Lengai Volcanic Ash for Removal of Hydrogen Sulfide and Ammonia from Biogas

Oldoinyo Lengai mountain located in Northern Tanzania is the only active natrocarbonatite volcano with unusually alkali-rich natrocarbonatites which are not found elsewhere in the world. Volcanic ash formed earlier during eruptions was collected from different sites along the mountain, and its potency to adsorb hydrogen sulfide (H2S) and ammonia (NH3) from biogas was investigated. The samples were calcinated at different temperatures (550 ̊C 850 ̊C) and were characterized by X-ray florescent, scanning electron microscopy and X-ray diffraction techniques. The on-site adsorption experiments were conducted at the biogas digester at ambient conditions. The calcinated ash was packed into the reactor bed, biogas allowed to pass through the adsorbent, and the inlet and outlet concentrations of H2S and NH3 were measured. The height of the site where the adsorbent was taken from, calcination temperature, biogas flowrate and mass of the adsorbent were variable parameters and found to influence greatly on the efficiency of H2S and NH3 removal. The efficiency is increased with calcination temperature raise and mass of adsorbent and decreased with flowrate increase. The samples collected from the top site of the mountain and calcinated at 850 ̊C exhibited the best sorption performance.


Introduction
Biogas produced by waste biomass becomes one of the vital substitute energy sources in recent years as it is obtained from non-fossil fuels [1]- [7].The biogas consists of CH 4 and CO 2 which are the main constituents, but also contains in-I.Kandola [11].The hydrogen sulfide in biogas stream stands as extremely toxic gas which causes corrosion, erosion, fouling for metal devices such as, cooking stoves, biogas plants and steam turbines [12].The presence of H 2 S in combustion process results in formation of sulfur dioxide which is harmful for environment [13] [14], and injurious for human and animal health as it causes irritation of mucous membranes, headaches, dizziness, nausea and sudden death.For these motives, substantial attention has been paid to removal of H 2 S from biogas stream prior to use.
Considering many benefits of biogas use as non-fossil fuel, it is imperative to develop cost effective adsorbents, easy way of removing toxic substances for application at medium and small-scale plants.Hence this current study aiming to investigate the removal of H 2 S and NH 3 from biogas using Oldoinyo Lengai volcanic ash (OLA) at ambient temperature conditions.The Oldoinyo Lengai is the only active natrocarbonatite volcano and it is a unique source of alkaline ash in the world; the OLA is abundant and readily available [35] [36] [37] [38].The experiment was conducted on-site at The Banana investment company Ltd, Arusha, Tanzania where characteristics of H 2 S and NH 3 removal from biogas stream were determined at natural variation of biogas composition.

Materials Collection and Preparation
Ashes used in this study were collected from Oldoinyo Lengai volcano which is located Tanzania, the Rift Valley at 2˚45'S, 35˚55'E and 2000 m above the Serengeti plains [35] [37].Several samples of ashes were randomly collected from various places on top, medium and bottom of the mountain.Previously it was found that the samples contain compounds of K, Na, Ca, Zn and Fe [15] [38] [39] [40]

Materials Characterization
The oxides composition of the samples was determined by X-ray fluorescence spectrometer (XRF), model MiniPal4 (Pw4030)-Rh manufactured by PANalytical, using software provided with the instrument.A sample of 25 g of calcinated adsorbent was added to 100 ml of distilled water.The mixture was stirred and shaken for one hour by using mechanical shaker (model AS200, RETSH Company).The pH of samples was measured with a pH meter, model H199121.The moisture content of raw OLA ashes determined under standard procedures as has been reported elsewhere [32] [41] was found to be 22.2% ± 0.5%.Mineral phase analysis and elemental oxide composition were analyzed by X-ray diffraction (XRD) technique using D2 phaser-Bruker model and X-ray diffraction meter with a Cu-K α radiation source in a 2θ range between 10˚ and 80˚ at a scanning rate of 2˚ min −1 and analyzed using EVA software provided with the instrument.
The surface micromorphology of samples was investigated using a scanning electron microscope (SEM), modal JEOL JSM-6335F with resolution of 500 nm, at 200 kV at the Department of Materials Science and Engineering, University of Connecticut, USA.

Measurements of H2S and NH3 Removal and Evaluation of Sorption Capacity of Absorbents
The sorption experiment of H 2 S and NH 3 was carried out at the Banana Investments company Ltd in Arusha, Tanzania where the biogas was produced from winery effluent banana industrial waste.The biogas was liberated from upflow anaerobic sludge blanket between 100 and 120 m 3 daily, the composition of the gas was determined with the biogas 5000 g as analyzer to be 82% -89% CH 4 , 12% -15% CO 2 , <1% O 2 , 5 -48 ppm of NH 3 , and 24 -60 ppm of H 2 S [42].
A reactor made of plastic tube, 6.5 cm length, 1.The performance of material adsorbent was specified as percentage removal R: Sulfur sorption capacity (SC)of the OLA samples, in grams of sulfur per 100 grams of sorbent, was determined as described in [16] [43] [44]; ( ) where GHSV is the gas hourly space velocity which is the volume of the gas flowing hourly through the reactor with 1 g of absorbent (L•h −1 •g −1 ); V mol is the molar volume of the gas (L•mol −1 ) under standard conditions, M is the atomic mass of sulfur; C in and C out are the H 2 S concentrations before and after sorption (ppm); t is the breakthrough time in hours.

