Tar Removal from Biomass-Derived Fuel Gas by Pulsed Corona

Ind. Eng. Chem. Res. 2004, 43, 1649-1658

1649

Tar Removal from Biomass-Derived Fuel Gas by Pulsed Corona Discharges. A Chemical Kinetic Study S. A. Nair,*,† K. Yan,† A. J. M. Pemen,† E. J. M. van Heesch,† K. J. Ptasinski,‡ and A. A. H. Drinkenburg‡ Faculty of Electrical Engineering and Faculty of Chemical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Tar (heavy hydrocarbon or polycyclic aromatic hydrocarbon) removal from biomass-derived fuel gas is one of the biggest obstacles in its utilization for power generation. We have investigated pulsed corona as a method for tar removal. Experimental results have indicated the energy density requirement to be ∼400 J/L for naphthalene removal (model tar compound) from synthetic fuel gas (CO, CO2, H2, CH4, N2) at a temperature of 200 °C. For the process to be competitive and economical, the energy consumption needs to be reduced. This requires an understanding of the chemical processes involved. In previous studies, we have investigated the primary processes at a temperature of 200 °C. The present study aims to identify the main reactions involved by means of a sensitivity analysis. Results indicate that, apart from CO, which apparently seems to be the most terminating species for the reactive O radical at a temperature of 200 °C, formaldehyde, formed during radical reactions, also acts as a major quencher. Introduction Biomass is increasingly being investigated to satisfy the growing need for energy. Although it has been used as a fuel since the last century, only recently has it gained prominence due to increasing awareness about the problems associated with global warming. Biomass offers an excellent alternative to conventional fuels such as coal, natural gas, and petroleum. Apart from this, it enables us to recycle CO2, which is a known greenhouse gas. One of the more efficient techniques to utilize biomass as a fuel is gasification, which converts it into low calorific value gas (4-5 MJ/N‚m3). On the other hand, gasification also produces undesirable byproducts such as tars (PAHs) and particulates, which need to be removed to facilitate the use of the gas for various purposes, such as for power production or for synthesis by Fischer-Tropsch. Today, tar removal remains the main bottleneck for efficient use of biomass. Various gas-cleaning options exist, such as operating the gasifier at optimum conditions or incorporating catalysts within the gasifier to reduce the tar content. These options are being investigated.1 However, they tend to be fuel specific, which limits the flexibility of operation for the gasifier considering the immense variations in biomass fuels. Apart from this, there are other possibilities to physically remove tars, by means of condensing them and scrubbing the gas. These methods add to the problems rather than abating them, since it creates an additional wastewater stream which also poses a problem for cleaning. Specially designed reactors are needed for abatement of tar-containing solution.2 A better solution should be to separate and recycle tars, which is also being looked at.3 The existing methods to treat the gas * To whom correspondence should be addressed. Phone: +31 40 247 4494. Fax: +31 40 245 0735. E-mail: S.A.Nair@ tue.nl. † Faculty of Electrical Engineering. ‡ Faculty of Chemical Engineering.

outside the gasifier in a secondary reactor are by catalytic or by thermal cracking methods. Thermal cracking requires temperatures in the order of 12001400 °C.4 One drawback of thermal cracking is that part of the enthalpy of the gas is lost. For catalytic gas cleaning,5 dolomites, olivine, steam re-forming catalysts, etc., are the common candidates. This process requires high temperatures (850-900 °C), which are close to the gasifier exit temperatures. Critical issues are long-term reliability as well as catalyst lifetime. Nonthermal plasma techniques offer an alternative to existing methods, where radicals produced by energetic electron-molecule collisions initiate reactions. By means of plasma techniques (i.e., nonthermal), reactions can be initiated at any temperature. Feasibility studies were carried out where a nonthermal plasma reactor was placed after a gasifier to demonstrate that tar and dust can be removed simultaneously.6,7 Laboratory investigations have quantified the energy requirements for removal of “model” tar components. To promote industrial applications, it has to overcome stiff barriers in terms of economics over other methods. With regard to the present application, i.e., at high temperatures as well as high dust concentration, pulsed corona would be the most ideal plasma technique for fuel gas cleaning. To reduce the investment costs, significant milestones have been attained for development of reliable and comparatively less costly pulsed power sources8-11 and at the same time overcoming the engineering problems for running a corona plasma system until 800 °C.12 The operating costs for the gas cleaning is ∼20% of the final electrical output from biomass gasification at the investigated temperatures of 200 °C, and the goal is to reduce it to 5% of the electrical output. One of the most widely investigated processes by use of nonthermal plasma techniques would be combined NOx and SOx removal.8,13 In addition, this would be the most likely process to bridge the gap between research and industry application. One of the crucial turning points regarding research in this area was the understanding of the process chemistry involved. Fundamen-

