Partial Oxidation of Filter Cake Particles from Biomass Gasification

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Partial oxidation of filter cake particles from biomass gasification process in the simulated product gas environment Junfei Jiang, Lin Lang, Leteng Lin, Huacai Liu, Xiuli Yin, and Chuangzhi Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01100 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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Energy & Fuels

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Partial oxidation of filter cake particles from biomass

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gasification process in the simulated product gas environment ※ Jun-fei Jianga, Lin Langa , Le-teng Linb, Hua-cai Liua, Xiu-li Yina, Chuang-zhi Wua

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a

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Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of

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Sciences (CAS), Guangzhou 510640, China

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CAS Key Laboratory of Renewable Energy, Guangdong Key Laboratory of New and Renewable

b

Built Environment and Energy Technology, Linnaeus University, Växjö, Sweden

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Abstract

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Filtration failure occurs when filter media is blocked by accumulated solid particles. Suitable

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operating conditions were investigated for cake cleaning by partial oxidation of filter-cake particles

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(FCP) during biomass gasification. The mechanism of the FCP partial oxidation was investigated in a

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ceramic filter and by using thermo-gravimetric analysis through a temperature-programmed route in a

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2 vol. % O2–N2 environment. Partial oxidation of the FCP in the simulated product gas environment

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was examined at 300–600°C in a ceramic filter that was set and heated in a laboratory-scale fixed

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reactor. Four reaction stages, namely drying, pre-oxidation, complex oxidation and non-oxidation,

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occurred in the FCP partial oxidation when the temperature increased from 30°C to 800°C in a 2 vol.%

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O2–N2 environment. Partial oxidation was more effective for FCP mass loss from 275 to 725°C.

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Experimental results obtained in a ceramic filter indicated that the best operating temperature and FCP

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loading occurred at 400°C and 1.59 g/cm2, respectively. The FCP were characterized by

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Fourier-transform infrared spectroscopy, scanning electron microscopy and Brunaeur–Emmett–Teller

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before and after partial oxidation. Fourier-transform infrared spectroscopy analysis revealed that

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partial oxidation of the FCP can result in a significant decrease in C–Hn (alkyl and aromatic) groups 1

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and an increase in C=O (carboxylic acids) groups. The scanning electron microscopy and Brunaeur–

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Emmett–Teller analysis suggests that during partial oxidation, the FCP underwent pore or pit

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formation, expansion, amalgamation and destruction.

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Key word: Biomass gasification; Hot gas filtration; Partial oxidation; Filter cake particles

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1. Introduction

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Biomass gasification is one of the most promising technologies for converting biomass to

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product gas, which is suitable for heat supply, power generation or synthesis-gas production. Product

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gas usually contains various amounts of condensable hydrocarbons (i.e., tars), inorganic impurities

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and solid particles1. Concentrations of unwanted species will be affected by the characteristics of the

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original biomass feedstock, reactor configurations and operating conditions2. According to the

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literature3, solid-particle concentrations in product gas from a conventional biomass gasification plant

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could weigh up to 3 and 10% of the biomass feedstock. Morgalla et al. characterized particulate matter

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(PM) formed during wood-pellet gasification in an indirect bubbling fluidized-bed gasifier4. The fine

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and intermediate PM modes were formed mainly through nucleation and condensation of tar and

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alkali vapors. The coarse PM mode was original particles from ash, bed material, fragmented char and

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soot5. Normally, the carbon content in PM from the raw product gas is very high.

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During initial product-gas cleaning and upgrading, PM must be removed to ensure that

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downstream catalysts and reformers work properly. Tar components and other inorganic impurities,

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such as sulfur and nitrogen compounds, are removed by catalytic reforming or by wet scrubbing,

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depending on the process design. For PM removal, industries normally use combined methods such as

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cyclones, filters, electrostatic precipitators and scrubbers according to process requirements.

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Electrostatic

precipitators

are

not

an

economical

solution

2


for

the

relatively

smaller

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biomass-gasification-plant scale. A combination of cyclones and hot-gas filtration is often used, which

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could avoid the cooling of too much product gas, and maintains the overall process efficiency. The

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particle collection efficiency can exceed 90%, and even 99.5% when rigid barrier filters are applied1.

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However, filtration failure can occur when filter media is clogged and the pressure drops out of

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control. A conventional regeneration method is to introduce periodic back-pulsing, by sending a short

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pressure pulse in the reverse flow direction. Frequent back-pulsing could increase particle penetration

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and filter erosion, and reduce filter life expectancy6. When the system runs at a high temperature,

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some other effects from accumulated filter-cake particles, such as dust sintering7, soot formation8 and

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alkali corrosion9 10 could result in a high and permanent residual pressure drop. New alternatives to

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handling the pressure drop created by filter-cake particles are still required to achieve a long-term

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durability.

