INTRODUCTION

Renewable alternative energy sources are becoming more economically viable, partly as a result of concerns about the effect of CO2 from fossil fuel on global warming and the rising cost of oil and natural gas. Biogas from the anaerobic digestion of organic waste from livestock operations and food processing has long been used as a source of fuel in developing countries such as China and India.

New advances in aerobic digesters and biogas cleaning technology over the last few years have led countries such as Germany to forecast that 17% of their energy requirements for natural gas will be supplied from biogas by the year 2020.

DESCRIPTION OF THE CHALLENGE

Biogas from anaerobic digesters contains approximately 65% methane; 30% CO2, H2O, and N2; and up to 1% H2S.  H2S converts to sulfuric acid with moisture, and this is very corrosive to biogas processing equipment.  New advances in H2S control technologies have made it possible to remove H2S from biogas so that it can be used as clean fuel.  One such technology is the next generation of venturi scrubbers developed by ALTECH Technology Systems with the patented System REITHER venturi scrubber.

Based on field trials, theoretical calculations, and full-scale installations, our design incorporates a venturi scrubber with a second-stage nozzle scrubber, using NaOH to remove high levels of H2S from biogas with over 99% efficiency.  This design limits the absorption of NaOH by CO2 by using the short residence time of the biogas in the venturi, which favors the reaction with H2S over CO2.  This greatly reduces NaOH consumption and the associated costs common in packed tower scrubber technology.

DESIGN AND PROCESS

System REITHER is a wet scrubbing process for air pollution control that uses a patented venturi throat design for the control of fine dusts, mists, aerosols, and gases. This new, patented advance in venturi scrubbing achieves removal efficiencies that usually are higher than 99%.

New advances in throat design have led to significant improvements in removal efficiencies, the ability to respond to variability in flow, and the ability to remove gases and submicron particles and aerosols.

The principle of this patented design is illustrated in Figure 1, in which it is shown that the contaminated airstream flows through the duct and is forced by two outside cylinders into the middle region.  Through the addition of a third cylindrical displacer on a manually adjustable shaft, a double split is formed, creating two venturi throats.  The vertical movement of the displacer can vary the throat to the optimum width for maximum efficiency even during variations in flow.

The scrubbing liquid is introduced through the main spray headers positioned over the venturi throat.  These nozzles have relatively large openings, and this introduces scrubbing fluid to the venturi throat.  The design is based on maintaining an air velocity or speed through the throat of 100 meters per second, which shears the water droplets and creates atomization.  This “shear” velocity breaks the liquid aerosols apart or shears them into a very fine mist, maximizing the opportunity for the scrubbing liquid to collide with submicron particulates and/or gases.  Because of the intimate contact between the airstream and the scrubbing liquid, the use of an additive such as sodium hydroxide or hydrogen peroxide can achieve an instantaneous catalytic reaction with gases such as hydrogen sulfide and other associated compounds.

After the venturi, the airstream enters a secondary nozzle scrubber to provide additional retention time and scrubbing fluid for high volumes of H2S.  As the reacted scrubbing mist enters, the heavier specific gravity pulls the captured gases down into the reservoir. The clean biogas is discharged at the top. Although there are a number of demister designs, the cyclonic demister is the best choice for this situation because there is no chance for plugging and reduced performance.

This project focuses on the design and operation of a scrubber to remove H2S from a municipal sewage pumping station and from biogas generated from covered anaerobic wastewater lagoons at a beef-processing plant.

FIELD TRIALS

H2S Control at a Municipal Wastewater Pumping Station in Ontario, Canada

A municipal wastewater pumping station in a residential location was receiving numerous complaints about odors.  It was situated downstream from a chicken-processing facility, and wastewater from the cleaning cycle was going anaerobic during low flows at night.  H2S was the main odor component, with levels up to 200 ppm recorded over the wet well channels.

Altech conducted a pilot plant demonstration using System REITHER technology to control hydrogen sulfide and other odorous compounds.  Low concentrations of sodium hydroxide and hydrogen peroxide were used with water to remove hydrogen sulfide; the chemical reaction is as follows:

4H2O2 + H2S = H2SO4 + 4H2O

2NaOH + H2SO4 = Na2SO4 + 2H2O

Chemical scrubbers using packed tower technology have been used traditionally for this application.  This technology typically uses a two-stage scrubber system to produce the efficiencies needed to remove more than 99% of the hydrogen sulfide that is generated.