10.1021/ie034066p CCC: $27.50 © 2004 American Chemical Society Published on Web 02/25/2004

1650 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

tal reactions were outlined14 and thereby energy consumption could be reduced. The present work relates to kinetic modeling of tar removal from biomass gasification and aims at developing a comprehensive yet brief set of reaction schemes, largely governing the process. Method Description In principle, the nonthermal plasma chemical process involves active generation of electrons, metastables, and ions and their induced chemical reactions. All the physical processes contribute to generation of radicals. All the radicals and excited species that are generated by means of collisions between energetic electrons (>5 eV) and molecules and those that survive the quenching process will act as reaction initiators for the purpose to be achieved. To depict an actual process, one needs to have a “perfect” streamer model to describe the distribution of the species produced in both the axial and radial directions. Unfortunately, this is far from reality. Hence, in this paper, a fitting parameter is used to evaluate the amount of radicals necessary for the desired conversion of tar as observed experimentally. Radical production in reality is pulsed; that is, radical generation takes place during a short period between two pulses. On the other hand, it can also be said that radical concentration decays to zero between two pulses. Another simplification is to have a continuous model instead of a discrete model.15 Here radical production is incorporated in the rate constant of the process. Typical radical lifetime is on the order of 10-100 µs, which is equivalent to the time scale for radical diffusion from the streamer channel. Therefore, both phases in the reactor, viz. the plasma phase and the bulk gas phase, are well mixed; hence, the model can be termed as a pseudohomogeneous model. The reactor used for the experiments is a tubular flow reactor or a differential reactor and it forms a part of a closed-loop system.6,16 The overall configuration represents a wellmixed system. We assume the following: (1) Radicals produced are well mixed with reactants. (2) Radicals once produced are uniformly distributed within the whole volume, i.e., a zero-dimensional model. (3) The chemistry of the process is solely described by radical behavior. (4) Initial radical production is directly proportional to the related bulk gas composition. For description of the chemical reactions, 121 reactions and 20 species are taken into account: viz. O, OH, H, CO, N, CH3, O2, H2O2, HO2, HCO, CH4, CH2O, naphthalene, toluene, phenol, naphthyl, naphthoxy, 2-βnaphthol, indenyl, and indene. The aim of the calculation is 2-fold: (1) to develop a chemical kinetic reaction scheme; (2) to identify termination and competitive reactions. The general reaction scheme and the format of the mass conservation equations are as follows:

dt and otherwise

)

dt

∑j kji[Xi][Xj]

(4)

where GR is the radical production (mol J-1 m-3), expressed as KRλR[M], KR is the dissociation constant (J-1), M is the bulk gas concentration (mol/m3), λR is the gaseous volume fraction, Ep is the energy per pulse (J/pulse), f is the pulse repetition rate (pulse/s), k is the reaction rate constant (s-1), [Xi], [Ri] is the species concentration (mol/m3), and [A] is the gas component such as H2, CO2, or H2O. According to the above hypothesis we have,

GO ) KCO2λCO2[M] GOH ) KH2OλH2O[M] GH ) KH2λH2[M] + KH2OλH2O[M] Equation 3 presents a mass balance of radicals. The first term on the right-hand side indicates the production term, and the second term represents the formation or consumption of radicals via reactions. Equation 4 represents the general scheme for secondary radicals during corona processing. KR is introduced as a fit parameter. Its value is adopted to achieve a good fit between the calculated and the experimental value. The G value or the radical yield can then be evaluated according to

G′ (molecules/100 eV) ) 1.6 × 10-17GRV

(5)

where V is the volume of the reactor (m3) and the conversion factor is 1 eV ) 1.6 × 10-19 J. The experiments are performed for near ∼99.99% conversion, and the treatment time is determined by eq 6

t)

ED V Epf

(6)

where t is the treatment time, ED is the energy density (J/L), and V is the reaction volume (L). Substituting time with the corona energy density, eqs 3 and 4 can be expressed as

d[Ri] dED

V ) GRV -

d[Xi]

∑j kij[Ri][Xj] Epf

∑j kji[Xi][Xj]

(7)

V )

dED

Epf

(8)

AfR

(1)

R + X f products

(2)

As a function of time and the energy density, radicals and gaseous concentrations can be calculated via eqs 3 and 4 and via eqs 6-8, respectively. Naphthalene removal can therefore be estimated as a function of energy density. This is compared with the experimental results. The model-experimental data fitting is done by the method of least squares.