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Filter cake that is formed during biomass gasification consists of ash particles, unconverted char

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particles, tar condensate, bed material and soot, depending on the process conditions. Because the

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carbon content could be as high as 88%2, there is a possibility that carbon in filter-cake particles (FCP)

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could be oxidized partially by introducing a small amount of oxygen to reduce the pressure drop over

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the filter cake. Many publications have reported that the oxidation of carbonaceous components in

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particulates is very effective to reduce the pressure drop in the field of PM control during diesel

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combustion11 12. Molintas et al raise the concept of a char eliminator that is set up downstream from

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the biomass gasifier where air and pure oxygen are used as oxidants to reduce the amount of char

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particles that are produced during gasification13. However, they do not describe if the suggested char

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eliminator is a separate combustor or a combined functional unit with a filter. In their article, carbon

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black is chosen as a model material for char particles to study oxidation kinetics. This is not the

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optimum because of large differences in reactivity between carbon black and the FCP. 3


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We propose a partial-oxidation concept by introducing a low concentration of oxygen to reduce

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the carbon content in FCP in a product-gas environment. Complexity is created by the competition

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and interaction between homogeneous and heterogeneous reactions. To our best knowledge, literature

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on this topic is scarce. The aim of this study was to establish the optimum reaction conditions where

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the FCP could be oxidized to a maximum while maintaining a minimum degradation of product gas.

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Experiments were carried out in the simulated product gas (SPG) environment. To understand the

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evolution of FCP partial oxidation, the FCP before and after oxidation were analyzed besides the gas

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components.

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2. Experimental

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2.1 The sample preparation.

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The FCP sample used in investigation were collected from the hot gas filter (400–600°C)

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downstream a Coupled Drafting Fixed Bed Gasifier using sawdust as feedstock. The gasifier was

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operated around 750–900°C using air as gasification agent. Even though the FCP sample were

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collected from the hot gas filter that worked above 400°C, there would be substantial amount of tar

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condensed during the cooling period before taking the FCP sample out of the gasifier filter. Thermal

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pretreatment at 400°C became necessary to remove most of tar condensate that could seriously clog

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the pipes in our test rig. Therefore, before the experiments, the FCP were pretreated according to the

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following steps. Firstly, they were sieved into small particles with the diameter ranging from 0.15 mm

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to 0.20 mm and then dried at 105°C in a vacuum drying oven for 12h. Secondly, the FCP were heated

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up in N2 atmosphere from room temperature to 400°C with a heating rate of 10K/min. When the

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temperature reached 400°C, the FCP were taken out immediately and cooling down in a dryer as the

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testing sample. The ultimate analysis of the sample was done and the results were showed in table 1. 4


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According to the proximate analysis, the moisture content, ash, volatile component and fixed carbon

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were 2.51%, 19.58%, 23.01% and 54.80%, respectively.

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In addition, SPG sample in tests was mixed according to the raw gas components14 from biomass

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gasification with air and purchased from GuangZhou Pu Yuan Gas Company Limited. Components of

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gas products A (product gas) were 10 vol. % H2、15 vol. % CO2、22 vol. % CO、3 vol. % CH4 and 50

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vol. % N2 and components of gas products B and C were N2 and O2, respectively.

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2.2 Test rig

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A lab-scale fixed-bed reactor was used to investigate the partial oxidation characteristics of the

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FCP in the product gas at temperatures ranging from 300°C to 600°C. As shown in Fig.1, the

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experimental system consisted of gas mixed tank, quartz reactor, condenser, and gas chromatography.

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The quartz reactor installed vertically in the electric furnace had internal and external diameters of 20

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mm and 25 mm, respectively. Temperature of filtration (TF) was measured by the thermocouple and

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regulated by controlling the heat supply of the furnace. The bottom of thermocouple was put into the

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middle layer of the FCP in tests for its partial oxidation. When passing through the reactor, the gas

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entered a series of three gas-washing bottles in the condenser. Water was used as the additive in the

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first bottle, and quartz gel in the other two bottles. After purification, gas components were analyzed

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by the gas chromatography (GC-2014, Shimadzu). Argon was used as the carrier gas, and the

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chromatographic columns were Porapak N and molecular sieve types. The Porapak N column was

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used to separate the CO2 from the mixed gas while the molecular sieve column was to determine the

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concentration of H2, CO, CH4, O2 and N2. During GC detection, temperature of column was kept at

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45°C for 5 min, then increased to 105°C with a heating rate of 10K/min and then kept for 5min again

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before terminating detection. 5


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2.3 Experiments scheme

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Partial oxidation behavior of the FCP in the 2 vol. % O2-N2 environment would involve a

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complex chemistry of homogenous and heterogeneous reactions. In order to understand those

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reactions better, experiments have been planned and performed in three steps: partial oxidation of SPG,

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partial oxidation of the FCP in 2 vol.% O2-N2 environment and partial oxidation of the FCP in 2 vol.%

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O2-SPG environment. In this paper, the O2 concentration of 2 vol. % was chosen according to some

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trying investigations in the pilot experiment (Fig.S1 and table S1). The explosive limit of the SPG was

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also calculated before the experiments and a low O2 concentration of 2 vol.% was far beyond the

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explosive limit. Moreover, the stabilization of the SPG in different O2 concentration (2–8% vol.% )

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were tested and the results showed 2% seemed to be safer between 300-500 °C (Fig.S2).

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2.3.1 SPG partial oxidation

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In this part, SPG partial oxidation was investigated in the high temperature filter. A quartz filter

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reactor with ceramic media about 5 mm thick which could hold the FCP on its surface was used to

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seek suitable conditions for the FCP partial oxidation. Before the experiments, system air-tightness

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was examined to ensure the safety. During the testing, the TF of the reactor was increased from 25°C

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to 600°C with a heating rate of 3K/min. The mixed gas (the 98 vol. % SPG and 2 vol. % O2) with a

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total flow of 100 mL/min was introduced into the quartz filter reactor as the TF reach 300°C. It was

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noted that no the FCP were on the surface of ceramic media in SPG partial oxidation. When exiting

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from the reactor, each component concentration in the product gas was determined by the

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GC-2014(Gas Chromatograph) which started detecting from 300°C using 50°C as an interval. Gas

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compositions presented in the paper were averaged from three times measurements.

