Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter

doi:10.4236/sgre.2012.31008

Paper Menu >>

Journal Menu >>

Smart Grid and Renewable Energy, 2012, 3, 56-61

http://dx.doi.org/10.4236/sgre.2012.31008 Published Online February 2012 (http://www.SciRP.org/journal/sgre)

Lowering Gasifier Tars and Particulates Using Heated

Dololmite Catalyst and a Particulate Filter

Christopher O. Akudo, Beatrice G. Terigar, Chandra S. Theegala*

Department of Biological and Agricultural Engineering, Louisiana State University, Baton Rouge, USA.

Email: *[email protected]

Received December 21st, 2011; revised January 31st, 2012; accepted February 7th, 2012

ABSTRACT

For this study, a fixed-bed, down-draft gasifier was designed to investigate the effect of a dolomite catalytic bedon tar

removal. Pine pellets and wood chips (cypress) were used to produce syngas from the down-draft gasifier. For the gas

conditioning, a combination of a heated dolomite (bed temperature at 850˚C for catalytic cracking of tars) and a par-

ticulate filter (for particulate removal) was used. Investigation of temperature effects on dolomite activity between

650˚C and 950˚C bed temperatures, showed optimum catalytic efficiency at approximately 850˚C. At the optimum con-

ditions, gravimetric tar and particulate concentrations in syngas produced from pine pellets were 0.85 g/Nm3 (±0.16)

and 4.75 g/Nm3 (±0.07), respectively before gas condition ing, and 0.09 g/Nm3 (±0.02) and 2.01 g/Nm3 (±0.13), respec-

tively after gas conditioning . Syngas fro m wood ch ips con tained 1.63 g/Nm 3 (± 0.45) and 3 .84 g/Nm3 (±1.16) of tars and

particulates, respectively before gas cleaning and 0.19 g/Nm3 (±0.02) and 2.27 g/Nm3 (±0.27) tars and particulates, re-

spectively after gas conditioning. The combustible portion of the gas constituted carbon monoxide (12% - 14%), hy-

drogen (11% - 12%), and methane (~2%). These results suggest that syngas produced from gasification of pine pellets

and wood chips in a down-draft biomass gasifier can be effectively cleaned using a heated catalyst bed and a particulate

filter. However, the benefits of gas conditioning will be offset by the need to maintain a heated catalyst bed for tar

cracking.

Keywords: Gasification; Dolomite; Tars; Particulates; Syngas

1. Introduction

Biomass gasification is a thermo-chemical process that

produces relatively clean and combustible gases through

pyrolytic reaction. Depending on the gasifying ag ent (air,

steam, oxygen) the raw gas produced at the gasifier exit

has different compositions. The most commonly used

agent at demonstration and commercial scale is air, with

equivalence ratios of 0.2 - 0.3 [1]. By use of air, the flue

exit gas contains, on a volumetric basis, about 50% N2,

8% - 20% H2and CO (each) and 2% - 4% CH4, with the

remainders being CO2 and H2O [1]. This gas composition

is mostly useful only for heat generation and electricity

production. The exact gas composition at the gasifier exit

in the process depends on other operating variables, and

it has been well studied by different authors, at different

operating scales and with different gasifiers [2-4]. In ad-

dition to these gases, a condensable mixture of organic

compounds referred to as tars are formed [5]. Tars are

undesirable because of the problems associated with

condensation in the downstream processes where they

form tar aerosols, and polymerization to form more com-

plex structures which can damage internal combustion

engines, gas turbines, and other machinery. Therefore,

the syngas needs to be cleaned of tars and other solid

particles before it can be used in any fuel conversion

device. The final gas quality dictates the end use or ap-

plication suitabilit y of the generated syngas.

There has been extensive research on tar elimination

using catalysts that aid in the decomposition of the hy-

drocarbons found in tar. The most widely used catalysts

are non-metallic oxides such as dolomite, olivine, mag-

nesite, and zeolite [6]. Dolomite is a calcium magn esium

ore with the general chemical formula (CaMg)(CO3)2

that contains approximately 20% MgO, 30% CaO, and

45% CO2 on a weight basis [6]. The use of calcined dolo-

mites in biomass gasification for tar cracking and re-

moval has been the subject of interest in hot gas cleaning.

