH3, and VOC adsorption. Moreover, a higher Na2CO3 concentration will be a benefit by decreasing the water consumption and maintaining the temperature of the scrubber; therefore, 5 wt% of Na2CO3 solution was selected for further investigations. A concentration that is too high will cause excessive Na2CO3 loss when discharging waste solution to maintain the salt concentration. As the L/G ratio increased, the concentration of the three pollutants decreased accordingly, as it can be proven by the comparison between Scenario 4 and Scenario 5 in Figure 7. However, at 25 °C, the H2S concentration in the purified

syngas does not meet the acceptable level of 20 ppm (Table 1). Carbonate solution at a temperature range of 25−80 °C is often used in industrial applications,23,36,37 and hot carbonate solutions are more favorable for oil removal as they improve the hydrolysis of oil. Herein, the scrubber with Na2CO3 solution of 5 wt% concentration running at 70 °C and L/G of 6 was investigated for further contaminant removal. The choice of 70 °C was made because when 5 wt% of Na2CO3 solution with an L/G of 6 is applied, Na2CO3 solution can be warmed by the hot volatile compounds to around 70 °C in the first condenser when Na2CO3 solution is used as the cooling agent according to the heat balance calculation (SI Supplement II). It is well-known that NH3, HCN, and HNCO gases are the main NOx precursors and NOx can be easily generated during combustion when they are present.20 During the experiments it has been found that the concentrations of gaseous HCN, NO2, and NO in the syngas were negligible (not shown here). For this reason, these three species were not considered in this study. The detected pollutants in the syngas are shown in Figure 9, indicating that, after scrubbing with hot Na2CO3 solution, the H2S concentration in the cleaned syngas decreased to 15.74 ppm with a removal efficiency of 96%, which is acceptable for downstream applications (Table 1). HCNO, NH3, and HCl are all removed efficiently in the hot Na2CO3 solution scrubber due to their hydrophilic and polar characteristics. VOC, mainly BTX (benzene-toluene-xylene), can be partially absorbed by the hot Na2CO3 solution with a removal efficiency of 67%, and the concentration of SO2 decreased to 14 ppm. Finally, the tar concentration in the syngas decreased from 144 to 38 mg/m3N with a removal efficiency of 73%. As

Figure 7. Real-time emissions of gaseous pollutants after Na2CO3 solution scrubber. (a) Emissions of H2S. (b) Emissions of NH3. (c) Emission of VOC. G

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particulate collection, and it is the first time that the combination of a condenser and an oil scrubber is highly effective for both tar and particulate matter removal. However, the oil scrubber only contributed to about 15% of removal of the total particulates in the volatile and 1% removal of the total oil phase, ensuring its long life span. The hot Na2CO3 solution scrubber is a good solution in the final step to remove all of the gaseous pollutants. In practice all of the concerned pollutants in the syngas should be cleaned, and the use of a series of individual device appears necessary. Here a successful solution has been provided with the combination of the four stepwise devices, including a hot char filter, a condenser, an oil scrubber, and a hot Na2CO3 solution scrubber, to treat the volatile product from MSW pyrolysis to obtain high quality syngas, and this combination can be recommended in practice. All of the pollutants appearing in the volatile or syngas can be cleaned to an acceptable level for later applications.

Figure 9. Scrubbing performance of hot Na2CO3 solution (70 °C).

tars of Class 4 disappeared and tars of Class 2 only accounted for 3.4% in the total tar remaining in the cleaned syngas, the dew point of tar was estimated to be 25−30 °C, which is lower than the practical process temperatures, acceptable in practice. All of the data showed that Scenario 6 in Table 3 provided a good solution to final step cleaning of syngas. 3.5. Stepwise Cleaning Effects and Their Contributions to the Total Removal Efficiency. Table 5 provides the various contaminant levels in the volatile or syngas after different treatment units, and the contribution of different scrubbing units to their total removal efficiency E. E1 is the efficiency for evaluating the contribution of individual device to the total removal, and the sum of E1 of all devices is the total efficiency E, while E2 is the removal efficiency of the concerned step. For H2S, NH3, and VOC, their concentrations in the volatiles cannot be easily measured, and then these measurements are made only after the condenser. It can be seen that a hot char filter is effective for both particulate removal and tar conversion, and a condenser is important for separating tar from the syngas, intercepting AAEMs, and helping to capture particulates in a later step at the same time; it can also warm up the Na2CO3 solution. The oil scrubber is not only effective for tar removal but also a perfect unit for