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2.3.2 Partial oxidation of the FCP in the 2 vol. % O2-N2 environment 6


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The goal of this part was to provide the lowest temperature for partial oxidation of the FCP in the

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2 vol. % O2-SPG environment. The experiments were performed in the atmosphere of 2 vol. %O2 and

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98 vol. % N2 with a total gas flow of 100 mL/min. Before the experiments, FCP loading of 0.64g/cm2

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were laden on the surface of ceramic media. During the experiments, a flow of 100 mL/min mixed gas

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was introduced into the reactor when the temperature began increasing from 25°C to 600°C with a

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heating rate of 3K/min. The GC-2014 started detecting the gas components when experiments begin.

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The repeated runs showed good repeatability.

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Additional experiments were conducted with a Thermo-gravimetric Analyzer (Netzsch STA 449

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F3 Jupiter) in order to elucidate the pyrolysis and partial oxidation mechanisms of the FCP in pure N2

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and in the mixed gas (2 vol. %O2 and 98 vol. % N2), respectively. The total flow rate of gas which

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included the N2, O2 and purge gas was 100mL/min. For each run, 8.0 mg of the FCP were heated from

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room temperature to 900°C at a constant heating rate of 10 K/min. The thermo-gravimetric (TG) plots

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depicted the weight loss of the FCP as it was heated to different temperature. The differential

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thermo-gravimetric (DTG) plots showed the derivative of the FCP mass with time.

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2.3.3 Partial oxidation of the FCP in the 2 vol. % O2-SPG environment

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The focus of this part was to investigate the FCP partial oxidation in the 2 vol. % O2-SPG

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environment. The mixed gas was 2 vol. % O2 and 98 vol. % gas product A. Effects of the TF, FCP

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loading and FCP residence time were investigated, respectively. For each experiment, the FCP were

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laden on the surface of ceramic media. A flow of 100 mL/min N2 was introduced into the reactor to

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protect the FCP from oxidation during the heating procedure. The reactor was heated up from room

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temperature with a heating rate of 3 K/min. The mixed gas with a total flow of 100 mL/min was

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switched into the reactor as soon as the TF reached the required level. The exited gas from reactor was

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first washed by water and dewatered by quartz gel, and then measured by GC-2014. After certain time 7


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as required, the mixed gas was switched to pure N2 immediately in order to protect the FCP from

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continuous partial oxidation until the temperature of the reactor dropped below 100°C. For FCP

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partial oxidation, the suitable TF is important because both the homogeneous and heterogeneous

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reactions are sensitive to the temperature. The FCP mass loading, which also could be regard as the

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particles height on the surface of filter media, can reveal the relationship between the O2 concentration

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and particles content. Therefore, different TF (300–600°C) and FCP loading (0.16–3.2 g/cm2 filter

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media) were selected. For the purpose of exploring the oxidation process, the different FCP residence

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time (5–24 h) in filter was chosen.

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2.4 FCP characterization

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The elemental analysis of FCP was conducted according to the standard method ASTM D 5373

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by using a Vario EL cube elemental analyzer. And moisture content, volatile matter content, fixed

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carbon and ash were obtained according to standard methods GB/T 28731-2012. In the test of the

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specific surface area, the FCP were degassed at 280°C for 10 h and then tested in an automatic

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adsorption instrument manufactured by Quantachrome. The total surface area was determined by BET

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equation and calculated by the adsorption branch of the isotherm over the pressure range 0.050 < p/p0

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< 0.300, ending in a correlation coefficient (r2) of 0.999. Apparent morphology of the FCP was

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analyzed by scanning electron microscopy (SEM) using a Hitachi S-4800 for the images, and the

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examination conditions were included in each image. The functional groups change of the FCP were

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analyzed using Bruker tensor 27 Fourier transform infrared spectrometer (FTIR). Detail sample

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preparation and analysis method referred to the reference15.

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2.5 Data analysis 8


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The flow rate of the gas which is introduced into reactor is 100 mL/min, and the exited gas flow

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is determined based on the N2 balance. The low heating value (LHV) of SPG is defined as q (MJ/m3),

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and the ratio of LHV between SPG exited from the reactor and SPG entered into the reactor is qr,

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which are estimated according to equation (1) and (2), respectively. Meanwhile, the concentration

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change ratio of each gas component is defined as △X (%). The positive value of △X represents

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increasing ratio while the negative is the opposite, which are calculated by equation (3). The definition

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△C (mmol/min) is the increment of carbon element in SPG after reaction, acquiring from equation (4).

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The Ogas and Osolid represent for the O2 proportions which react with SPG and the FCP, respectively.

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The Oleft is the rest O2 proportion after reaction. They are calculated according to equations (5)–(7).

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Additionally, weight loss of the FCP after reaction was measured by the weighing method, showing in

195

equation (8).