Delgado and co-workers [7] studied the use of calcined

dolomites in biomass gasification with steam. The cata-

lytic decomposition of biomass tars using calcined dolo-

mites was also reported by Devi and co-investigators [8].

Calcination of dolomite involves decomposition of the

carbonate mineral, eliminating CO2 to form MgO-CaO.

*Corresponding a uthor.

Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter 57

Complete dolomite calcination occurs at fairly high tem-

peratures and is usually performed at 800˚C - 900˚C [6]

and restricts its effective use to these relatively high tem-

peratures. Aznar and co-authors [9] performed experi-

ments involving a bed of calcined dolomite placed after a

biomass fluidized bed gasifier in which gasification was

made with steam-oxygen mixtures to clean the raw syn-

gas.

An alternative to the dolomite is the naturally occur-

ring particles of olivine, which are a mineral containing

magnesium, iron oxide and silica. Rapagna and co-work-

ers [10] have indicated that the tar reforming activity of

olivine was comparable to calcined dolomite. Olivine is

advantageous in terms of its ability to withstand friction

and does not easily break [8]. However, there is still am-

biguity on the prospective use of olivine as a tar decom-

posing catalyst. Nickel based catalysts have been found

to almost completely remove the tar and are also very

effective for NH3 removal at temperatures above 800˚C

[11]. The main limitation of using nickel based catalysts

is the tendency to lose their catalytic effect because of

carbon fouling. This fouling effect occurs mainly when

the catalyst is placed right after the gasifier. High tar

concentration is one good example of the devastating

effect on catalyst activity [12]. The use of a catalytic re-

actor downstream of the gasification reactor has proven

to be a more effective approach to tar destruction [13].

From an energy balance point-of-view, the energy con-

tained in the tars is fractional when compared to the en-

ergy content in the exiting gases. Due to these reasons,

tar cracking is not aimed at enhancing the energy content

of the gas, but primarily to improve the suitability of the

generated syngas and to minimize problems with the

downstream equipment. This fact was corroborated by

Corella and co-workers [14], who concluded that there is

almost no difference in the heating value of the gas pro-

duced af ter tar cracking. They indicated that the increase

in the hydrogen production is compensated by a decrease

in carbon monoxide, and there is hardly any change in

methane production .

This study presents the results of further investigation

of the use of dolomite for tar removal. The effect of cat-

alytic bed temperature on tar removal was investigated.

Also, since previous research focused on the use of do-

lomite in a fluidized bed system, its use in a fixed bed

downdraft biomass gasifier is being explored in this pa-

per.

2. Experimental Section

2.1. Gasifier System Operation

The fixed bed gasifier system used in this project con-

sisted of a feeder unit, down-draft gasifier, gas flare

chamber, temperature monitoring system, catalytic bed

and a in house developed protocol [15,16] for impurity

sampling as shown in Figure 1. The gasifier feeder unit

was designed to allow for b atch feeding of biomass. Two

15 cm diameter knife-gate valves (with 68 kg rating)

attached 45 cm apart on the 15 cm diameter metal feed-

ing pipe was used as an airlock. This airlock allowed

periodic feeding of biomass without the loss of internal

pressure. The raw materials used were cypress mulch

woodchips (less than 5 cm long, bought at local hardware

stores) and pine pellets (Tractor Supply Co., Zachary, LA)

that were less than 1.3 cm long and 0.6 cm diameter. The

moisture con tent of the woodch ips varied between 11% -

17% during storage, while the pellets maintained fixed

moisture content of 3.2%. After the feed was weighed, it

was manually loaded into the system. Using the gate

valves and feed level detector, materials were reloaded

during gasification. Six K-type thermocouples were

connected to the wall of the furnace from top to bottom

with approximately 7 cm distance between adjacent

thermocouples. The majority of the gas that was pro-

duced from the gasifer (6 - 8 CFM) was flared in a flar-

ing-unit. A small slip stream was drawn from the exiting

Figure 1. Gasification experimental setup consisted of a gasifier, flaring unit (for disposing the majority of the syngas), a slip-

stream connected to a catalyst bed (in a muffle furnace), a filter assembly (in a heated oven) and a tar quantification assem-

ly (series of acetone bottles). b

Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter

syngas and was used for tar-cracking and particulate

cleaning experiments as discussed below.