4. CONCLUSION Step-wise cleaning of the volatile compounds produced from pyrolysis of MSW was investigated, with a specific attention to the cleaning effects of hot char filtration, condensation, oil, and Na2CO3 solution scrubbing. The combination of this stepwise cleaning system can improve the syngas quality in both syngas components conditioning and pollutant removal. Hot char filtration removed about 71% of the particulates in the volatile and converted about 32% of the oil phase into gases. Condensation was a simple but effective cleaning step, able to separate 67% of the oil phase and more than 14% of particulates in the original volatile compounds and greatly reducing the burden for the successive scrubbers. Oil scrubbing removed 100% of the particulates and 96% of the tar entering the system, especially the tars belonging to the Classes 2 and 4, and greatly reduced their dew point, ensuring the cleaning effects but the oil scrubber only took in slight shares of the total particulates and oil in the volatile, therefore ensuring its long life span at the same time. A Na2CO3 solution scrubber operating at 70 °C appeared highly effective in removing NH3,

Table 5. Contaminant Concentrations in the Volatile Product or Syngas, and Their Stepwise Removal Efficiencies process Position Efficiency definition

pyrolysis reactor Total efficiency

E= Contaminant Particulate (mg/ m3N)

→

hot char filter

a

→

E1 = E2 =

→

condenser

b Ca − Cb a Ca

Ca − Ce Ca

→

oil scrubber

c

Na2CO3 solution scrubber

d

E1 =

Cb − Cc Ca

E1 =

Cc − Cd b Ca

E1 =

Cd − Ce b Ca

E2 =

Cb − Cc Cb

E2 =

Cc − Cd Cc

E2 =

Cd − Ce Cd

100%

7644

E1 = E2 = 70.68%

2241

Tar (water free, mg/m3N)

99.99%

339205

E1 = E2 = 31.90%

231000

H2S (ppm)

96.34%

-

-

-

E1 E2 E1 E2 -

NH3 (ppm)

97.38%

-

-

-

-

VOC (ppm)

97.60%

-

-

-

-

= = = =

14.53% 49.58% 67.03% 98.42%

E1 = 14.79% E2 = 100% E1 = 1.03% E2 = 96.06% E1 = E2 = 1.26%

0

-

143.59

62.15

E1 = E2 = 20.29%

49.54

296.60

E1 = E2 = 77.49%

66.75

E1 E2 E1 E2 E1 E2 E1 E2

1130 3640 429.80

→ e

424.40

0 = = = = = = = =

0.03% 73.22% 95.08% 96.29% 77.09% 96.71% 20.11% 89.36%

38.46 15.74 1.63 7.10

a

E1 is the removal efficiency contributed to the total contaminant concentration; E2 is the stepwise removal efficiency of the concerned step. bFor H2S, NH3, and VOC their initial concentrations in the volatile is difficult to measure, Cc is used instead of Ca. H