XRF Analysis and pH of Raw OLA Samples
Results on the XRF analysis of the raw OLA samples (

XRD Analysis of Calcinated OLA Samples
The XRD spectra were measured for several samples; an example for the OLA-TP-850 is shown in Figure 2. The data indicate the crystalline solid form of the materials where compounds of iron, sodium, calcium and potassium are identified as main components by peaks of magnetite (2θ = 35.5˚,43˚, 57˚, 63˚), analcime (2θ = 16˚, 18˚, 26˚), epidote (2θ = 31˚, 56˚) and nepheline (2θ = 21˚, 23˚, 27˚, 29.8˚, 37˚), respectively.These results are in accordance with the data reported previously [39].For the sample after absorption, the additional peaks were recorded at 2θ = 22˚, 31˚, 32.5˚ and 49˚ which correspond to sodium hydrogen sulfate Na 3 H(SO 4 ) 2 .The formation of this compound indicates interaction of hydrogen sulfide with the adsorbent and hence chemisorption occurrence.Our results differ from those published [19] in which red mud was used for H 2 S removal, and after sorption FeS 2 , FeS, CaSO 4 •2H 2 O, sulfur, sodium sulfide and bisulfide were formed.
Apparently, the distinction of our results with literature [19] may be attributed to the different mineralogical and chemical composition of the adsorbents.

SEM Surface Micromorphology Analysis
The surface morphology of samples was investigated using SEM; an example shown in Figure 3   porosity which evidently may provide efficient physisorption.
It was observed earlier that the porous structure and surface chemistry of the adsorbent had significant effect on adsorption and oxidation of hydrogen sulfide [33].Besides the morphology of ashes is controlled by calcination temperature, and porosity increases with T c increase [51].We may expect that our samples which were calcinated at higher temperatures would demonstrate better adsorption performance.2 for different OLAs.The effect of calcination temperature and height of sites the samples were collected on the sorption capacity is depicted also in the diagram (Figure 4).As is seen, the SC is increased with calcination temperature raise; e.g., the ashes taken without calcinations, OLA-TP-RT, and calcinated at 850˚C, OLA-TP-850, result in the SC of ~0.2 and ~1.0 g S/100g of adsorbent, respectively.It is also observed that the SC increases with height of site; the samples collected at the top and calcinated at 850˚C, OLA-TP-850, demonstrate the best performance.The breakthrough time relates to the life time of the adsorbent; the longer is the BT, the higher is the working capability of the material.One can see the BT increases greatly with height (from 5 min for OLA-BN-650 to 120 min for OLA-TN-650) and calcination temperature raise (from 55 min for the OLA-TP-RT up to 177 min for OLA-TP-850).Thus, the OLA-TP-850 samples possess the longest working time.

Adsorption Performance of OLA Samples
The adsorption efficiency removal R was measured both for H 2 S and NH 3 and analyzed for different samples with respect to time interval between the measurements of initial and final concentrations of the adsorbates while biogas flowed through the reactor.The plots for Rvs time for the samples collected at different heights and calcinated at 850˚C are shown in Figure 5. Results indicate the removal efficiency decreases with time for all samples.For the OLA-TP-850, R reduces by ~10% both for H 2 S and NH 3 after one hour of the adsorbent's use.bent's use, the R of OLA-TP-RT descents to 37%, while the OLA-TP-850 holds 60% for H 2 S removal; for ammonia, the values of R are 40% and 80%, respectively.Based on the results of [33] [52] [53], we suggest that the increase in adsorption efficiency with temperature T c may be caused by increase of number of pores created in adsorbents.On the other hand, the removal efficiency decreases in time because the surface of adsorbent is being occupied with the adsorbate molecules and the porosity is reduced as the pores are clogging with the gas molecules [32].
If compare the plots of Rvs time for H 2 S and NH 3 , they look differently: monotonic decay for the former and step-like for latter.The step-like behavior for ammonia may be attributed to rather low concentration (C in ~0.005 mg NH 3 per 1 g of the biogas, that is one order less than of H 2 S) measured at the sensitivity limit of the gas analyzers.Moreover, according to Equation (2), the lower input concentration of ammonia also brings bigger value of R.
Therefore, among all OLA samples considered the ashes collected at the top site and calcinated at 850˚C, the OLA-TP-850, demonstrated the best removal efficiency retaining it high during the exploitation time of the adsorbent.