2.2. Maintaining the Integrity of the

Specifications Evaluation of the Gas

Cleaning Component

A small slip-stream of 94.4 cm3/s (0.2 CFM) was used

for all gas cleaning experiments. The first component of

the gas cleaning assembly consisted in a dolomite bed,

set inside an electrically heated muffle furnace. The

temperature of the dolomite bed was varied from 650˚C

to 950˚C by altering the thermostat setting of the muffle

furnace. A fixed-bed reactor (2.36 mm diameter) was

used for all catalyst experiments. The fixed-bed had an

internal diameter of 19 mm and length of 152.4 mm

made from a steel pipe and was placed inside a muffle

furnace. Catalyst depth in the pipe was 101.6 mm

supported by wire mesh at both ends of the pipe. Prior to

the experimentation involving dolomite, calcination of

the dolomite was done at 850˚C in a furnace for 2 hours.

The second component of the gas cleaning assembly

included a glass fiber filter (Whatman: GF/F Cat No

1825 090) and a filter holder assembly placed inside an

electrically heated oven for particulates removal and

quantification. The filter assembly was heated to 250˚C

to prevent tar condensation on the filter paper. The

particulates collected on the filter paper were quantified

gravimetrically.

2.3. Sampling and Analysis

The syngas sampling was initiated after steady state

temperatures were reached inside the gasifier. The steady-

state point was also indicated by a self-sustainable flame

in the flaring unit.As indicated earlier, the bulk portion of

the produced syngas (6 - 8 CFM) was flared and all the

gas conditioning experiments were conducted on a small

94.4 cm3/s (0.2 CFM) slip-stream of the generated syn-

gas. The sampled gas was passed through insulated 0.32

cm copper tubing to the filter assembly, where the par-

ticulates were collected. The filter assembly in the oven

was maintained at 250˚C, which prevented tars from

condensing on the filter. The gas exiting the filter oven

was passed through a series of four impinger bottles

containing acetone, which collected the tars by dissolu-

tion (Figure 2). Passing the gas through a series of bot-

tles ensured total collection of tars and number of bottles

to be used in series was decided based on the amount of

tars collected in the various impinger bottles during the

trial runs.

Due to restrictions and depositions on the filter paper,

the sample gas flow rate dr opped from 94.4 cm3/s to 78 .7

cm3/s after 30 minutes of sampling. The lowering of air

flow was anticipated prior to running the experiments

and appropriate adjustments were made during the quan-

tification of tar and particulate concentrations.

The syngas exiting the acetone bottles was drawn

through a volatile organic carbon absorber and a flow

regulator before entering the vacuum pump. The gas ex-

iting the vacuum pump was tested for syngas composi-

tion. The gas samples were drawn using a 10 ml syringe

with a 0.3 µm syringe filter. The collected sample was

then analyzed using a gas chromatograph (SRI, MG#1)

equipped with a 0.32 cm stainless steel silica gel-pack

column and a thermal conductivity detector. An amount

of 1 mL of gas sample was injected into the gas chroma-

tography and ultra-high purity helium at a flow rate of 10

mL/min was used a carrier gas [15,16].

3. Results and Discussion

3.1. Gasifier Temperature Profile

The temperature of the gasifier during operation was

continuously measured and recorded by a PC based data

acquisition system. Temperature averages were calcu-

lated within a 30 seconds interval and average tempera-

ture profiles were created. Figure 3 shows a typical

steady-state gasification temperature profile within the

gasifier at various locations from the bottom grate. Tem-

perature 1 represents the thermocouple immediately

above the grate, and Temperature 2 to Temperature 6

represents the temperatures from thermocouples that

Figure 2. An in-house built particulate and tar quantification system was used for the present study. The particulate

quantification unit consited of an stainless steel assembly placed inside an oven maintaiend at 250˚C. A series of four

impinger bottles containing acetone collected the tars, and the syngas exiting the acetone bottles was drawn through a volatile

organic carbon absorber and a flow regulator before entering the vacuum pump and tested for composition.

Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter 59

Figure 3. Temperature profile as gasification approaches

steady state. Temperature 1 represents the thermocouple

immediately above the grate, and Temperature 2 to Tem-

perature 6 represents the temperatures from thermocouples

that were placed approximately 7 cm higher than the pre-

ceding thermocouple.

were placed approximately 7 cm higher than the preced-

ing thermocouple.

Based on the construction of the gasifier and the inlet

ports (which were place between thermocouples 3 and 4),

the highest steady-state temperatures (or oxidation zone)

will be experienced by Temperature 2. All temperatures

measured were relatively constant. However, Tempera-

ture 1 had a sharp decrease at about 35 min due to a

malfunction of the grate which lead to a small accumula-

tion of ash close to the thermocouple. This problem was

promptly fixed resulting in a more accurate temperature

profile.

Based on experimental observations, true gasification,

as indicated by a self-sustainable flame in the flaring unit,

occurs when the Temperature 2 temperatures exceed

700˚C [17]. The high reactor temperature obtained also

reduces the producti on of de tectable tar species [18].

3.2. Gas Composition and Heating Value

The average yield of measured syngas components from

wood chips and pine pellets are shown in Figure 4. The

average concentrations of the combustible gases namely:

carbon monoxide (CO), hydrogen (H2), and methane

(CH4) obtained were within the limits reported in previ-

ous work [3]. However, the high percentage of nitrogen

(N2) present in the gas mixture could be explained by the

fact that air was used as the gasification medium. The use

of pure oxygen will result in higher percentages of the

combustible gases; however, no additional efforts were

made to lower the nitrogen content for this study. The

attempt to minimize the air flow from the blower did not

seem to have any noticeable effect on the gas composi-

Figure 4. Volumetric concentration of combustible gases in

the syngas from wood chips and pine pellets after gasifica-

tion.

tion (data not shown) but rather reduced the velocity of

the syngas flowing from the gasification chamber.

The heating value of a gas cannot be measured directly,

but only with respect to a reference state. The most widely

used is th e Lower Heating Value (LHV), parameter that-

defines the potential energy available per unit volume of

the biomass. This uses water vapor as its reference state.

The heating value of the syngas was calculated from the

concentration of combustible gases in the mixture.

Lower Heating Value (LHV) = %H2 × LHVH2 + %CH4

× LHVCH

4 + %CO × LHVCO

The heating values obtained were within the range of

previous work using woodchips as raw materials. A com-

parison with data published on gasification with similar

systems is presented in Table 1.

3.3. Gravimetric Tar and Particulates

The process of tar and particulate sampling was initiated

only after the temperatures in the gasifier approached

steady state and there was evidence of combustible gases

produced, indicated by a self-sustaining flame. Figure 5

Table 1. Comparison of wood chips and pine pellets syngas

composition from experimental results with literature pub-

lished data.

Parameters Published

Data [3]Experimental Results

Wood Chips Experimental Results

Pine Pellets

H2 (% Vol.)15 - 2111.56 11.44

CO (% Vol.)10 - 2212.09 13.49

CO2 (% Vol.)11 - 1312.50 12.98

CH4 (% Vol.)1 - 5 1.70 2.05

N2 (% Vol.)39 - 6362.16 59.73

LHV(MJ/Nm3)4.0 - 5.63.38 3.67

Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter

Figure 5. Concentration of tar and particulates in the wood chip and pine pellet syngas before and after the gas cle aning pro-

cedure.

shows the concentrations of tars and particulates both

before and after gas conditio ning in the syngas generated

from wood chips and pine pellets.

A comparison of tar concentration in the syngas before

and after treatment with dolomite at 850˚C showed a

significant decrease (p < 0.05) of 90% and 92% for the

wood chips and pine pellets respectively, after passing

the syngas through the dolomite bed.