This odor control design uses high-efficiency venturi technology to remove more than 99% of the hydrogen sulfide in a single-stage system.   High-velocity airflows in the venturi throat atomize the scrubbing fluid to create a large scrubbing fluid surface area that forces contact with contaminates.  Figure 2 shows the results of the pilot demonstration in regard to hydrogen sulfide removal.

One of the challenges of this study was to determine the effectiveness of both sodium hydroxide and hydrogen peroxide as additives to the scrubbing liquid.  Hydrogen peroxide is a strong oxidation agent, whereas sodium hydroxide is used to convert the H2S catalytically to the more inert sodium sulfate.  Both compounds were used in low percentages that ranged from 0.5% to 1.5%.  Both compounds were effective; however, hydrogen peroxide required a slightly higher concentration to perform effectively.  Sodium hydroxide appeared to react effectively with the H2S as a catalyst to reduce the compound to sodium sulfate.  The inlet and outlet gas was analyzed by real-time H2S monitors that could demonstrate the results of the reaction immediately.  Also, the scrubbing liquid led to a buildup of small solid particles of a consistent size that would be indicative of the deposits of sodium sulfate.  The conclusion, then, was to use sodium sulfate as the scrubbing system additive only at a percentage of 0.5%.

Major Beef-Processing Facility in the Midwestern United States: H2S from Biogas Generated from Anaerobic Lagoons

A pilot study using System REITHER technology to remove H2S from biogas at a beef-processing plant was conducted on February 10, 2009.  Hydrogen sulfide levels were around 4,600 ppm from biogas generated in covered anaerobic wastewater treatment lagoons.  Biogas was being treated by using iron sponge technology, which was not designed to handle the H2S loading.  A breakthrough was occurring every 30 days of operation.

System REITHER technology demonstrated 90% removal of H2S using a solution of 1% NaOH with water as the scrubbing fluid.  NaOH outperformed hydrogen peroxide, sodium hypochlorite, chlorine dioxide, and ferric chloride as the chemical additive to remove H2S at these levels.  However, based on these levels, the sodium hydroxide was consumed very quickly.

It was noted that as the pH of the scrubbing fluid dropped, removal efficiencies of H2S were reduced as the NaOH was consumed.  It was clear that the sodium hydroxide was very effective at treating the H2S; however, at these concentrations, the pH of the scrubbing fluid could drop from 13 to 1 in 20 minutes.  Clearly, the treatment efficiency decreased accordingly.  The challenge, then, was to achieve reliable high treatment efficiency consistently on an ongoing basis.

The project team designed a dosing system that would measure the pH in the reservoir of the scrubber system and continually injected sodium hydroxide to maintain a high pH.  This approach optimizes the consumption of sodium hydroxide while ensuring high treatment performance.  Table 1 shows the trial results.

SUMMARY

Chemical air scrubbers have been shown to be very effective by chemically treating odorous gases such as hydrogen sulfide and ammonia.  Although traditional packed tower chemical scrubbers have been shown to be effective in controlling some odors, there are drawbacks such as a large footprint, packing media that have to be replaced over time, and, in the case of biogas, a longer retention time that results in high caustic consumption created by absorption by the high percentage of CO2.

Venturi air scrubbers traditionally have been used to control airborne particulates and aerosols from contaminated air.  The System REITHER advanced design has been demonstrated to be very effective in controlling gases such as hydrogen sulfide that chemically react with additives in scrubbing fluid together with fine airborne particulates.

Based on data from the study, sodium hydroxide was the most effective scrubber addition and had good treatment performance at low percentages in the scrubber liquid.  Consumption was governed by the concentration of H2S, which can be highly variable in biogas.  In the case of the beef processing facility, it was consistently 4,600 ppm, which is extremely high.

The benefits of this technology include the following:

• Scrubs many gases and airborne particulate contaminates simultaneously.
• Small footprint and overall size compared with other technologies.
• Simple compact design with no moving parts and low maintenance requirements.
• No media to replace; no channeling or breakthrough.
• High-efficiency technology can achieve >99% removal of odorous gases and particulates in a single-stage system,
• Handles variable airflows with patented adjustable venturi throat.
• Scrubbing fluid can be recirculated without nozzle clogging because of the unique design.

REFERENCES

Table 1:  H. J. Taback, G. C. Quartucy, and R. J. Goldstick, eds. Alkaline and Stretford Scrubbing Tests for H2S Removal From In-Situ Oil Shale Retort Offgas. KVB, Inc., Engineering and Research Division, Irvine, CA, 1985.