(3)

Experimental Conditions

For the initial radical

d[Ri]

d[Xi]

) GREpf -

∑j kij[Ri][Xj]

A schematic of the experimental arrangement is shown in Figure 1. The experiments are performed in

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1651

Figure 1. Experimental setup.

a reactor with a L (length)/D (diameter) ) 12, (L ) 3 m, D ) 25 cm). The reactor is a part of a circulation loop, and gases are circulated within the reactor until the desired conversion is achieved. The total volume of the system is 300 L. A weighed sample of tar can be vaporized into the system by means of a boiler, and its concentration can be monitored by means of Fourier transform infrared spectroscopy. The system can be filled with the gas of required compositions by monitoring their partial pressures. Tar removal experiments are performed in varying gas mixtures, to have an insight into the mechanisms. Fuel gas, similar to the composition (Table 1) obtained in a typical gasification process, is used for determining the energy requirement for tar removal under “real” conditions. Details about the diagnostics used for the measurement and the procedures used can be found in the relevant literature.6,16 Results (a) Estimation of G Value or the Radical Yield. (a) For a N2 + CO2 Gas Mixture. As investigated in refs 16 and 17, the primary process for naphthalene removal is via the dissociation of CO2, followed by the

Table 1. Experimental Conditions Experimental Operation: temperature 200-210 °C pressure 1 bar biogas composition tar components CO CO2 H2 CH4 N2

20% 12% 17% 1% rest

naphthalene toluene phenol

Batchwise pulse power properties 70-80 kV peak voltage 20 ns rise time, 100 ns pulse width 50-80 MW peak power 1-1.2 J/pulse 50 pulses/s 80% efficiency (from mains to corona)

oxidation of naphthalene. The initial step is therefore to estimate the radical yield in a gas mixture of N2 + CO2. The parameter fitting was done by the method of least squares. For O radical generation the fit parameter is estimated to be

KCO2 ) 2.417 × 10-7

and

GO ) 3.75 × 1017 molecules m-3J-1 and the radical yield can be calculated as

G′O (G value) ) 1.134 molecules/100 eV

1652 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

Figure 2. (a) Comparison between experimental and calculated results in terms of energy density. (b) Naphthalene removal in terms of square root of the energy density in a N2 + CO2 gas mixture.

Using the above G value, naphthalene removal was calculated and compared with experimental results as shown in Figure 2a. Calculations do agree with the experimental results, where a linear relation describes the removal during the initial phase. Reactions under such conditions can be evaluated by use of the global corona plasma kinetic model.8 This model is an efficient tool, since from the measured component removal characteristics, it is possible to have direct insight into the radical behavior. As is described in the model, a specific process has its own characteristic behavior with respect to the energy requirement. The process shows linear or nonlinear radical terminations by its dependency on the energy density. Figure 2 (b) re-plots the results in terms of the square root of the energy density. It indicates nonlinear terminations play a dominant role. For the initial phase (Figure 2a), the linear relation indicates an ideal case for the removal where the dominant mechanism is radical attack on the targeted molecule. However, as the concentration of tar component starts to decrease, radical-radical termination starts to dominate and then it causes the nonlinear dependency. This is the case when the bulk CO concentration or the amount produced by the dissociated CO2 is too high, leading to termination of the reactive O species. Initial radicals are mainly generated around streamer heads during their propagation. The streamer head and its propagation velocity are around 100 µm and 107108 cm/s,18 respectively. A high-density O radicals is generated in the N2 + CO2 mixture by decomposing CO2 to CO and O, where CO and O are generated simultaneously. O radical termination and utilization are determined by the following three competing reactions:

O + naphthalene f products k ) 2.32 × 10-11 exp(-7500/(RT)) (104) CO2 f O + CO O + CO + M f CO2 + M

(a) (18)

The characteristic time for radical generation is in the order of 0.1-1 ns. However, reaction time scales are of the order of 10-100 µs, which is equivalent to the time for radical diffusion. Evidently, reactions take place both in and out of the channel. When the concentration of naphthalene is high enough, reaction 104 is dominant; otherwise, termination reactions a and 18 are important. Because in such a gas mixture, CO and O are

Figure 3. Comparison between experimental and calculated results for a gas mixture of N2 + H2.

generated simultaneously, the net effects of the reactions 18 and a are equal to radical-radical terminations. The simple model describes the process where linear terminations play an important role. In cases where nonlinear terminations play a role (Figure 2b), streamer distribution also needs to be included. The difference between the calculated and the experimental results as seen in Figure 2a thus may be caused by ignoring the time and spatial distributions. (b) For a N2 + H2 Gas Mixture. Naphthalene removal in the N2 + H2 mixture also shows the same characteristic behavior of nonlinear terminations (Figure 3). Radical yield for H species can be evaluated in a similar approach as described above. The fit parameter was calculated to be KH2 ) 1.915 × 10-7 and GH ) 2.971 × 1017 molecules m-3 J-1. Therefore, the radical yield can be estimated to be

G′H (G value) ) 0.898 molecules/100 eV The deviations from the experimental results can result from the same reasons as mentioned in the above section. (c) For a N2 + CO2 + H2 Gas Mixture. In the case of this mixture, it was observed experimentally16,17 that H2 seems to play a less pronounced role for naphthalene removal. Radical yield in such a gas mixture should be similar to that for a real fuel gas mixture. The primary mechanism is driven by CO2 dissociation. The G value for O radicals can be expected to be the same. Hence, the parameter that needs to be investigated would be

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1653

Figure 4. Comparison between experimental and calculated results for a gas mixture including both H2 and CO2.