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(12) Abdoulmoumine, N.; Adhikari, S.; Kulkarni, A.; Chattanathan, S. A review on biomass gasification syngas cleanup. Appl. Energy 2015, 155, 294−307. (13) AQSIQ and SAC Manufactured gas; General administration of quality supervision, inspection and quarantine of the People’s Republic of China and Standardization administration of the People’s Republic of China; GBT13612−2006; Government Printing Office: Beijing, 2006. (in Chinese). (14) Shen, Y.; Yoshikawa, K. Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis -A review. Renewable Sustainable Energy Rev. 2013, 21 (5), 371−392. (15) González, J. F.; Román, S.; Engo, G.; Encinar, J.; Martínez, G. Reduction of tars by dolomite cracking during two-stage gasification of olive cake. Biomass Bioenergy 2011, 35 (10), 4324−4330. (16) Zhang, L. X.; Matsuhara, T.; Kudo, S.; Hayashi, J.; Norinaga, K. Rapid pyrolysis of brown coal in a drop-tube reactor with co-feeding of char as a promoter of in situ tar reforming. Fuel 2013, 112 (3), 681− 686. (17) Han, J.; Kim, H. The reduction and control technology of tar during biomass gasification/pyrolysis: An overview. Renewable Sustainable Energy Rev. 2008, 12 (2), 397−416. (18) Phuphuakrat, T.; Namioka, T.; Yoshikawa, K. Tar removal from biomass pyrolysis gas in two-step function of decomposition and adsorption. Appl. Energy 2010, 87 (7), 2203−2211. (19) Chen, H.; Namioka, T.; Yoshikawa, K. Characteristics of tar, NOx precursors and their absorption performance with different scrubbing solvents during the pyrolysis of sewage sludge. Appl. Energy 2011, 88 (12), 5032−5041. (20) Bergman, P. C. A.; van Paasen, S. V. B.; Boerrigter, H. The novel “OLGA” technology for complete tar removal from biomass producer gas; Pyrolysis and Gasification of Biomass and Waste, Expert Meeting; Strasbourg, France; September 30 − October 1, 2002. (21) Phuphuakrat, T.; Namioka, T.; Yoshikawa, K. Absorptive removal of biomass tar using water and oily materials. Bioresour. Technol. 2011, 102 (2), 543−549. (22) Cooney, D. O.; Olsen, D. P. Absorption of SO2 and H2S in small-scale venturi scrubbers. Chem. Eng. Commun. 1987, 51 (1−6), 291−306. (23) Wallin, M.; Olausson, S. Simultaneous absorption of H2S and CO2 into solution of sodium carbonate. Chem. Eng. Commun. 1993, 123 (1), 43−59. (24) Aravind, P. V.; de Jong, W. Evaluation of high temperature gas cleaning options for biomass gasification product gas for solid oxide fuel cells. Prog. Energy Combust. Sci. 2012, 38 (6), 737−764. (25) Fuentes Cano, D.; Gómez-Barea, A.; Nilsson, S.; Ollero, P. Decomposition kinetics of model tar compounds over chars with different internal structure to model hot tar removal in biomass gasification. Chem. Eng. J. 2013, 228 (28), 1223−1233. (26) Mašek, O.; Sonoyama, N.; Ohtsubo, E.; Hosokai, S.; Li, C.; Chiba, T.; Hayashi, J. Examination of catalytic roles of inherent metallic species in steam reforming of nascent volatiles from the rapid pyrolysis of a brown coal. Fuel Process. Technol. 2007, 88 (2), 179−185. (27) The editorial board of the State Environmental Protection Administration. Air and emission monitoring and analysis methods; China Environmental Science Press: Beijing, 2003 (In Chinese). (28) AQSIQ and SAC. Proximate analysis of coal; General administration of quality supervision, inspection and quarantine of the People’s Republic of China and Standardization administration of the People’s Republic of China; GB/T476−2008; Government Printing Office: Beijing, 2008 (in Chinese). (29) Yu, G.; Feng, Y.; Chen, D.; Yang, M.; Yu, T.; Dai, X. In situ reforming of the volatile by char during sewage sludge pyrolysis. Energy Fuels 2016, 30 (12), 10396−10403. (30) Kurkela, E.; Ståhlberg, P.; Laatikainen, J.; Simell, P. Development of simplified IGCC-processes for biofuels: supporting gasification research at VTT. Bioresour. Technol. 1993, 46, 37−47. (31) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Release of

H2S, HCl, and tar, and guaranteed a high quality syngas for practical applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02388. Review of syngas cleaning technologies (I); Evaluation of temperature of hot Na2CO3 solution in operation (II) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 21 65985009. Fax: +86 21 65982786. ORCID

Dezhen Chen: 0000-0003-3421-3998 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work has been financially supported by National Natural Science Foundation of China (Granted No.51776141) and Shanghai Municipal Science & Technology Commission Fund for improving economy in the Yangtze River Delta region (Granted No. 12195811100).

