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Research Paper

Hydrogen sulfide removal from biogas by bio-based ironsponge

Phil Cherosky, Yebo Li*

Department of Food, Agricultural, and Biological Engineering, The Ohio State University, Ohio Agricultural Research and Development

Center, 1680 Madison Ave., Wooster, OH 44691-4096, USA

a r t i c l e i n f o

Article history:

Received 7 June 2012

Received in revised form

19 October 2012

Accepted 31 October 2012

Published online 30 November 2012

* Corresponding author. Tel.: þ1 330 263 385E-mail address: li.851@osu.edu (Y. Li).

1537-5110/$ e see front matter ª 2012 IAgrEhttp://dx.doi.org/10.1016/j.biosystemseng.20

The iron sponge process, a technology used for removing hydrogen sulphide (H2S) from

biogas, can potentially use various biodegradable wastes as the supporting material for the

H2S adsorption media, providing improved flexibility and cost-effectiveness. In this study,

ground garden waste, digested garden waste, and spent tobacco were evaluated as sup-

porting materials of the H2S adsorption media. It was found that both particle size and

moisture content had significant effects on H2S removal performance when ground garden

waste was used. The optimum moisture content of the ground garden waste system was

determined to be 15%. The optimum moisture content for the digested garden waste

system was found to be 25%. Iron sponge with either ground garden waste or digested

garden waste at their optimum conditions had an H2S removal performance comparable to

a commercial product (SulfaMaster�). Iron sponge using spent tobacco, however, was

found to be only about 20% as efficient as SulfaMaster�. Ground or digested garden waste

could be an alternative supporting material for the iron sponge system.

ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved.

1. Introduction Wilkins, 1988). As the biogas flows through the iron sponge,

Hydrogen sulphide, a major contaminant in biogas produced

from anaerobic digestion of organic materials, can cause

corrosion of metal parts, degrade engine oil, and form

poisonous sulphur dioxide (H2S) during combustion for elec-

tricity production (Hamilton, 1985; Muche & Zimmerman,

1985; Sublette & Sylvester, 1987). In order to increase the

practical use of biogas, H2S must be economically removed.

Several methods for H2S removal are available, including

biological, physical and chemical methods. One of the most

well-knownmethods is an iron sponge process which uses an

H2S adsorption media, typically ferric oxide or hydroxide,

coated onto a supporting material traditionally comprised of

wood chips or wood shavings (Walsh, Ross, Smith, Harper, &

5.

. Published by Elsevier Lt12.10.010

the hydrated iron oxide reacts with H2S forming iron sulphide,

thus removing H2S from the gas (Dillon, 1990). Commercial

products, such as SulfaMaster�, Sulfur-Rite�, Media-G2�(Zicari, 2003, pp. 120) and SulfaTreat� (Harman, 2011), are

major iron sponge systems in which the iron hydroxides are

coated onto different supporting materials including dried

manure, ceramic beads, diatomaceous earth, or unspecified

proprietary materials. Most of these are produced at central

locations requiring expensive shipment, except SulfaTreat�which can use dried digested dairy manure as the supporting

material. The produced sponge can be used on-site for puri-

fication of biogas from anaerobic digesters fed with dairy

manure (Harman, 2011). Another advantage of SulfaTreat� is

its biodegradability, which minimizes landfill demand for

d. All rights reserved.

Table 1 e Characteristics of supporting materials tested.

Test material Particle size Moisturecontents tested

Control (SulfaMaster�) 2.4 mme1.4 mm 9%, 46%, 55%

Garden waste 19.1e6.4 mm 19.1 mme6.4 mm 4%, 15%, 25%, 35%

Garden waste 4.8e2.4 mm 4.8 mme2.4 mm 4%, 15%, 25%, 35%

Garden waste 2.4e0.5 mm 2.4 mme0.5 mm 4%, 15%, 25%, 35%

Digested garden waste 2.4 mme0.5 mm 25%, 35%, 45%

Spent tobacco 2.4 mme0.5 mm 15%, 25%, 35%

b i o s y s t em s e n g i n e e r i n g 1 1 4 ( 2 0 1 3 ) 5 5e5 956

disposal of the spent iron sponge media. Therefore, it is

reasonable to envisage that other materials, especially low

cost biodegradable wastes, might potentially be used for the

iron sponge system, making this technology more adaptable

to different locations.