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q (M J/Nm 3 ) = (η CO × 126.36 + η H 2 × 107.98 + η CH 4 × 358.18)/1000

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q r = qout / qin × 100%

(1)

(2)

∆ X = ( xout − xin ) / xin × 100% (3)

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∆C = (CO+CO2 +CH4 )out − (CO+CO2 +CH4 )in

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oleft = ( O2 )out / ( O2 )in ×100%

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1 1 ( H 2 )in - ( H 2 )out  + 2 ( CO )in - ( CO )out  +2 ( CH 4 )in - ( CH 4 )out  2 Ogas = ×100% ( O2 )in

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Osolid = 100% -Oleft -Ogas

(4)

(5) (6)

(7)

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Wloss = [(Wfiter +WFCP )before reaction - (Wfiter +WFCP )after reaction] / (Wfiter )before reaction * 100% (8)

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Where qin (MJ/m3) is the LHV of SPG which enters into reactor, qout (MJ/m3) is the LHV of SPG

205

which exits from reactor, ηH , ηCO and ηCH are the volume percentage of H2, CO, CH4 in SPG, x

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(mmol/min) is one of the gas components.

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3. Results and discussion

2

4

9


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3.1 Partial SPG oxidation

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The potential effect of the ceramic media and vessel material on SPG composition was

210

investigated by allowing SPG to pass through a heated reactor. As shown in Fig. 2a, the SPG

211

compositions were nearly retained at the initial level of between 300–600°C, which indicates that the

212

SPG is rather stable to the filter and reactor material under the investigated conditions. During partial

213

oxidation of the SPG (as shown in Fig. 2b), a significant composition of simulated gas occurred above

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450°C when 2 vol.% O2 was mixed with the 98 vol.% SPG. The different chemical composition of the

215

SPG showed different responses as the TF increased. The H2 concentration decreased gradually to

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500°C and then decreased rapidly thereafter. The concentrations of CO and CH4 showed an obvious

217

decrease at ~550°C, possibly because of the gas reducibility of each component, namely, H2 > CO >

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CH4. These results reveal that the TF was a critical parameter that influenced the partial oxidation of

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the SPG. A suitable TF for the FCP partial oxidation in the 2 vol.% O2-SPG environment should not

220

exceed 500°C.

221 222

3.2 Partial oxidation of the FCP in the 2 vol.% O2–N2 environment

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The effect of TF on the change in CO, CO2 and O2 concentrations was plotted in Fig. 3. The

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concentration change of each component in the product gas indicates that the higher TF was more

225

conducive to partial oxidation of the FCP. From 50°C to 600°C, the O2 concentration decreased

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rapidly and no O2 was detected by gas chromatography when TF reached 500°C. CO2 appeared at

227

150°C, and CO was produced after 225°C. Both concentrations increased as the TF increased, which

228

indicates an ongoing oxidation of the FCP. CH4 was detected at 350°C and increased slightly as TF

229

increased. This result may indicate the start of the decomposition of heavy tar condensate or the

230

pyrolysis of other volatile hydrocarbons. 10


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Fig. 4 shows the partial oxidation of the FCP in a 2 vol.% O2–N2 environment. The

232

thermo-gravimetric behavior of its pyrolysis is plotted in Fig. 4 using a dotted line. The differences

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between the thermo-gravimetric curve of pyrolysis and partial oxidation are thought to result from the

234

O2 because these experiments were conducted under nearly identical conditions, with the exception of

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O2 addition. For partial oxidation of the FCP, obvious mass loss commenced at ~275°C, whereas mass

236

loss occurred at 350°C for the FCP pyrolysis. The mass loss of the FCP terminated at 725°C in partial

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oxidation, and the remaining mass was ~22.16% of the initial. This indicates that the FCP had nearly

238

burned out. For the pyrolysis case, however, only 23.15% mass loss was obtained at 725°C.

239

According to Figs 3 and 4, four sequential reaction stages could be defined. Below 100°C, in the

240

first drying stage, water was released so that the thermo-gravimetric curve in Fig. 4 dropped and the

241

gas concentration in Fig. 3 exhibited almost no change. From 100 to 300°C, a second pre-oxidation

242

stage resulted, where the thermo-gravimetric curve showed a tiny increase because some O2 was

243

assimilated by the FCP to form possible complexes such as carboxylic acid or estolide16 17. Those

244

complexes could be thermally decomposed at these temperatures, which lead to a small production of

245

CO and CO2. A similar mass increase was reported by Zhan et al.18 who found that the mass of a coal

246

sample reached a maximum at 265°C, with one magnitude exceeding the initial weight. Between 300

247

and 725°C, a third complex oxidation stage resulted, which included the partial oxidation and

248

pyrolysis of the FCP. During this stage, most FCP were oxidized to generate CO and CO2, and the

249

oxidation was enhanced with an increase in temperature. The FCP were pyrolyzed partially into

250

volatiles, so that CH4 was detected above 350°C. During the last non-oxidation stage above 725°C,

251

the residue in the crucible was mainly from fly ash, which was relatively stable at a high temperature

252

and did not lead to further mass loss. Therefore, partial oxidation of the FCP should be investigated

253

further in a more realistic temperature range above 300°C. 11


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3.3 Partial oxidation of the FCP in the 2 vol.% O2-SPG environment

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3.3.1 Effect of TF

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Fig. 5 shows the changes in gas components, excluding N2, after partial oxidation of the FCP

258

(0.64 g/cm2) in the 2 vol.% O2-SPG environment at different TF. The total duration of each condition

259

was 5 h and each point was the average of the results over 5 h. According to Fig. 5a, for 300–450°C,

260

the CH4 concentration changed slightly, whereas the H2 and CO concentrations showed a more

261

obvious decrease at 400°C. The CO2 concentration increase was much higher than the decrease in CO

262

concentration, which indicates that the carbon content in the FCP was oxidized to CO2 at 300–450°C.