Figure 5. Effect of terminations by H2 for a gas mixture consisting of N2, H2, and CO2.

the radical yield from H2. Comparing calculated and experimental results (Figure 4), the fitting parameter is evaluated as

KCO2 ) 2.417 × 10-7

and

GO ) 3.75 × 1017 molecules m-3 J-1 KH2 ) 1.5 × 10-9

and

GH ) 2.327 × 1015 molecules m-3 J-1 Figure 4 as well indicates the effect of H2 concentration. Increase of H2 concentration leads to increase in the energy requirement for the removal of naphthalene. This is due to O radical consumption via the following reaction

O + H2 f OH + H

(21)

Figure 5 shows the same calculation as presented in Figure 4, but without including reaction 21. As can be seen, without reaction 21, no effect of termination caused by H2 can be seen. Apparently, the positive influence of generation of secondary radicals such as OH and H does not balance the loss of O radicals, which is finally terminated to H2O. The following factors lead to the experimental observation of Figure 4: (a) unfavorable kinetics for removal of naphthalene by H or (b) termination of generated radicals by CO resulting from

Figure 6. Effect of termination by CO.

CO2 dissociation; (c) nonlinear radical terminations of produced OH and H to H2O formation or a combination of the above. Termination by CO may be substantially weaker in the present case, as seen in Figure 2a, where the effect was dominant for the last fraction of naphthalene removal. Reaction 21 represents the main termination pathway for O radicals (Figure 5) in the present gas mixture. Increase of H2 concentration leads to termination of O radicals and its further conversion to H2O (plasma-assisted combustion via OH and H radicals). The respective G values can be evaluated as 1.134 molecules/100 eV (for O radicals) and 0.007 molecules/ 100 eV (for H radicals). In such a gas mixture, CO2 seems to be selectively dissociated at the conditions of the experiment. (d) Effect of CO. Figure 2 depicts an ideal condition for naphthalene removal, i.e., in the absence of any terminating species. However, as can be seen from Figure 2 and Figure 6, the energy requirement becomes higher in the real case, and this largely is the effect of terminating species, by background bulk molecules. Figure 6 indicates the effect of CO concentration, which is largely responsible for the termination of the reactive O species. Calculations show a trend similar to that observed experimentally; however, it does not show the same sensitivity to CO concentration. Experimental investigations on the effect of CO showed that 2% CO can lead to significant terminations than can be seen from the calculated results. (e) For a N2 + CO2 + CO + H2 Mixture. From an energy density point of view, processes with or without H2 do not differ much, for naphthalene removal and estimation of radical generation. However, from a chemical processes point of view, they are different. Figure 7 indicates the effect of increasing H2 concentration for a N2 (10%) + CO2 (10%)+ CO (10%) mixture. As H2 concentration increases, more radical quenching occurs now by way of both H2 (by reaction 21) and CO (by reaction 18). Apart from that, secondary radicals such as OH are also terminated by way of CO.

CO + OH f CO2 + H

(32)

It is well known from combustion research19,20 as well that OH radical quenching by CO is higher as compared to O radical quenching. The positive influence of generation of H radicals is not seen, apparently because of its unfavorable kinetics for naphthalene removal.

1654 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

Figure 7. Effect of H2 concentration for a gas mixture of N2, CO, CO2, and H2.

Figure 8. Experimental and calculated results for naphthalene removal from a fuel gas.

Verification of Parameters for a Fuel Gas Mixture. Using the above estimated G values, calculations are performed for the case of the fuel gas mixture of 12% CO2, 20% CO, 17% H2, 1% CH4, and the rest N2. Figure 8 indicates the calculated and the experimental results for removal of naphthalene. The estimated G values and the proposed chemical kinetics adequately describe the process of naphthalene removal from the fuel gas mixture. It is clear that the calculations describe the relative influence of the different radicals on naphthalene during corona processing

Figure 9. Naphthalene decomposition scheme.