Garden waste (or in US terminology yard trimmings) is

a desired feedstock for biogas production via anaerobic

digestion because it is widely available and tipping fees are

commonly associated with its disposal (Yazdani, Barlaz,

Augenstein, Kayhanian, & Tchobanoglous, 2012). Spent

tobacco is another abundant biomass available as a feedstock

for biogas production (Kapadiya, Shilpkar, & Shah, 2010).

However, to date, no studies have been conducted to investi-

gate H2S removal performance of iron sponge using spent

tobacco or garden wastes as supporting materials of the H2S

adsorption media.

The active element of an iron sponge is the hydrated iron

oxide/hydroxide whichmust bemaintained in amoist state to

ensure that the iron will remain in hydrated form (Anerousis

& Whitman, 1985). Supporting materials of the H2S adsorp-

tion media used in an iron sponge must absorb moisture and

have enough surface area to support the iron oxide/hydroxide

(Anerousis & Whitman, 1985). Therefore, moisture content

and particle size should be optimised to improve the perfor-

mance of the iron sponge media.

In this study, ground garden waste, digested garden waste,

and spent tobacco were used as supporting material of the

iron sponge. The effects of particle size and moisture content

on H2S removal were investigated to obtain optimal operating

conditions for each supporting material. The H2S removal

performance of iron sponges with each supporting material

was compared with the commercial product SulfaMaster�.

2. Materials and methods

2.1. Supporting materials of H2S adsorption media

Fresh garden wastes collected in August 2011 were tree trim-

mings from the Ohio Agricultural Research and Development

Center (OARDC) campus in Wooster, OH. The garden waste

was oven dried at 40 �C for 7 days to obtain a moisture level of

less than 10%. The samples were then ground and separated

using a hammer mill and respective screens. Digested garden

waste was the digestate from a solid-state anaerobic digester

which had been ground prior to digestion. After digestion the

digested garden waste was dried and run through a series of

sieves to determine particle size. The spent tobacco was

a waste product used for pesticide research and was supplied

by quasar energy group (Cleveland, OH, USA). The spent

tobacco was neither ground nor dried prior to iron sponge

formulation as it was initially at a fine particle size and had

a moisture content of < 10%. Table 1 gives the particle sizes

tested for each supporting material.

2.2. Preparation of iron sponge media

The iron sponge was developed by combining iron chloride

(FeCl3) with aqueous sodium hydroxide (NaOHaq) to form iron

hydroxide (Fe(OH)3) and sodium chloride (NaCl). NaCl has no

effect on the H2S removal process. The reaction equation is:

FeCl3 þ 3NaOH / Fe(OH)3 þ 3NaCl. Once the reaction

completed, the solution was allowed to cool and was

combined with each supporting material under constant

mixing to reach an iron concentration of 12.5% (by dryweight),

the same as the control media (SulfaMaster�). Because NaCl

has no effect on H2S absorption, washing or cleaning was not

necessary. After the new supporting material was thoroughly

mixed, it was placed in a drying oven at 40 �C for 24e48 h. The

total solids (TS) contentwas determined via drying the sample

in an oven at 103 �C for 24 h, then distilled water was added to

the media to bring it to the desired moisture content. Table 1

shows the characteristics of each media tested.

2.3. Experimental setup

Four laboratory scale biogas purification columns (setup in

parallel) were made using clear plastic piping. The laboratory

columns had an interior diameter of 51 mm and a total height

of 152 mm. The media was filled in the columns to a height of

69 mm. Rubber caps on each end created an air tight seal

allowing biogas to flow through the designated ports only. The

iron sponge was held in place within the columns by a wire

mesh that allowed for gas flow without media loss. A 25 mm

long by 6 mm diameter tube was located in the centre of the

mesh to help distribute biogas into the centre of the media. A

schematic diagram of the system is shown in Fig. 1.

Biogas was collected in 100 l Tedlar bags from the quasar

energy group digester located in Wooster, OH. The digester

was fed with food processing wastes and operated under

mesophilic conditions with a hydraulic retention time of 28

days. The concentration of H2S in the biogas was measured

prior to entering the columns. A vacuum pump, with attached

flow meter, was connected to the columns via a series of

tubes. The biogas was pumped through the columns at

1.0 l min�1, allowing the biogas to have a contact timewith the

media of 8.3 s. Experiments were conducted over an extended

time with approximately 350 l of biogas passing through to

ensure that H2S removal efficiencies did not dramatically

change during continuous operation. Biogas was sampled

before and after it flowed through the iron sponge, and the

removal efficiency was determined as the average percentage

of H2S removed from the biogas of three replicates.