263

The O2 concentration tended to decrease from 300°C and became nearly exhausted at 400°C. The

264

fractions of O2 consumption are depicted in Fig. 5b; most O2 was consumed by the FCP. A maximum

265

Osolid was reached at 400°C where the Oleft nearly decreased to zero and the Ogas was at its minimum.

266

To identify the optimum TF for partial oxidation of the FCP under this condition, the qr and △C in

267

the gas were calculated at different TF, as shown in Fig. 6. At 300–400°C, the qr was higher than 100%

268

and the △C increased visibly (The qr was higher than 100% at 400°C because the volume of CO and

269

CH4 was increased relatively, showing in Table S2). However, at TF ≥ 450°C, the qr was lower than

270

100% and the △C decreased. These results suggest that extensive SPG had already been oxidized at TF

271

≥ 450°C, and contributed to a decrease in qr and △C. Based on these results, the optimum TF for

272

partial oxidation of the FCP should be 400°C where the FCP could be oxidized to a significant extent

273

without much SPG loss, i.e., the LHV of the SPG is maintained at a high level.

274

3.3.2 Effect of FCP loading

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The effect of the FCP loading was investigated at 400°C to maximize the utilization of O2. Fig. 7

276

shows the change in gas components, except for N2, as the SPG that was mixed with 2 vol.% O2 12


Page 12 of 31

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277

passed through various FCP loadings. The total duration of each condition was 5 h, and each point

278

was the average of 5 h. Fig. 7a shows that the CO and CH4 concentrations exhibited very little change,

279

which indicates their stability under these conditions. The H2 concentration showed an obvious

280

declining decrease over the range. However, the CO2 concentration exhibited a large and significant

281

increase when the FCP loading increased from 0.16 g/cm2 to 1.59 g/cm2, and was constant after 1.59

282

g/cm2. As the FCP loading increased, the O2 concentration in the exit gas decreased rapidly, and was

283

nearly exhausted at 0.64 g/cm2, which was also proven by the Oleft change in Fig. 7b. The change in

284

Osolid showed a maximum at a 0.64 g/cm2 loading, and more than 90% O2 reacted with the FCP.

285

Although a high level was maintained after 0.64 g/cm2, the Osolid decreased slightly because of the

286

gradual increase in O2 consumption by the SPG partial oxidation. In general, the partial oxidation of

287

the FCP and H2 was promoted as the FCP loading increased from 0.16 g/cm2 to 0.64 g/cm2. A further

288

increase in FCP loading yielded no apparent change to all gaseous products.

289

Fig. 8 reports the effect of FCP loading on △C and qr in the gaseous products. An increase in FCP

290

loading promoted the FCP partial oxidation but reduced qr because of H2 consumption by the

291

oxidation reaction as discussed above. Using a loading amount above 1.59 g/cm2 was unnecessary,

292

because the △C and qr were nearly constant. The FCP loading of 1.59 g/cm2 could maximize the O2

293

utilization, whereas the LHV of the product gas was still reasonably closer to the initial level.

294 295

3.4 FCP partial oxidation mechanism

296

3.4.1 Discussion for the oxidation process

297

To illustrate the FCP partial oxidation in the 2 vol.% O2-SPG environment better, long

298

residence-time tests were carried out at 400°C. To analyze the change in gas components, an FCP

299

loading of 0.32 g/cm2 was chosen because a higher FCP loading would require a longer time to reach 13


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

300

steady state, and could establish a longer stable process for a given gas flow and O2 ratio in the SPG.

301

Fig. 9 shows the evolution of gas components, except N2, during this process. CO, H2 and CH4

302

concentrations were maintained at an almost constant level but the O2 and CO2 concentration changed

303

significantly over the period. The increase in CO2 in the exit gas was high initially and then decreased

304

gradually, whereas the O2 concentration showed a nearly inverse tendency with CO2, which suggests

305

that the reaction, C + O2 → CO2, was dominant during the test period.

306

Products from the FCP partial oxidation included CO2 and other oxides. In Fig. 9, the empty

307

triangle represents the calculated CO2, assuming that the O2 reacted only with the H2 in the SPG and

308

elemental C in the FCP. The O2 decrease should be equivalent to the increase in CO2 after excluding

309

that used for H2 partial oxidation. However, the CO2 experimental value was lower than its calculated

310

value within 600 min, especially at the beginning of the experiments, which implied an O elemental

311

loss in gas composition. After 600 min, no obvious difference existed between the calculated and

312

experimental value. One reason was the partial H and S oxidation. According to an ultimate analysis

313

of the FCP in Table 1, the H and S contents exhibited a significant decrease alongside the C content,

314

which indicated an oxidation of these elements. The complex formation discussed in Section 3.2 may

315

be another reason for O element loss. Those results implied that the tar condensate included in FCP

316

might be more active for partial oxidation in the initial period of the hot gas filtration. It is very

317

important because the tar condensate removal will decrease the pressure drop in filtration.