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REFERENCES

(1) Dong, J.; Chi, Y.; Tang, Y.; Ni, M.; Nzihou, A.; Weiss-Hortala, E.; Huang, Q. Effect of operating parameters and moisture content on municipal solid waste pyrolysis and gasification. Energy Fuels 2016, 30 (5), 3994−4001. (2) Arena, U. Process and technological aspects of municipal solid waste gasification. A review. Waste Manage. 2012, 32 (4), 625−639. (3) Psomopoulos, C. S.; Bourka, A.; Themelis, N. J. Waste-to-energy: A review of the status and benefits in USA. Waste Manage. 2009, 29 (5), 1718−1724. (4) Dong, J.; Chi, Y.; Tang, Y.; Ni, M.; Nzihou, A.; Weiss-Hortala, E.; Huang, Q. Partitioning of heavy metals in municipal solid waste pyrolysis, gasification, and incineration. Energy Fuels 2015, 29 (11), 7516−7525. (5) Chen, D.; Yin, L.; Wang, H.; He, P. Pyrolysis technologies for municipal solid waste. A review. Waste Manage. 2014, 34 (12), 2466− 2486. (6) Zhang, Q.; Chang, J.; Wang, T.; Xu, Y. Review of biomass pyrolysis oil properties and upgrading research. Energy Convers. Manage. 2007, 48 (1), 87−92. (7) Wang, N.; Chen, D.; Arena, U.; He, P. Hot char-catalytic reforming of volatiles from MSW pyrolysis. Appl. Energy 2017, 191, 111−124. (8) Milne, T. A.; Abatzaglou, N. Biomass gasifier “tars”: their nature, formation, and conversion; Summary report for the National Renewable Energy Laboratory; NREL, 1998; subcontract TP-570-25357. (9) Baratieri, M.; Baggio, P.; Bosio, B.; Grigiante, M.; Longo, G. The use of biomass syngas in IC engines and CCGT plants: A comparative analysis. Appl. Therm. Eng. 2009, 29 (16), 3309−3318. (10) Ohtsuka, Y.; Tsubouchi, N.; Kikuchi, T.; Hashimoto, H. Recent progress in Japan on hot gas cleanup of hydrogen chloride, hydrogen sulfide and ammonia in coal-derived fuel gas. Powder Technol. 2009, 190 (3), 340−347. (11) Okuno, T.; Sonoyama, N.; Hayashi, J.; Li, C.; Sathe, C; Chiba, T. Primary release of alkali and alkaline earth metallic species during the pyrolysis of pulverized biomass. Energy Fuels 2005, 19 (5), 2164− 2171. I

DOI: 10.1021/acs.energyfuels.7b02388 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels inorganicconstituents from leached biomass during thermal conversion. Energy Fuels 1999, 13 (4), 860−870. (32) Hirohata, O.; Wakabayashi, T.; Tasaka, K.; Fushimi, C.; Furusawa, T.; Kuchonthara, P.; Tsutsumi, A. Release behavior of tar and alkali and earth metals alkaline during biomass steam gasification. Energy Fuels 2008, 22 (6), 4235−4239. (33) Feng, D.; Zhao, Y.; Zhang, Y.; Sun, S.; Meng, S.; Guo, Y.; Huang, Y. Effects of K and Ca on reforming of model tar compounds with pyrolysis biochars under H2O or CO2. Chem. Eng. J. 2016, 306, 422−432. (34) Energy Research Center of the Netherlands (ECN). Thersites: the ECN tar dew point site; http://www.thersites.nl/, 2009. (35) Könemann, H. W. J.; van Paasen, S. V. B. OLGA tar removal technology; 4MW commercial demonstration. In 15th European bio mass conference and exhibition, Berlin, Germany, 2007. (36) Knuutila, H.; Juliussen, O.; Svendsen, H. F. Density and N2O solubility of sodium and potassium carbonate solutions in the temperature range 25 to 80 °C. Chem. Eng. Sci. 2010, 65 (6), 2177−2182. (37) Garner, F. H.; Long, R.; Pennell, A. The selective absorption of hydrogen sulphide in carbonate solutions. J. Chem. Technol. Biotechnol. 2010, 8 (5), 325−336.

J

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