2.4. Analytical methods

The percentage of TS of iron sponge media were analysed

according to the Standard Methods for the Examination of

Water and Wastewater (APHA, 2005). H2S concentrations in

69 mm152 mm

51 mm

Fig. 1 e Schematic diagram of laboratory-scale biogas purification columns.

b i o s y s t em s e ng i n e e r i n g 1 1 4 ( 2 0 1 3 ) 5 5e5 9 57

biogas were measured by bubbling 20 ml of gas through 20 ml

of 2 N Zinc Acetate solution to remove all H2S. The solution

was then analysed using the methylene blue method for

sulphide determination with a HACH spectrophotometer

(HACH, 2010). This method was verified using Drager Tubes

and was shown to be accurate within 2%. The iron concen-

tration in the SulfaMaster� media was determined using

inductively coupled plasma-mass spectrometry (ICP-MS)

(Agilent 7500, Agilent Technologies, Wilmington, DE, USA).

Samples for ICP-MS analysis were prepared by digestion using

a microwave digester (MARSXpress�, CEM Corporation,

Matthews, NC, USA) programmed with a 15-min ramp-up

time to 200 �C and then maintained for 15 min.

2.5. Statistical analysis

Statistical significancewas determined by analysis of variance

(ANOVA) using SAS software (Version 8.1, SAS Institute Inc.,

Cary, NC, USA) with a threshold p-value of 0.05.

Fig. 2 e Effect of moisture content (m.c.) of ground garden

waste on H2S removal at various particle sizes (initial H2S

concentration 700e1000 ppm; 350 l of biogas passed).

3. Results and discussion

3.1. Effect of particle size and moisture content on H2Sremoval by iron sponge

Fig. 2 shows the effects of particle size and moisture content

on H2S removal using ground garden waste as a supporting

material of the H2S adsorption media. At the largest particle

size (19.1 mme6.4 mm), the material was not able to effec-

tively absorbwater and retain the impregnated iron hydroxide

due to the low total surface area and ridged structure of the

garden waste. When more water was added, some of the iron

hydroxide bound to the surface of the material was removed

in thewater. The coarse gardenwaste based adsorptionmedia

operated most effectively at 4% and 15% moisture removing

84e85% of H2S, but showed significantly decreased perfor-

mances at higher moisture contents.

At the medium particle size (4.8e2.4 mm), similar results

were obtained. H2S removal efficiencies (85e87%) under low

moisture conditions were much higher than those under high

moisture conditions. Although the surface area was increased

compared to the largest particle size (19.1e6.4mm), it was still

not enough to significantly improve the absorption of water

and retention of iron hydroxide at high moisture contents.

Moreover, the H2S removal efficiency at 25% moisture was

even lower than that for the largest particle size, because the

particles were clumped together by the excess unabsorbed

water, which caused portions of the gas to flow around the

media and partially short circuit the system.

At the finest particle size (2.4e0.5 mm) for the ground

gardenwaste, themedia operated better (82% of H2S removed)

at high moisture contents than it did at larger particle sizes.

The most significant improvement ( p < 0.05) in H2S removal

efficiency, compared to ground garden waste at 19.1e6.4 mm

and 4.8e2.4 mm particle sizes, was found at a 35% moisture

content. The possible reason is the ability to absorb water was

further increased by the smaller particle size, while the

unabsorbed water was enough to facilitate the H2S þ Fe(OH)3reaction but not enough to cause clumping or iron hydroxide

removal. At the lower moisture content (4%), most of the

water was absorbed by the media, leaving little water on the

surface to properly facilitate the conversion of H2S to Fe2S3,

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

69 129 205 271 350

H2S

Rem

oval

Biogas Volume Passed (L)

Control 15% m.c. 25% m.c. 35% m.c.

Fig. 3 e Effect of moisture content of tobacco on H2S

removal (initial H2S concentration 700e1000 ppm).

Control Dig. Yd Waste Yd Waste Tobacco

b i o s y s t em s e n g i n e e r i n g 1 1 4 ( 2 0 1 3 ) 5 5e5 958

and causing a significant decrease in H2S removal

performance.

As shown in Fig. 2, at 4.8e0.5 mm particle size and 15%

moisture, the ground garden waste material had its best

performance, removing 87% of the H2S whichwas comparable

to the control (SulfaMaster�).