318

3.4.2 Further characterization of the FCP samples at different conversion levels

319

Table 1 summarized the mass-loss measurements when a fixed amount of FCP was oxidized

320

partially with a 2 vol.% O2 at 400°C. The mass of the FCP exhibited a significant loss, which occurred

321

mainly because of the partial oxidation of the FCP. Thermal cracking may play a role, but not

322

influence the results significantly as expected based on the pyrolysis results (Fig. 4). Partial oxidation 14


Page 14 of 31

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Energy & Fuels

323

of the FCP shows great potential for engineering applications because it may increase the product-gas

324

LHV under certain conditions as discussed above. It would also be an effective method to reduce the

325

filter-cake mass and the pressure drop over the filter.

326

Fig. 10 shows the Fourier-transform infrared absorption spectra curves of the FCP before and after

327

partial oxidation. The major functional group change included C–Hn(alkyl and aromatic) stretching

328

vibration (2860–2970 cm-1) and C=O (carboxylic acid) stretching (1700 cm-1)

329

and aromatic) groups could be oxidized completely and removed because the peak at 2920 cm-1

330

decreased significantly within 5 h and almost disappeared after 10 h. This result contributed to the

331

elemental C and H decease in Table 1. The C/H value showed a significant increase before 5 h and

332

then a slow decrease, which showed that the rate of elemental H loss exceeded that of the elemental C.

333

Therefore, the C–Hn (alkyl and aromatic) stretching bonds in FCP were much easier to oxidize.

334

Oxidation of the C=O (carboxylic acids) groups increased with time. This result suggested that the

335

amount of C=O (lactones, carboxylic acids) stretching increased gradually. According to reports 16 17,

336

this kind of oxygen-containing group could be formed by carbon-particle oxidation and some could

337

decompose to CO2 at 400°C. This behavior explained why the CO concentration changed slightly at

338

400°C, whereas the CO2 exhibited a significant increase.

16 19

. The C–Hn (alkyl

339

Fig. 11 shows the FCP morphologies during partial oxidation. The initial morphologies (Fig. 11a)

340

showed a flat and non-porous appearance of FCP with small particles on the surface and a Brunaeur–

341

Emmett–Teller (BET) surface area of only 4.18 m2/g (Table 1). The FCP exhibited a poorly developed

342

pore structure as reported by Miguel et al 2. However, a remarkable change in enormous amount of

343

pores or pits arose on particle surfaces with the same shape after 5 h of partial oxidation (Fig. 11b),

344

which was also indicated by the BET surface area of 318.50 m2/g. Subsequently, pores or pits were

345

enlarged gradually and amalgamated to produce larger pores or pits (Fig. 11c) and the BET surface 15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

346

area continued increasing. With pore or pit growth, particles began to crack and form scale-like

347

structures on the particle surface (Fig. 11d), which led to pore destruction and contributed to a

348

reduction in the BET surface area. This phenomenon indicated that the FCP partial oxidation included

349

pore formation, expansion, amalgamation and destruction. For further applications, it will be helpful

350

to decrease the filtration pressure and the FCP after partial oxidation will be easier to removal by

351

introducing a reversed stream (Fig.S3).

352

4. Conclusions

353

The FCP removal by partial oxidation was examined on the surface of filter media by adding a

354

low concentration of O2 to the product gas. The following conclusions were made:

355

1. In the 2 vol.% O2–N2 environment, four sequential reaction stages that included drying,

356

pre-oxidation, complex oxidation and non-oxidation could be defined in the FCP partial oxidation

357

when the temperature increased from 30°C to 800°C. FCP partial oxidation, compared with

358

pyrolysis, was proved to be more effective for the FCP mass loss from 275 to 725°C.

359

2. In the 2 vol.% O2-SPG environment, FCP partial oxidation was feasible because significant FCP

360

were lost. The optimum TF for the FCP filtration–oxidation was 400°C at 1.59 g/cm2, 100 mL/min

361

and 2 vol.% O2. Partial oxidation of the FCP would result in a significant decrease in C–Hn (alkyl

362

and aromatic) groups and an increase in C=O (carboxylic acid) groups, which was determined by

363

Fourier-transform infrared spectroscopy. Partial oxidation of the FCP was by pore formation,

364

expansion, amalgamation and destruction as proven by scanning electron microscopy and BET

365

analysis.

366 367

AUTHOR INFORMATION

368

Corresponding Author 16


Page 16 of 31

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

369

Energy & Fuels

* Telephone: +86-20-37218289. E-mail: [email protected].

370 371

Acknowledgments

372

The authors acknowledge the financial support provided by National Natural Science Foundation

373

of China (Grant Nos. 51676192, 51661145022) and Science and Technology Program of Guangzhou

374

(Grant Nos.201707010237)

375 376

References

377

1.

378

BIOENERG 2013, 52 (3), 54-84.

379

2.

380

of solid particles produced at a biomass gasification plant. BIOMASS BIOENERG 2012, 47 (4), 134-144.

381

3.

Knoef, H., Handbook biomass gasification. The Netherlands: BTG BiomassTechnology Group 2005.

382

4.

Morgalla, M.; Lin, L.T.; Seemann M.; Strand, M. Characterization of particulate matter formed during wood

383

pellet gasification in an indirect bubbling fluidized bed gasifier using aerosol measurement techniques. FUEL

384

PROCESSING TECHNOLOGY 2015, 138, 578-587.