in a fuel gas mixture. The relevant kinetic scheme can also be studied by means of sensitivity analysis, which was the main aim of this investigation. The model for naphthalene removal was completed by inclusion of a decomposition scheme as shown in Figure 9, which is proposed according to the product distribution observed during experiments.16,17 Byproducts were mainly formed by an oxidation mechanism. Intermediate compounds seen in Figure 9 lead to byproduct formation of naphthalenedione and phthalic anhydride, which was observed by GC/MS analysis, as well.16 The main path for ring opening is via naphthoxy formation and its decomposition to indenyl via a thermal mechanism, which largely governs the decomposition scheme. Naphthoxy formation can be initiated via different steps, by H attack followed by OH substitution. The most favorable pathway is, however, direct attack by an O species. As can be seen, a higher H concentration is beneficial (in terms of initial attack); at the same time, it leads to naphthalene formation by back reaction with β-naphthol. The estimated value of the parameters was also compared with the experimental results for removal of toluene and phenol. The experiments are performed under the same conditions as that for naphthalene. Figure 10 shows their comparison. The calculations show good agreement for both of them. In the case of phenol, deviations are seen for the last remaining fraction. This may be due to one of the following factors or a combination of both: (a) phenol removal due to thermal dissociation; (b) chemical reactions to describe phenol removal may not be complete. Theoretical investigations on the relative influence of the different radicals on naphthalene removal during corona processing in the fuel gas mixture lead further to a sensitivity analysis on the kinetics. Reaction Path Investigations Naphthalene removal by pulsed corona discharges, as indicated above, is a radical-dominated process. From the above results, the importance of the specific radicals is known. However, the specific reactions that influence the radical concentrations to an extent to influence naphthalene removal need to be investigated. This is done by means of linear sensitivity analysis.21 Here each reaction rate constant is increased by a factor (results

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1655

Figure 10. Comparison of experimental and calculated results for removal of (a) toluene and (b) phenol.

in terms of the energy density.

S ) η - η0

Figure 11. Relative effects of various reactions on naphthalene removal for a 10 times increase in each reaction rate constant.

where η0 is the fraction removed at 400 J/L, η is the fraction removed at 400 J/L after the reaction rate constants were changed by 10 times its initial value, amd S is the coefficient. The coefficient S indicates the influence of a particular reaction rate on the removal and therefore the energy density. This gives a clue as to which reactions need to be controlled for improving the process. Although the analysis is also very sensitive to the proposed G values and the corona energy density, the above derived or estimated G values and an energy density of 400 J/L were used in the analysis. The following reactions are found to be more sensitive to the overall process of naphthalene removal.

O + CO + M f CO2 + M

are included when rate constants are changed by a factor 10), and its influence on naphthalene removal is obtained. The present approach is used so as not to influence the radical production terms by gas discharge itself, since this was estimated from experimental results. To represent the results of this investigation, instead of a sensitivity coefficient, a parameter is defined that indicates the effect on naphthalene removal

(18)

O + H2 f OH + H

(21)

O + CH2O f HCO + OH

(26)

HCO + HCO f CH2O + CO

(53)

HCO + M f H + CO + M

Figure 12. Main reaction pathways for reactive radicals in corona processing of fuel gas for naphthalene removal at 200 °C.

(9)

(54)

H + CO + M f HCO + M

(4)

H + CH2O f HCO + H2

(11)

The effects of the above sensitive reactions on naphthalene removal are shown in Figure 11. The reactions 18, 26, 21, 53, and 4 show negative influence on naphthalene removal, since the fraction remaining is higher as compared to the case where the values of the rate constants are unchanged. Reactions 11 and 54 show a positive influence on removal. The effect of reaction 18 and 21, in fact, can be directly observed from the experiments. Figure 11 indicates reaction 26 dominates O radical termination. Although it produces reactive OH radicals, CO in turn consumes this via reaction 32. Hence, reactions that influence the concentration of terminating species CH2O exhibit sensitivity toward naphthalene removal. Thus, reaction 53, which leads to formation of terminating species such as formaldehyde as well as CO, causes a negative influence by reducing the O radical density. Reaction 11, which consumes a certain fraction of CH2O, by

1656 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 2. Appendixa k ) A 0‚

( ) ( T 298

n

Ea exp R*T

)

no. 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 61 62 63 64 65 67 68 69 70

H H H H H H H H H H H H H H H H O O O O O O O O O O O O O H O CO CO CO CO OH OH OH OH OH OH OH OH OH OH OH OH OH H2O2 H2O2 HCO HCO HCO HCO CH4 CH2O H2O CO2 N O2 O2 O2 O2 O2 O2 O2 H2 H2 H2

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

O2 H O CO OH O2 HO2 H2O2 HCO CH4 CH2O H2O CO2 N N2 HCO O CO OH O2 H2 HO2 H2O2 HCO CH4 CH2O H2O CO2 N OH HCO OH O2 HO2 N OH O2 H2 HO2 H2O2 HCO CH4 CH2O N N OH OH