3.2. Effect of supporting material on H2S removal

The spent tobacco was a very poor material for binding with

iron hydroxide. Once impregnated with iron hydroxide and

dried, the spent tobacco crumbled and fell apart, resulting in

the separation of iron hydroxide from the supporting mate-

rial. When moisture was added to the media, the tobacco

quickly absorbed all of the water and became saturated. This

caused themedia to easily clump, even at lowmoisture levels,

which in turn led to ineffective H2S removal. The spent

tobacco had substantially lower H2S removal efficiency than

the control at all moisture contents tested (Fig. 3), removing

between 10% and 25% H2S. For biogas volumes of 129 l and

350 l passed through the system, H2S removal improved as the

moisture content of the tobacco increased; however, this

result was not found for biogas volumes of 69 l, 205 l, and 271 l.

The digested garden waste performed at a much higher

efficiency compared to the tobacco based media (Fig. 4). At

25% moisture, the digested garden waste media removed

89e92% of the H2S, which was not significantly different

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

65 132 202 278 350

H2S

Rem

oval

Biogas Volume Passed (L)

Control 25% m.c. 35% m.c. 45% m.c.

Fig. 4 e Effect of moisture content of digested garden waste

on H2S removal (initial H2S concentration 700e1000 ppm).

( p > 0.05) from the control. Increasing moisture content to

35% and 45% had significantly negative effects on H2S removal

efficiency ( p < 0.05) due to saturation and clumping. At 25%

moisture, as the total volume of biogas passing through the

systemwas increased to 350 l, the material’s ability to remove

H2S did not decrease despite the increased accumulation of

Fe2S3.

In order to gain an accurate comparison of all supporting

materials, the materials and conditions with the best perfor-

mance from each experiment were tested simultaneously.

Fig. 5 shows the results of H2S removal by iron sponge with

different media concurrently tested with the same biogas

flowing through all columns. The materials tested were:

digested garden waste at 25% moisture; ground garden waste

at a particle size 4.8 mm and 15% moisture; tobacco at 25%

moisture; and SulfaMaster� (control). The digested garden

waste and the control material each removed up to 91% of the

H2S from the biogas. Ground garden waste also performed

similarly to the control, removing up to 89%H2S. Therewas no

significant difference between the control, ground garden

waste, and digested garden waste ( p > 0.05). The spent

tobacco system was not effective, removing only 12e22% of

the H2S. After a total volume of 371 l of biogas passed through

the system, the performance of all of the iron sponges on H2S

removal had not deteriorated significantly ( p > 0.05). There-

fore, iron sponges with ground garden waste or digested

garden waste can be alternatives to the control. Considering

that garden waste is also a feedstock for the production of

biogas, it may be more cost-effective to also use it after

digestion.

3.3. H2S removal by digested garden waste system atdifferent H2S concentrations

The digested garden waste was further tested along with the

control at various H2S concentrations. The purpose of

these tests was to determine the effectiveness of the iron

sponge media over a wide range of initial H2S concentrations.

As shown in Fig. 6, at low H2S concentrations (250e550 ppm),

the control performed better than the digested garden

waste based media by up to 2%. The removal efficiencies of

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

76 163 225 308 371

H2S

Rem

oval

Biogas Volume Passed (L)

Fig. 5 e Effect of supporting material on H2S removal (initial

H2S concentration 700e1000 ppm). Supporting material

tested: digested garden waste (Dig. Yd Waste), ground

garden waste (Yd Waste), tobacco, and SulfaMaster�(control).

0%10%20%30%40%50%60%70%80%90%

100%

H2S

Rem

oval

Initial H2S (ppm) Concentration

Control Dig. Yd Waste

Fig. 6 e Effect of initial H2S concentration on H2S removal

by iron sponge media with digested garden waste.

b i o s y s t em s e ng i n e e r i n g 1 1 4 ( 2 0 1 3 ) 5 5e5 9 59

both materials significantly increased at concentrations

higher than 450 ppm H2S. At H2S concentrations of

550e1150 ppm, the digested garden waste-based media

slightly outperformed the control. Statistically, there was no

significant difference between the digested garden waste and

the control at either the low or high H2S concentrations

( p > 0.05). As a result, the iron sponge media with digested

garden waste appears to be a good alternative to the control

commercial product at a wide range of H2S concentrations

(250e1150 ppm).

4. Conclusion

The H2S removal performance of garden waste based media

was affected by both particle size and moisture content.

Compared to a commercial product (SulfaMaster�), iron

sponge with either ground garden waste or digested garden

waste as the media support material had comparable H2S

removal performance at their optimum particle size and/or

moisture content. Meanwhile, iron sponge with the spent

tobacco support material showed a poor performance for H2S

removal. Digested garden waste may be the best alternative

supporting material of iron sponge for H2S removal, particu-

larly considering that garden waste is also a feedstock for the

production of biogas.

r e f e r e n c e s

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