385

5.

386

atmospheric fluidized bed biomass gasifiers. Biomass and Bioenergy 2011, 35, Supplement 1 (0), S71-S78.

387

6.

388

cleaning process for syngas. Fuel 2013, 108 (6), 42-53.

389

7.

390

Filtration. Energy & Fuels 2006, 20 (4), 1629-1638.

391

8.

Woolcock, P. J; Brown, R.C. A review of cleaning technologies for biomass-derived syngas. BIOMASS

Miguel, G. S.;, Domıńguez, M.P.; Hernańdez, M.; Sanz-Pe´rez, F. Characterization and potential applications

Gustafsson, E.; Lin, L.T.; Strand, M. Characterization of particulate matter in the hot product gas from

Sharma, S.D; M. K., McLennan, K.; Dolan, M.; Nguyen,T.; Chase D. Design and performance evaluation of dry

Hurley, J.P; Mukherjee, B. Assessment of Filter Dust Characteristics that Cause Filter Failure during Hot-Gas

Tuomi, S.; Kurkela, E.; Simell, P.; Reinikainen, M. Behaviour of tars on the filter in high temperature filtration 17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

392

of biomass-based gasification gas. Fuel 2015, 139 (139), 220-231.

393

9.

394

candles in gasification environment. J. Eur. Ceram. Soc. 2014, 34 (3), 575-588.

395

10. Bläsing, M.; Schaafhausen,S.; Müller, M. Investigation of alkali induced corrosion of SiC filter candles at high

396

temperature, in gasification environment. J. Eur. Ceram. Soc. 2014, 34 (4), 1041-1044.

397

11. Choi, S.; Oh, K.C.; Lee, C.B. The effects of filter porosity and flow conditions on soot deposition/oxidation and

398

pressure drop in particulate filters. Energy 2014, 77(1), 327-337.

399

12. Liati, A.; Eggenschwiler, P. D. Characterization of particulate matter deposited in diesel particulate filters:

400

Visual and analytical approach in macro-, micro- and nano-scales. Combustion and Flame 2010, 157(9), 1658–1670.

401

13. Molintas, H.; Gupta A. K. Kinetic study for the reduction of residual char particles using oxygen and air.

402

Applied Energy 2011, 88(1), 306-315.

403

14. Narvaéz, I; Orío, A; Aznar, M.; Corella,J. Biomass Gasification with Air in an Atmospheric Bubbling Fluidized

404

Bed. Effect of Six Operational Variables on the Quality of the Produced Raw Gas. Ind. Eng. Chem. Res 1996, 35 (7),

405

2110-2120.

406

15. Zhao, S.H.; Luo, Y. H.; Zhang, Y.L.; Long, Y. F. Experimental investigation of the synergy effect of partial

407

oxidation and bio-char on biomass tar reduction. J. Anal. Appl. Pyrolysis 2015, 112 (3), 262-269.

408

16. Figueiredo, J. L; Pereira, M. F. R; Freitas, M.M.A; Órfão, J. J. M. Modification of the surface chemistry of

409

activated carbons. Carbon 1999, 37 (9), 1379-1389.

410

17. Shen, W. Z.; Li, Z. J.; Liu, Y. H. Surface chemical functional groups modification of porous carbon. Recent

411

Patents Chem Eng 2008, 1 (1), 27-40.

412

18. Zhan, J.; Wang, H. H.; Song, S. N.; Hu, Y.; Li, J. Role of an additive in retarding coal oxidation at moderate

413

temperatures. P COMBUST INST 2011, 30 (2), 2515-2522.

414

19. Yang, H.P.; Yan, R.; Chen, H.P.; Lee, D.H.; Zheng, C. G. Characteristics of hemicellulose, cellulose and lignin

Schaafhausen, S.; Yazhenskikh, E.; Heidenreich, S.; Müller, M. Corrosion of silicon carbide hot gas filter

18


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Energy & Fuels

415

pyrolysis. Fuel 2007, 86 (12-13), 1781-1788.

416

FIGURE CAPTIONS

417 418

FIGURE 1. Schematic diagram of quartz filter reactor.

419 420

FIGURE 2. Effect of TF on gas components in quartz filter reactor with a media: a.

421

without O2; b.2 vol. % O2

422 423

FIGURE 3. Effect of temperature on the gas components in the 2 vol.% O2–N2

424

environment

425 426

FIGURE 4. Thermo-gravimetric behavior of FCP pyrolysis and partial oxidation

427 428

FIGURE 5. Effect of TF on gas components change (a) and O2 consumption after

429

reaction (b) in the 2 vol.% O2-SPG environment

430 431

FIGURE 6. Values of qr and △C at the different TF.

432 433

FIGURE 7. Effect of FCP loading on gas components change (a) and O2 consumption

434

after reaction (b) in the 2 vol.% O2-SPG environment

435 436

FIGURE 8. Values qr (A) and △C (B) at different FCP loading

437 19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

438

FIGURE 9. Effect of residence time on concentration of gas components

439 440

FIGURE 10. The FTIR spectrogram of the FCP after reaction

441 442

FIGURE 11. SEM analysis showing different morphologies in the sample before and

443

after oxidation at 400 ℃（A：the FCP material, B：the FCP after 5 h residence time, C：

444

the FCP after 10 h residence time, D：the FCP after 20 h residence time）

445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 20


Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

461 462 463

TABLE CAPTIONS

464

Table 1 Ultimate, BET analysis and Weight loss of the FCP in reaction

465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483

21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

484 485 486

4 5 3 6 8 11 9

2

7

1

10

N2

O2

487 488

1.Flow meter 2.Massflow controller 3.Gas mixed tank 4.Piezometer 5.Thermocouples 6. Quartz Tube

7.Heater 8.FCP layer 9.Ceramics Filter 10.Condensor 11.Gas chromatography

Gas product A

Fig.1. Schematic diagram of quartz filter reactor.