+ + + + + + +

HCO M CH4 H2O HCO M N

+ + + + + + + + + + + + +

N N N H2 H2O2 HCO CH4 CH2O H2O N HO2 H2O2 HCO

+ +

M M

+

M

+ +

M M

+

M

+ +

M M

+ +

M M

+

M

f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f

HO2 H2 OH HCO O OH OH HO2 CO CH3 HCO OH CO NH NH CH2O O2 CO2 H O3 OH OH HO2 H CH3 HCO OH CO NO H2O CO CO2 CO2 CO2 CN H2O O H H2O H2O CO CH3 HCO NH NO H2O2 H2O O CH2O OH CH2O CH2O CH2O H HCN HCO OH CO N2 H HO2 CO CH3 HCO HO2 NO HO2 H2O CH2O

reaction with H radicals, shows a positive effect on naphthalene removal. Hence, a higher concentration of

+ + + + + + + + + + + + +

M M H2 O OH H2 H2 H2 H2 H2 OH M N

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

M M O2 M H O2 OH CO2 OH OH OH O2 M M OH H O OH O O HO2 H 2O O2 HO2 H2O H2 O H 2O O H M M H HO2 OH CH3 OH CO CO H2 + H NH NO M HO2 HO2 HO2 HO2 HO2 OH O H OH H

H

Ao (cm3/mol‚s)

n (temp exp)

4.08 × 10-32 9.10 × 10-33 4.36 × 10-32 5.29 × 10-34 6.86 × 10-14 1.66 × 10-9 2.81 × 10-10 8.00 × 10-11 2.01 × 10-10 9.86 × 10-13 8.72 × 10-12 5.16 × 10-12 2.51 × 10-10 5.02 × 10-32 8.60 × 10-11 7.77 × 10-14 5.21 × 10-35 6.50 × 10-33 4.33 × 10-11 3.39 × 10-34 1.52 × 10-13 2.91 × 10-11 1.42 × 10-12 5.00 × 10-11 8.75 × 10-12 1.78 × 10-11 1.25 × 10-11 2.81 × 10-11 5.02 × 10-38 4.38 × 10-30 5.00 × 10-11 5.40 × 10-14 4.20 × 10-12 2.51 × 10-10 3.84 × 10-9 1.02 × 10-12 3.70 × 10-11 9.40 × 10-13 8.05 × 10-11 2.91 × 10-12 5.00 × 10-11 2.77 × 10-13 4.75 × 10-12 1.88 × 10-11 4.70 × 10-11 1.05 × 10-32 6.87 × 10-31 4.00 × 10-9 1.69 × 10-13 2.03 × 10-3 1.36 × 10-13 8.54 × 10-13 3.00 × 10-11 1.03 × 10-9 2.51 × 10-14 1.69 6.03 × 10-11 3.20 × 10-13 1.38 × 10-33 2.41 × 10-10 9.00 × 10-11 8.50 × 10-11 6.71 × 10-11 3.40 × 10-11 7.72 × 10-12 2.36 × 10-11 5.00 × 10-11 4.00 × 10-11 2.66 × 10-13

-0.8 -1.3 -1 0 2.8 -0.9 0 0 0 3 1.77 1.9 0 0 0.5 0 0 0 -0.5 -1.2 2.8 0 2 0 1.5 0.57 1.3 0 0 -2 0 1.5 0 0 0 1.4 0 2 -1 0 0 2.4 1.18 0.1 0 -0.76 -2 0 0 -4.86 2.85 1.35 0 -1.1 0 -6.9 1.2 0 0 0 0 0 0 0 0 0 0 0 2

Ea (J/mol) 0 0 0 -3076 -16213 -72752 -3658 -33258 0 -36667 -12555 -76992 -110583 0 -593653 18957 7483 -12555 -249 0 -24777 1663 -16629 0 -36002 -11557 -71504 -220334 0 0 0 2079 -199547 -98942 -299321 1663 -220334 -12389 0 -1330 0 -8813 1871 -88965 0 0 0 -415724 -29018 -222828 -93954 -108920 0 -71172 0 -404083 -160469 -14218 4182 -236963 -166289 -7067 -237794 -162964 -310130 -44233 -108920 -16629 -74581

H radicals will benefit the process. Too high a concentration, however, leads to more HCO formation via

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1657 Continuation of Table 2, the Appendix no. 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107b 108b 109b 110b 111b 112b 113b 114b 115b 116b 117b 118b 119b 120b 121b

H2 HO2 HO2 HO2 HO2 HO2 HO2 HO2 HO2 HO2 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 OH OH OH OH H H H H H O O O H H O2 OH CH3 H H OH H O H OH O H

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

N H 2 O2 HCO CH4 CH2O H2O N M HO2 H O H OH O2 HO2 HO2 CH3 OH CO H2 HCO H2O N naphthalene toluene phenol toluene naphthalene toluene toluene phenol phenol naphthalene phenol toluene naphthalene naphthyl naphthyl naphthyl naphthyl naphthoxy 2-β-naphthol 2-β-naphthol 2-β-naphthol naphthoxy indenyl indenyl indene indene indene

