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Keywords:

  • biomass;
  • NO reduction;
  • tar;
  • experimental study

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL SECTION
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. LITERATURE CITED

The character of biomass tar to reduce nitric oxide (NO) under conditions relevant for the reburning process is investigated experimentally. Flow reactor experiments on reduction of NO by phenol, a model compound of tar from updraft biomass gasification, are conducted from fuel lean to rich conditions, covering temperatures of 1173–1573 K and equivalence ratio of 0.34–1.73. Under the oxygen rich conditions, with the temperature increased, the efficiency of NO reduction was first increased which may be caused by the increasing quantity of hydrocarbon and non-hydrocarbon radicals produced by cracking of phenol. With a further increase in temperature, the thermal crack of phenol was enhanced and less oxygen is needed. The crack products were consumed mostly through reacting with the rich oxygen, and inhibit the NO reduction reactions. Under the fuel rich conditions, the efficiency of NO reduction increased continuously with the temperature because the increased temperature promotes phenol decomposition as well as the reactions of NO reduction by the crack products. NO reduction efficiency decline with the decrease of phenol concentration. The different cracking and polymerization action by phenol and benzene presented the different NO reduction characters of tar. And comparing with other fuels shed light on the effect of tar on NO reduction. © 2014 American Institute of Chemical Engineers Environ Prog, 34: 47–53, 2015


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL SECTION
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. LITERATURE CITED

There has been an increasing interest in biomass fuel because it is renewable and CO2 neutral. Using as fuel it can reduce both NOx and SOx emission in contrast with coal. However, the dispersed distribution and low heat value make biomass unsuitable for using as the main fuel to generate power and heat. Gasification is considered as one of the most promising routes for the utilization of the rich biomass resources in China [1]. What's more, the corrosion and blockage caused by the tar, which was produced during gasification and hard to be efficiently removed, have seriously hindered the development of the technology [2, 3].

Fortunately, tar has been found to have a positive effect on nitric oxide (NO) reduction in the previous study [4]. Under the high temperature environment, tar species will crack to light hydrocarbons and non-hydrocarbons [5-11], such as CO, H2, CxHy, etc., which can reduce NO at different extent. In view of this, the technical routine of gasification syngas reburning was chosen which can solve both tar and NO problems. That is, placing a gasifier directly adjacent to a boiler and gasification syngas in which included all the tar species is input into the boiler as the reburning fuel to reduce NO. From pilot scale experiment, good NO reduction effect (up to 80%) was gained under suitable conditions [12].

The main combustible compositions of biomass syngas include the light gases, such as CO, H2, CH4, other C2[BOND]C3 hydrocarbons, and tar. Among them, the light hydrocarbons (C1–3) which are effective reducing agents to NOx have been investigated extensively [13-17], and the non-hydrocarbons, such as CO and H2, also have been found to have the ability to reduce NOx from research results [18, 19].

NO reduction by tar is studied in this article. In view of the complexity of tar species, model compound should be selected. In previous study, NO reduction character by benzene was studied first considering the benzene ring representing the basic tar structure [20]. To generate real gasification tar, updraft biomass gasifier was build and tested because its conveniences in operation and high efficiency in thermal conversion. Gas chromatography-mass spectrography GC-MS analysis of tar samples from updraft gasifier have shown that phenolic compounds (phenol and its derivates) cover a large part in the whole tar, approximately 40% [12], so phenol was selected as tar model compound in this article. Experimental results of NO reduction by phenol was interpreted by a mechanism combined phenol destruction and NO reduction with the same method like Guarneri et al. [21].

EXPERIMENTAL SECTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL SECTION
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. LITERATURE CITED

Experimental Setup

A tubular flow reactor made of a corundum tube (60 cm in length with 50 cm inside the furnace, 0.8 cm in internal diameter) was used to investigate the characteristics of NO reduction by phenol. The experimental setup (Figure 1) was composed of a feeding system, a reaction system, and a continuous analysis system.

image

Figure 1. The tubular flow reactor system. 1. Phenol generator; 2. pre-heater; 3. tubular furnace; 4. corundum reactor; 5. filter; and 6. FTIR. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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NO (10%), high-purity N2, and O2 (>99.999%) were supplied from the pressured gas cylinders. Phenol is solid at room temperature, so phenol saturator which was heated in a water bath supplied the phenol vapor and high-purity N2 carried the phenol vapor into the pre-heater. To avoid the condensation of phenol vapor, the inlet pipe was maintained at 523 K. The concentration of phenol was measured by a high-performance liquid chromatography (HPLC) (Agilent 1100 series). After mixed and preheated with other gases in the pre-heater, all reactant gas entered into the reactor. The product gases were quenched and measured online by a BRU741 UV spectrometer made by B&W Tek, with resolution of 2 cm−1. Soot and soot precursors generated in the high temperature conditions, so a filter in which equipped with allochroic silica and Whatman filter papers to eliminate water and the condensation products was used to protect the ultraviolet (UV) spectrometer.