Similarly, significant reduction (p < 0.05) was observed

in the particulates concen tration as well. A 41% and 57%

reduction in particulates concentration was achieved for

wood chips and pine pellets, respectively.

3.4. Effect of Temperature on Dolomite

Performance

The catalytic bed temperature was varied in 100˚C

increments from 650˚C to 950˚C to investigate its effect

on tar removal, expressed as the concentration left in the

gas. The results presented in Figure 6 shows an increase

in the performance of the dolomite as bed temperature

increased from 650˚C to 950˚C, for both pine pellets and

wood chips. However, after increasing the temperature

beyond 850˚C, the rise in temperature did not produce

any improvement on the catalyst performance. This indi-

cates that temperatures beyond 850˚C yield diminishing

returns (with respect to tar cracking), th ereby, making th e

overall process economically impractical.

The incremental cost of maintaining a catalyst at a

higher temperature also goes up significantly due to

higher heat losses at elevated temperatures. Furthermore,

at temperatures higher than 1000˚C, thermal cracking of

tars can take place to a certain degree purely due to the

high temperatures. Considering the heating costs and

cracking efficiency, catalyst bed temperatures of 800˚C -

850˚C appear to be the ideal temperature range for the

dolomite bed.

Figure 6. Performance of dolomite as bed temperature increases from 650˚C to 950˚C on tar removal for both pine pellets

nd wood chips, expressed asthe concentration left in the gas. a

Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter 61

4. Conclusion

Gasification of biomass for energy production requires

effective gas cleaning techniques that removes the tars

and particulates formed during process. In this study,

dolomite was investigated for its tar cracking capability

in a downdraft gasification system. Temperature studies

indicate an optimum catalytic bed performance at 800˚C

- 850˚C. The gravimetric analysis presented in this p aper

is an attempt to overcome the tar and particulate prob-

lems associated with biomass gasification. Additional

work is needed in order to investigate the tar cracking

potential of proprietary low temperature catalysts and a

possible combination of different catalytic bed reactors.

A chemical analysis of the tar components and deposits

on the spent catalyst (if any) is needed to better under-

stand and design a gasification system.

REFERENCES

[1] J. Gil, M. P. Aznar, M. A. Caballero, E. Frances and J.

Corella, “Biomass Gasification in Fluidized Bed at Pilot

Scale with Steam-Oxygen Mixtures. Product Distribution

for Very Different Operating Conditions,” Energy & Fu-

els, Vol. 11, No. 6, 1997, pp. 1109-1118.

doi:10.1021/ef9602335

[2] I. Narvaez, A. Orio, M. P. Aznar and J. Corella, “Biomass

Gasification with Air in an Atmospheric Bubbling Fluid-

ized Bed. Effect of Six Operational Variables on the

Quality of the Produced Raw Gas,” Industrial & Engi-

neering Chemistry Research, Vol. 35, No. 7, 1996, pp.

2110-2120. doi:10.1021/ie9507540

[3] A. J. M. Pemen, E. J. M. van Heesch, K. Yan, S. V. B.

van Paasen, K. J. Ptasinski, Z. Matyas, P. A. Huijbrechts,

B. O. E. Hultermans, A. Nicoletti, P. P. M. Blom and P.

Zacharias, “Experimental Program of the Pulsed Corona

Tar Cracker,” Conference Paper for Pulse Power Con-

ference. Digest of Technical Papers. 12th IEEE Interna-

tional, Monterey, CA, Vol. 1, 1999, pp. 515-518.

[4] J. Herguido, J. Corella and J. Gonzalezsaiz, “Steam Gasi-

fication of Lignocellulosic Residues in a Fluidized-Bed at

a Small Pilot Scale—Effect of the Type of Feedstock,”

Industrial & Engineering Chemistry Research, Vol. 31,

No. 5, 1992, pp. 1274-1282. doi:10.1021/ie00005a006

[5] T. A. Milne, N. Abatzoglou and R. J. Evans, “Biomass

Gasifier Tars: Their Nature, Formation, and Conversion,”

NREL Report, NREL/TP-570-25357, NREL, Golden, CO,

1998.