Effect of Biogas Flowrate
The effect of biogas flowrate on the samples adsorption performance was investigated; the flowrates were 0.0004, 0.001, 0.002, and 0.008 m 3 /min.The values of the removal efficiency of H 2 S and NH 3 are plotted vs time for the top-site samples OLA-TP-850 in Figure 7.One can see, with the flowrate raise from 0.0004  to 0.008 m 3 /min the R values decrease from 71% to 20% for H 2 S and from 100% to 58% for NH 3 , respectively, measured after 150 min adsorbent's use.
It seems at low flowrate (0.0004 m 3 /min) the materials get enough contact time for interaction between gas molecules and adsorbent; whereas at high flowrate (0.008 m 3 /min), H 2 S and NH 3 gas molecules can pass through without being adsorbed and reacted fully with adsorbent.Therefore, the contact time be-tween gas and adsorbent is significant in determining the adsorption capacity.This is in accordance with findings reported in [48].

Effect of Mass of Adsorbent Material
The effect of variation of the absorbent mass, from 0.5 to 2.0 g, on the removal efficiency of H 2 S and NH 3 ; was considered for the samples OLA-TP-850 (Figure 8).The results indicate that R values evidently increase with mass.Four-fold enlargement in mass leads to essential increase in removal efficiency, from 23 to 80% for H 2 S and from 50% to 100% for NH 3 measured after 150 min adsorbent's use.When the mass of adsorbent was 2 g, the material retained 100% H 2 S removal after 40 min and efficiency decreased to 80% after 150 min; while for NH 3 the adsorbent held 100% removal efficiency for all time measurements.

Comparison of OLA-TP-850 with Other Materials
The adsorption properties of our best sample OLA-TP-850 are compared to other materials reported in literature (Table 3).3] Municipal waste bottom ash 13 × 10 3 55.94 × 10 -3 0.30 9.80 [55] Red mud soil 5 5 × 10 -5 2.10 >13.00 [19] Coal ashes 10 14.20 × 10 -5 0.50 11.90 ± 0.01 [15] OLA-TP-850 1 2 × 10 -3 1.00 12.31 Current study 7 cm diameter, filled with adsorbent supported by cotton wool on both end sides of the reactor tube.The biogas allowed to flow through the reactor at ambient temperature varying the flowrate from 0.0004 to 0.008 m 3 /min, and mass of adsorbents from 0.5 to 2.0 g.The mass of adsorbent of 1 g and the biogas flowrate of 0.002 m 3 /min were used as standard parameters for this study.Schematic diagram and photos of the experimental setup are shown in Figure 1.The flowrate was controlled and monitored by using a flow meter, model JBD2.5-SA.The concentration of H 2 S and NH 3 was recorded at the inlet and outlet of the reactor with different time intervals from 5 to 150 min.Breakthrough time (BT) was noted when the outlet concentration C out of H 2 S and NH 3 reached 50% of the inlet concentration C in .The sorption tests were repeated at least twice for each sample.

Figure 1 .
Figure 1.Photos and schematic diagram of the sorption experimental setup.

3. 4 . 1 .
Effect of Calcination Temperature and Height the Samples Were CollectedThe values of sorption capacity calculated using Equation (2) and breakthrough time are given in Table

Figure 4 .
Figure 4. Sorption capacity of Oldoinyo Lengai ash materials for hydrogen sulfide removal; mass of adsorbent 1 g and the biogas flowrate 0.002 m 3 /min.

Figure 5 .
Figure 5.Effect of the site location on the removal efficiency of H 2 S (a) and NH 3 (b); for all samples T c = 850˚C, flowrate 0.002 m 3 /min, mass of the adsorbent 1 g.

Figure 6 .
Figure 6.Effect of calcination temperature on removal efficiency of H 2 S (a) and NH 3 (b); OLA-TP samples, flowrate 0.002 m 3 /min, mass of the adsorbent 1 g.

Figure 7 .
Figure 7. Effect of biogas flowrate on removal efficiency of H 2 S (a) and NH 3 (b); OLA-TP-850, mass of the adsorbent 1 g.
et al.

Table 1 )
[45] that Fe 2 O 3 , CaO and SiO 2 are most abundant components of all samples; alkali metal oxides are in essential amount, K 2 O ~3% -10% and Na 2 O ~2% -5%.Some of transition and rare-earth elements were also found.As is seen the content of these oxides changes with height the samples were collected.The composition of the OLA samples is rather promising for H 2 S removal due to, as it was observed earlier[15][41][45], Fe, K, Ca, Zn, Na, Ti, and Sr oxides may act as catalysts for oxidation of H 2 S.

Table 1 .
The composition (wt%) and pH values of Oldoinyo Lengai volcanic ash samples.

Table 2 .
Adsorption performance of different OLA samples for H 2 S removal: sorption capacity and breakthrough time; mass of adsorbent 1 g and the biogas flowrate 0.002 m 3 /min.