The electric-heated furnace provides heat to the reactor. The temperature distribution was measured by Pt–Rh thermocouples along the reactor. From Figure 2, uniform temperature zone was in the middle of the reactor (from 30 to 40 cm) and this zone is considered as the reaction zone.

image

Figure 2. Temperature profiles of the reactor under different reaction temperature.

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Experimental Conditions

Flow reactor experiments on reduction of NO by phenol were conducted from fuel lean to fuel rich conditions (ER = 0.34–1.73), covering temperatures of 1173–1573 K. Considering the NO emission level of a coal-fired boiler in China, the initial NO concentration was set at 1000 ppm. The residence time was kept at 0.13 s by changing the total gas inlet volume ( inline image) according to different reaction temperature. The inlet concentration of phenol was varied to study its influence on NO reduction effect (Table 1).

Table 1. Variables in experiments of NO reduction by phenol
NOinlet (ppm)ERPhenolinlet (ppm) inline image (mL/min)Temperature (K)Residence time ( inline image) (s)
10000.34–1.73320–2285110–5101173–15730.13

The NO reduction efficiency, η, is defined as

  • display math(1)

The bulk equivalence ratio (ER for short), is defined as

  • display math(2)

The nominal gas residence time, τ, is defined as

  • display math(3)

The percentage in the equation is the volume percentage. The subscript inlet, outlet, stoi, means the inlet and outlet of the reactor and the stoichiometric combustion respectively. Vreactor is the volume of reaction zone, inline image is the volume flow of reaction gas before entering the reactor.

The chemical equation for stoichiometric combustion of phenol is:

  • display math(4)

In other words,

  • display math(5)

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL SECTION
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. LITERATURE CITED

Mechanism of NO Reduction by Phenol

Phenol Pyrolysis

Phenol and phenoxy radical play important role in aromatic oxidation process, so there are a few studies have been reported on phenol pyrolysis [22-28]. Experimental results indicate that major intermediates in the phenol pyrolysis are carbon monoxide (CO) and cyclopentadiene (C5H6). Miner species include benzene (C6H6), acetylene (C2H2), naphthalene (C10H8), and methane (CH4) [28]. Destruction of phenol was summarized as Figure 3 from Refs. [22-28].

image

Figure 3. Crack routes of phenol.

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Phenol destruction will most likely occur initially via the thermal decomposition reaction 1 [26], producing phenoxy (C6H5O) and H, via route (A) in Figure 3:

  • display math(R1)

Another possibility for phenol destruction from Horn and Frank's [22] shock tube study is reaction 2, producing C5H6 and CO, via route (B) in Figure 3.

  • display math(R2)

Following initiation, phenol consumption most likely proceeds via reactions with the H atom through reactions 3 and 4, via route (A) and (C) individually.

  • display math(R3)
  • display math(R4)

And OH produced in the displacement reaction will abstract H from phenol to form water, via reaction 5.

  • display math(R5)

The phenoxy radical produced through reactions 1, 3, and 5 decomposes unimolecularly to CO and the cyclopentadienyl radical via reaction 6.

  • display math(R6)
Oxidation of Phenol

In addition to the phenol consumption reactions 1–5 discussed above, the reaction of phenol with molecular oxygen should be considered, also via route (A).

  • display math(R7)

Among the three routes of phenol destruction, routes (A) and (B) are predominant. That is, C5H6 and C6H5O are the main products. In view of C6H5O will most be converted to C5H6 and C5H5 in the next stage, the crack of C5H6 and C5H5 is important [29, 30]. According to Lovell's [23] study, acetylene is the important product of cyclopentadiene pyrolysis.

NO Reduction

Kinetic study on the interaction between hydrocarbon/NO indicates that it is the free radicals, mainly CHi and HCCO [13-19], who are effective reductants that react with NO. For tar, it is the pyrolysis and oxidation intermediates and products that reduce NO. So the crack extent of phenol will influence the NO reduction effect.

Experimental Results

Effect of ER

As is shown in Figure 4, there are two trends of NO reduction efficiency change with ER under the temperature range. Under lower temperatures especially at 1173 K, NO reduction efficiency fall off with ER increase from 0.34 to 1.73. While under higher temperatures such as 1373–1573 K, NO reduction efficiency increase with ER. In other words, oxygen rich environment is favored for NO reduction by phenol at low temperatures, while fuel rich environment is preferred at temperature higher than 1373 K from experimental results.

image

Figure 4. NO reduction efficiency change with ER. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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As has been discussed above, under the reburning conditions, phenol undergo cracking reactions first and the crack products will reduce NO, so crack extent of phenol directly affect NO reduction effect. At low temperatures, the thermal cracking of phenol was not enough. But with the decreasing of ER, in other words, with the oxygen concentration increase, oxidative cracking reactions will be promoted and more products will be produced which can reduce NO then. Brinzsky [18] studied pyrolysis and oxidation of phenol, experimental results showed that the crack products' concentration (such as CO, CH4, C2H2, and C2H4, etc.) increased with the increasing of oxygen concentration (ER from 1.73 to 0.64) at 1170 K.