[6] D. Dayton, “A Review of the Literature on Catalytic

Biomass Tar Destruction,” NREL Report, NREL/TP-510-

32815, NREL, Golden, CO, 2002.

[7] J. Delgado, M. P. Aznar and J. Corella, “Calcined Do-

lomite, Magnesite, and Calcite for Cleaning Hot Gas from

a Fluidized Bed Biomass Gasifier with Steam: Life and

Usefulness,” Industrial & Engineering Chemistry Re-

search, Vol. 35, No. 10, 1996, pp. 3637-3643.

doi:10.1021/ie950714w

[8] L. Devi, K. J. Ptasinski, F. Janssen, S. V. B. van Paasen,

P. C. A. Bergman and J. H. A. Kiel, “Catalytic Decompo-

sition of Biomass Tars: Use of Dolomite and Untreated

Olivine,” Renewable Energy, Vol. 30, No. 4, 2005, pp.

565-587. doi:10.1016/j.renene.2004.07.014

[9] M. P. Aznar, J. Corella, J. Gil, J. A. Martin, M. A. Cabal-

lero, A. Olivares, P. Perez and E. Frances, “Biomass Ga-

sification with Steam and Oxygen Mixtures at Pilot Scale

and with Catalytic Gas Upgrading. Part I: Perform- ance

of the Gasifier,” In: A. V. Bridgwater and D. G. B. Boo-

cock, Eds., Developments in Thermochemical Bio- mass

Conversion, Kluwer Academic Publishers, London, 1997,

pp. 1194-1208.

[10] S. Rapagna, N. Jand, A. Kiennemann and P. U. Foscolo,

“Steam-Gasification of Biomass in a Fluidised-Bed of

Olivine Particles,” Biomass & Bioenergy, Vol. 19, No. 3,

2000, pp. 187-197. doi:10.1016/S0961-9534(00)00031-3

[11] W. Y. Wang, N. Padban, Z. C. Ye, A. Andersson and I.

Bjerle, “Kinetics of Ammonia Decomposition in Hot Gas

Cleaning,” Industrial & Engineering Chemistry Research,

Vol. 38, No. 11, 1999, pp. 4175-4182.

doi:10.1021/ie990337d

[12] S. Anis and Z. A. Zainal, “Tar Reduction in Biomass

Producer Gas via Mechanical, Catalytic and Thermal Me-

thods: A Review,” Renewable & Sustainable Energy Re-

views, Vol. 15, No. 5, 2011, pp. 2355-2377.

doi:10.1016/jsser.2011.02.018

[13] E. Kurkela, P. Stahlberg, J. Laatikainen and P. Simell,

“Development of Simplified Igcc-Processes for Biofuels-

Supporting Gasification Research at VTT,” Bioresource

Technology, Vol. 46, No. 1-2, 1993, pp. 37-47.

doi:10.1016/0960-8524(93)90052-D

[14] J. Corella, M. P. Aznar, J. Gil and M. A. Caballero, “Bio-

mass Gasification in Fluidized Bed: Where to Locate the

Dolomite to Improve Gasification?” Energy & Fuels, Vol.

13, No. 6, 1999, pp. 1122-1127. doi:10.1021/ef990019r

[15] C. O. Akudo and C. S. Theegala, “Sampling and Analysis

of Tars and Particulates from a Biomass Downdraft Gasi-

fier,” Conference Paper for ASABE Annual International

Meeting, Minneapolis, Paper No. 074149, 17-20 June

2007, pp. 1-7.

[16] C. O. Akudo, “Quantification of Tars and Particulates

from a Pilot Scale, Downdraft Biomass Gasifier,” Masters

Thesis, Louisiana State University, Baton Rouge, 2008.

[17] L. Wei, “Experimental Study on the Effects of Opera-

tional Parame ters of a Downdraft Gasifier,” Masters The-

sis, Mississippi State University, Starkville, 2005.

[18] C. M. Kinoshita, Y. Wang and J. Zhou, “Tar Formation

under Different Biomass Gasification Conditions,” Jour-

nal of Analytical and Applied Pyrolysis, Vol. 29, No. 2,

1994, pp. 169-181. doi:10.1016/0165-2370(94)00796-9