+

M

+

M

f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f

NH2 OH OH CH3 HCO OH NO H H2O2 H2 CH2O CH4 CH3O CH2O CH3O CH4 C2H6 CH4 CH3CO CH4 CH4 CH4 products naphthyl benzyl phenoxy products products H2 benzene H2 OH naphthoxy products products naphthyl naphthalene naphthoxy naphthoxy 2-Me naphthyl 2-β-naphthol naphthalene naphthoxy naphthoxy indenyl C6H5CHCH indene indenyl indenyl indenyl

+ + + + + + + + + + + + + + +

H2O H H2O2 H2O2 H2O2 OH O2 O2 O2 H M H OH OH O2

+ + + + +

O M H CO OH

+ + +

H2O H2O H2O

+ + + + +

benzyl CH3 phenoxy benzene H

+

H2

+ + +

O H H

+ + + + +

OH H2O H2 CO CO

+ + +

H OH H2

+ +

O2 CO2

+

M

Ao (cm3/mol‚s)

n (temp exp)

Ea (J/mol)

1.00 × 10-26 5.00 × 10-11 5.00 × 10-11 3.01 × 10-13 3.30 × 10-12 4.65 × 10-11 2.19 × 10-11 2.41 × 10-8 3.01 × 10-12 1.10 × 10-10 1.30 × 10-10 6.01 × 10-29 1.60 × 10-10 5.65 × 10-13 3.30 × 10-11 5.99 × 10-12 4.38 × 10-11 3.22 × 10-14 7.83 × 10-29 2.52 × 10-14 2.01 × 10-10 1.20 × 10-14 4.30 × 10-10 9.96 × 10-13 2.56 × 10-12 4.35 × 10-13 6.74 × 10-13 4.15 × 10-10 2.14 × 10-13 9.60 × 10-11 1.91 × 10-10 1.41 × 10-11 2.32 × 10-11 2.82 × 10-11 5.23 × 10-12 7.55 × 10-22 1.66 × 10-10 8.31 × 10-13 3.32 × 10-11 1.69 × 10-10 1.66 × 10-10 1.18 × 10-11 4.90 × 10-18 2.62 × 10-11 1.23 × 10-12 1.66 × 10-10 1.66 × 10-10 5.69 × 10-15 3.01 × 10-11 3.64 × 10-18

0 0 0 0 0 0 0 -1.18 0 0 0 -1.8 0 0 0 0 -0.4 2.2 -7.56 3.12 0 2.9 0 2 1 2 2 0 3.44 0 0 0 0 0 1.21 3.3

0 0 0 -77740 -48806 -137189 0 -202873 0 -8896 0 0 0 -37415 0 0 0 -18624 -45646 -36417 0 -62192 -3492 -8057 -3658 5488 91.46 -66944 -13054 -33840 -51882 -24445 -7500 -12804 -10476 -23822.892

-0.3

548.4708

0 2 0 0

-22566.852 5493.0816 -25522.4 -183591.18

1.18 0 1.77

18714.996 -12895.344 -12560

a Reference (for all kinetic data unless otherwise mentioned): http://kinetics.nist.gov/chemistry (last cited: Dec 2002). b Kinetic equation k ) ATn exp(-E/RT), Reference: http://web.mit.edu/anish/www/mechanismsymp2002.doc (last cited: May 2003).

reaction 4, which contributes to more terminating species by way of reaction 53. The equilibrium between reactions 54 and 4 apparently seems to control HCO concentration that in turn influences formaldehyde formation and consumption. Based on the above discussions, a complete scheme for corona processing in a fuel gas mixture can now be established at the relevant conditions of the experiment as shown in Figure 12. Conclusions The paper proposes a chemical kinetic model for corona processing of fuel gas. Within the experiemental conditions, the following can be concluded: (a) The radical yield for O radicals in fuel gas was estimated to be 1.134 molecules/100 eV and that for H radicals to be 0.007 molecules/100 eV.

(b) The main reaction pathway for tar removal is via oxidation.

O + naphthalene f products (c) The terminating species identified are CO and CH2O generated by the secondary radical processes. The dominating termination reactions are

O + CO + M f CO2 + M O + CH2O f HCO + OH Acknowledgment The author acknowledges the support of SDE (Dutch foundation for sustainable energy) and the Dutch Energy Research center, ECN. We are grateful to Dr. E. M. van Veldhuizen, Faculty of Applied Physics,