With temperature increase from 1173 to 1273 K, the thermal cracking reaction enhanced and the effect of oxygen on NO reduction at 1273 K seems not obvious in contrast with 1173 K. When temperature increase to 1373–1573 K, thermal cracking of phenol seems to be sufficient enough. The crack products will be consumed in the combustion reactions when ER < 1 and they will reduce NO mostly under fuel rich environment.

Effect of Temperature

As is shown in Figure 5, when ER = 0.64, NO reduction efficiency increase with temperature increasing from 1173 to 1373 K and then decrease when temperature increase from 1373 to 1573 K. Under the oxygen rich environment, ER = 0.64, the increasing of temperature first promote phenol cracking, but when temperature was high enough (in this experiments more than 1373 K), the crack products will be consumed via reacting with the rich oxygen, and inhibit the NO reduction reactions.

image

Figure 5. NO reduction efficiency change with temperature. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Under the fuel rich environment, such as ER = 1.53 and 1.73, the increasing of temperature promote phenol cracking, causing NO reduction efficiency increase also. When temperature higher than 1373 K, fuel rich environment supply enough crack products which can react with NO and the reduction reaction was enhanced with the temperature increase. So NO reduction efficiency increase continuously with temperature in the temperature range of 1173–1573 K and reached the maximal value 86.3% at 1573 K.

Effect of Phenol Concentration

The inlet concentration of phenol influence the NO reduction effect a lot from experimental results, as shown in Figure 6. With phenol inlet concentration decrease from 1661 to 1084 and 647 ppm at 1473 K, NO reduction efficiency decrease from 75.3% to 61.1% and 42.1%. It can be interpreted that the products and intermediates decrease with the phenol concentration decline.

image

Figure 6. NO reduction efficiency change with inlet phenol concentration.

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Compare with other Fuels

As have been mentioned above, most researches concentrate on NO reduction by light gases (hydrocarbon and non-hydrocarbons), so a comparison was made in this paper to compare NO reduction character by different fuels. NO reduction efficiency change with ER by light gas mixture (H2/CO/CH4/C2H2/C2H4 = 1/1/0.37/0.09/0.18) from reference [19], tar model compound (benzene from reference [13], phenol in the present study), and biomass syngas with tar [31] at a temperature near 1473 K is shown in Figure 7.

image

Figure 7. Comparison of reburning efficiency by different fuels at 1200°C (pw: present work). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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For benzene, the simplest aromatic hydrocarbon, it is easy to form soot under high temperature conditions. The experimental results showed that oxygen rich environment is favored for its destruction and reduce NO. For phenol, with a hydroxyl attached on the benzene ring, its reaction activity is better than benzene, and soot formation is mild in the experiments. So fuel rich environment is helpful to supply hydrocarbons which can reduce NO.

The different cracking and polymerization action by tar species directly influence the NO reduction character from the comparison results. Figure 8 is a simplified reaction scheme of the NO reduction by tar. It can be concluded that: the cracking reactions of tar produce light hydrocarbons and non-hydrocarbons, which are favored agents for reduce NO, so the conditions helpful for tar cracking is preferred. The polymerization reactions of tar generate soot and soot-precursors, although soot also has been found to have NO reduction effect, more residence time is necessary to get a better effect because of the heterogeneous reaction. NO reduction by tar molecular directly play very little role.

image

Figure 8. Simplified scheme of the NO reduction by tar. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The NO reduction efficiencies of the light gas mixture and the real gasification syngas with tar are close under fuel rich conditions (in Figure 7), but with a proper increase in oxygen, the superiority of syngas reburning is clear. The hydrocarbon radicals from the oxidative cracking of tar (which share 30 wt % in the syngas) are thought to be responsible for it.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL SECTION
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. LITERATURE CITED

Experimental study on NO reduction by phenol, a model compound of tar from updraft biomass gasification, was conducted in a tubular flow reactor in the temperature range of 1173–1573 K. Effect of ER and temperature on the NO reduction was investigated in detail. For phenol, a relative high reaction activity species of tar, NO reduction character can be concluded that: under the oxygen rich conditions, with the temperature increased, the efficiency of NO reduction was first increased which may be caused by the increasing quantity of hydrocarbon and non-hydrocarbon radicals produced by cracking of phenol. With a further increase in temperature, the thermal crack of phenol was enhanced and less oxygen is needed. The crack products were consumed mostly through reacting with the rich oxygen, and inhibit the NO reduction reactions. Under the fuel rich conditions, the efficiency of NO reduction increased continuously with the temperature because the increased temperature promotes phenol decomposition as well as the reactions of NO reduction by the crack products.

NO reduction efficiency decline with the decrease of phenol concentration.

The different cracking and polymerization action by phenol and benzene presented the different NO reduction characters of tar. And comparing with other fuels shed light on the effect of tar on NO reduction.

ACKNOWLEDGMENT

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL SECTION
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. LITERATURE CITED

The authors gratefully acknowledge the support from the National High Technology Research and Development Program of China (863 Program) ((No.2008AA05Z312)).

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL SECTION
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. LITERATURE CITED
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