489 490

22


Page 23 of 31

20 a H 2;

CO2;

CH4;

CO

10

△X（ %）

0

-10

-20 300

350

400

450

500

550

600

Temperature / ℃ 491

20

40

b

H2 ;

CO2;

CO;

O2

CH4

20

10

-20

0

△O2 / %

0

△X / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-40 -10 -60 -80

-20 300

350

400

450

500

550

600

Temperature / ℃ 492 493

Fig.2. Effect of TF on gas components in quartz filter reactor with a media: a. without

494

O2; b.2 vol. % O2 23


Energy & Fuels

0.12 Concentration / mmol●min-1

H2 ;

0.10

O2 ;

CH4 ;

CO2;

CO

0.08 0.06 0.04 0.02 0.00 0

100

200 300 400 Temperature /℃

500

600

495

Fig.3. Effect of temperature on the gas components in the 2 vol.% O2–N2 environment

120

0.2 Partial oxidation Pyrolysis

100

0.1 0.0

80 60 40

-0.1 100

-0.2

98

-0.3

96

-0.4

20 100

100

200

200

300

400

300 400 500 600 Temperaure / ℃

700

Deriv. Weight /%●min-1

496

Weight / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

-0.5 800

497 498

Fig. 4. Thermo-gravimetric behavior of FCP pyrolysis and partial oxidation

499 500 24


Page 25 of 31

a

H2 ;

CO;

CH4;

O2

CO2

-60

10

-70

5

-80

0

-90

-5

-100 300

350

400

△O2 / %

△X / %

15

450

Temperature /℃ 501

Osolid；

b 100

Oleft；

Ogas

80

O2 / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60 40 20 0

300

350

400

450

Temperature / ℃ 502 503

Fig. 5. Effect of TF on gas components change (a) and O2 consumption after reaction (b)

504

in the 2 vol.% O2-SPG environment

505

25


102

80

101

70 60

100 qr/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

50 99 40 98

qr;

△C

30 20

97 300

350 400 Temperature/ ℃

450

506 507

Fig.6 Values of qr and △C at the different TF.

508 509 510 511 512 513 514 515 516 517 518

26


△C/µ mol●min-1

Energy & Fuels

Page 27 of 31

-40

20

a 15

H2 ;

CO;

CH4;

O2

CO2

△X / %

5 -80

△O2 / %

-60

10

0 -5

-100

-10 0.00

0.64

1.28

1.92

2.56

FCP loading / g●cm

3.20

-2

519

b

Osolid ;

100

Oleft ;

Ogas

80

O2 / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60 40 20 0

0.16

0.32

0.64

1.59

3.18

FCP loading / g●cm-2 520 521

Fig.7. Effect of FCP loading on gas components change (a) and O2 consumption after

522

reaction (b) in the 2 vol.% O2-SPG environment

523 27


Energy & Fuels

0.10

102

0.08

qr / %

100 0.06

99 98 97 0.00

0.64

1.28

0.04

△C

qr;

1.92

2.56

PBG load / g●cm

△C / mmol●min-1

101

3.20

-2

524

Fig.8. Values qr and △C at different FCP loading amount.

525

15

H 2;

CO;

CH4;

O2

CO2 experimental value

-20

CO2 calculated value

10

5

-60

0

-5

△O2 / %

-40

△X / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

-80

0

200

400 600 800 Time / min

1000

-100 1200

526 527

Fig.9. Effect of residence time on concentration of gas components.

528 28


Page 29 of 31

FCP-0h

Absorbance / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1700cm-1

2920cm-1

FCP-5h FCP-10h FCP-20h

3500

3000

2000

Wave numbers / cm

1500 -1

529 530

Fig.10.The FTIR spectrogram of the FCP after reaction

531 532 533 534 535 536 537 538 539 540 541

29


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A

B

542

C

D

543 544

Fig.11. SEM analysis showing different morphologies in the sample before and after

545

oxidation at 400 ℃（A：the FCP material, B：the FCP after 5 h residence time, C：the

546

FCP after 10 h residence time, D：the FCP after 20 h residence time）

547 548 549 550 551 552 553 30


Page 30 of 31

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 1 Ultimate, BET analysis and Weight loss of the FCP in reaction

554

Residence

BET（m2/g） Weight loss

Ultimate analysis (wt. %) a

time (h)

555

(wt. %) C

H

N

S

C/H (%)

0

62.15

3.57

0.70

0.71

17.41

4.18

0

5

51.61

2.28

0.80

0.28

22.64

318.50

31.78

10

43.76

2.07

0.93

0.31

21.14

362.09

45.37

20

35.04

1.87

0.96

0.29

18.74

286.21

56.57

a：dry basis

556

31


Partial Oxidation of Filter Cake Particles from Biomass Gasification

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