1658 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

Eindhoven University of Technology, Dr. E. A. Filimonova, Institute for High Tenmperatures, Russian Academy of Sciences, Moscow, Russia, and the reviewers for their suggestions and fruitful comments during the preparation of the manuscript. Literature Cited (1) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. A review of the primary measures for tar elimination in biomass gasification processes Biomass and Bioenergy 2003, 24 (2), 125-140. (2) http://www.volund.dk. (3) Bergman, P. C. A.; van Paasen, S. V. B.; Boerrigter, H. The novel “OLGA” technology for complete tar removal from biomass producer gas, Paper presented at Pyrolysis and Gasification of Biomass and Waste, Expert Meeting, Strasbourg, France, 30 September-1 October 2002. (4) Jess, A. Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels. Fuel 1996, 75 (12), 1441-1448. (5) Simell, P. Catalytic hot gas cleaning of gasification gas. Helsinki University of Technology, Espoo, Finland, 1997. (6) Nair, S. A.; Yan, K.; Pemen, A. J. M.; van Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. Tar removal from biomass derived fuel gas by pulsed corona discharges. Fuel Process. Technol. 2003, 84 (1-3), 161-173. (7) Van Heesch, B. E. J. M.; Pemen, G. U. A. J. M.; Yan, K.; Van Paasen, S. V. B.; Ptasinski, K. J.; Huijbrechts, P. A. H. J. Pulsed Corona Tar Cracker. IEEE Trans. Actions Plasma Sci. 2000, 28 (5), 1571-1575. (8) Yan, K. Corona Plasma Generation, Eindhoven University of Technology, Eindhoven, The Netherlands, 2001. (9) Yan, K.; Smulders, H. W. M.; Wouters, P. A. A. F.; Kapora, S.; Nair, S. A.; van Heesch, E. J. M.; van der Laan, P. C. T.; Pemen, A. J. M. A novel circuit topology for pulsed power generation. J. Electrost. 2003, 58 (3-4), 221-228. (10) Yan, K.; van Heesch, E. J. M.; Nair, S. A.; Pemen, A. J. M. A triggered spark-gap switch for high-repetition rate highvoltage pulse generation. J. Electrost. 2003, 57 (1), 29-33. (11) Winands, G. J. J.; Smetsers, R. C. J.; Pemen, A. J. M.; Yan, K.; Nair, S. A.; van Heesch, E. J. M. An Industrial 10-30 kW corona plasma system, Proceedings of the 16th International Symposium on Plasma Chemistry, Toarmina, Italy, 2003. (12) Pemen, A. J. M.; Nair, S. A.; van Gompel, F. M.; van Leuken, H. E. M.; Mertens, F. A. J.; Lekx, V.; Yan, K.; van Heesch,

E. J. M.; Ptasinski, K. J.; Thieman, H. P. E.; Geutjes, T. C. J.; Leijendeckers, P. Experimental program for pulsed corona processing of biomass derived fuel gases at 850 degree C, Proceedings of the 16th International Symposium on Plasma Chemistry, Toarmina, Italy, 2003. (13) McLarnon, C. R.; Mathur, V. K. Nitrogen Oxide Decomposition by Barrier Discharge. Ind. Eng. Chem. Res. 2000, 39, 2779-2787. (14) Filimonova, E. A.; Amirov, R. H.; Hong, S. H.; Kim, Y. H.; Song, Y. H. Influence of temperature and hydrocarbons on removal of NOx and SO2 in a diesel exhaust gas activited by pulsed corona discharge, HAKONE VIII, Proceedings of International Symposium on High-Pressure Low-Temperature Plasma Chemistry, Puhajarve, Estonia, 2002; p 337. (15) Eliasson, B.; Simon, F. G.; Egli, W. Hydrogenation of CO2 in a silent discharge. Non-thermal plasma techniques for pollution control, Part B; 1992; pp 321-337. (16) Nair, S. A.; Yan, K.; Pemen, A. J. M.; van Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. Chemical processes in tar removal from biomass derived fuel gas by pulsed corona generation. Plasma Chem. Plasma Process. 2003, 23 (4), 665-680. (17) Pemen, A. J. M.; Nair, S. A.; Yan, K.; van Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. Pulsed Corona discharges for tar removal from biomass derived fuel gas. Plasma Polym. 2003, 8, 209-224. (18) Kulikovsky, A. A. Production of chemically active species in the air by a single positive streamer in a nonuniform field. IEEE Trans. Plasma Sci, 1997, 25 (3), 439-446. (19) Yetter, R. A.; Dryer, F. L.; Rabitz, H. A comprehensive reaction mechnaism for carbon monoxide/hydrogen/oxygen kinetics. Combust. Sci. Technol. 1991, 79, 97-128. (20) Marinov, N. M.; Pitz, W. J.; Westbrook, C. K.; Vincitore, A. M.; Castaldi, M. J.; Senkan, S. M.; Melius, C. F. Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame. Combust. Flame 1998, 114 (1/2), 192-213. (21) Emdee, J. L.; Brezinsky, K.; Glassman, I. A kinetic model for the oxidation of Toluene near 1200 K. J. Phys. Chem. 1992, 96, 2151-2161.

Received for review August 14, 2003 Revised manuscript received October 26, 2003 Accepted December 17, 2003 IE034066P