Removing H2S from SynGas

Using Proven Technology in Japanese Waste Gasification Facilities


John Watson


Kenneth Jones

Merichem
Schaumburg, Illinois, USA

Introduction

In the past decade, Japan’s rapidly dwindling landfill capacity encouraged development of projects aimed at the gasification of municipal and industrial solid waste. The push to reduce dependence on traditional solid waste disposal methods combined with the continuing search for clean, flexible alternative fuel and chemical feedstock sources justified a renewed focus on gasification of solid waste by a variety of industries and municipalities. Gasification not only greatly reduces the space required for solid waste disposal but also eliminates many of the environmental issues associated with mounds of decomposing waste. The syngas generated in the process is used for power production or as feedstock for a range of chemical processes.

The gasification of this solid waste does produce some undesirable products, one of which is hydrogen sulfide (H2S), a corrosive, toxic product of the sulfur compounds found in the solid waste. The recovery of sulfur from syngas must be accomplished to complete the process of converting solid wastes to clean, environmentally friendly fuels and feedstocks. The total sulfur contained in these raw syngas streams is small in comparison to that produced in large oil refineries and natural gas processing plants. In those large plants, the Claus process has become the standard route to recovery of elemental sulfur. However, the high capital costs and operating complexity associated with small Claus units forced solid waste gasification developers to search for better alternatives. A number of gasification projects in Japan have relied on the LO-CAT® The successful implementation of LO-CAT technology in the solid waste gasification market in Japan provided the technical basis for extending the technology into other gasification markets around the world. The first European gasifier project utilizing LO-CAT is scheduled to startup this year, and LO-CAT units are currently under design and construction for coal gasification projects in China and the United States. Whenever the total sulfur contained in the raw syngas is less than 40 tonnes per day, LO-CAT is a valid option for purifying the syngas and recovering the sulfur in a useable form.

Background

Dealing with hydrogen sulfide (H2S) in fuel gas streams is by no means a new issue. Hydrogen sulfide is an extremely toxic, corrosive and odorous gas, causing safety and materials issues in its unaltered form. High levels of H2S in many raw natural gas streams have long required processing to reduce the contained acid gases before transport and distribution of the fuel to market. The H2S remaining in the fuel is oxidized to sulfur dioxide (SO2), which along with SO2 generated from combustion of various solid and liquid fuels, is a major contributor to acid rain. While sulfur removal from gas streams has been an issue since the first use of sour gas, it has received ever-increasing attention as an environmental issue. Increased use of gasification to turn sulfur bearing materials into useful synthesis gas provides the opportunity to capture the sulfur before it is converted to SO2 and further diluted in flue gases. The body of knowledge gained by processing hydrogen sulfide bearing fuel gases across a wide range of industries has become a part of the expanding field of gasification technology.

Early on, scientists recognized iron as an excellent oxidizing agent for the conversion of H2S to elemental sulfur. However, due to the very low solubility of iron in aqueous solutions, the iron had to be present in the dry state (iron sponge) or in suspensions (the Ferrox process) or compounded with toxic materials such as cyanides. In the 1960’s development work was begun in England to increase the solubility of elemental iron in aqueous solutions. This work led to the introduction of the CIP process, CIP being an acronym for “Chelated Iron Process.” However, it wasn’t until the late 1970s that a system of chelates was developed that had sufficient oxidative resistance to be technically stable and commercially successful. This development work led to the introduction of the LO-CAT process.

For close to thirty years, this technology has served a number of industries. Starting with oil and gas production (upstream), and oil refining (downstream), the basic process has been continually improved and modified to allow for expanded use in other markets and industries. There are more than 180 process installations around the world that depend on Gas Technology Products to remove H2S from their sour gas streams. On a combined basis, these installations remove over 1,250,000 pound of sulfur per day. From petrochemicals to metals (coke oven gas and direct reduced iron off gas), from waste water treatment (municipal and industrial) to carbon dioxide products (food and beverage), this simple robust technology has found many niches. More recently alternative energy arenas such as geothermal, landfill gas, biogases, and gasification have been added to the portfolio of applications successfully using LO-CAT for sulfur recovery.

LO-CAT in Gasification

The first application of LO-CAT technology in gasification was started up in Japan in 2001. The high cost of waste disposal by landfill in Japan encouraged the development of waste gasification projects, with the additional benefit of providing new sources of alternative energy. The success of the first LO-CAT project in this field quickly led to 5 additional units in the Japanese solid waste gasification industry. A similar project will soon start up in Europe, and the experience gained in processing syngas from waste gasifiers has led to the selection of LO-CAT for use in coal gasification projects in the United States and China. These coal based projects are currently in design and fabrication and are scheduled to come on line in 2008.

A summary of LO-CAT units processing syngas is shown in Table 1. As can be seen, the technology been applied to syngas derived from an array of raw materials and in units serving the spectrum of syngas uses as fuel and feedstock. The LO-CAT units operate across a range of pressures, removing H2S both directly from syngas and from amine acid gas extracted from syngas. As has been the case with the introduction of LO-CAT into each new market, the projects started out small with sulfur recovery as low as 100 kg/day and have quickly grown to more than 4 tonnes of recovered sulfur per day. The growth in unit sizes is expected to continue; currently the technology is being considered for gasification projects around the world with sulfur recovery rates from 100 kg to 36 tonnes per day.

Table 1. LO-CAT Gasification Applications


Location

Start Date

Type of Gasification

Pressure/Unit Type

SynGas Use

Flow rate
MMSCFD

Inlet
H2S
ppmv

OutletH2S
ppmv

China

2008

Coal Gasification

High/ Direct

Acetic Acid Production

26.89

4070

<0.5

USA

2008

Coal Gasification

Low/ Indirect

Fischer Tropsch Fuels

0.36

77,400

<1

Italy

June 2006

MSW
Gasification

Low/ Direct

Power Generation

17.92

2,632

<20

Japan

2006

MSW (Municipal Solid Waste)
Gasification

Low/ Direct

Power Generation, Methanol, Ammonia, Hydrogen

17.26

4,050

<40

Japan

2005

23.73

3,000

<40

Japan

2005

5.15

400

<20

Japan

2004

15.32

400

<20

Japan

2003

Waste Plastics

High/ Direct

Ammonia Fertilizer

28.2

140

<1

Japan

2001

Car Shredder Dust

Low/ Direct

Confidential

6.4

296

<10

Process Chemistry

The LO-CAT process was developed to provide an isothermal, low operating cost method for carrying out the modified Claus reaction:

H2S + 1/2 O2 H2O + S°  

As embodied in the LO-CAT process, the basic modified Claus reaction is divided into five sequential steps:

Absorption of H2S
H2S (Gas) + H2O (Liquid) H2S (Aq) + H2O (Aq) (1)
Ionization of H2S
H2S (Aq) H+ + HS (2)
Sulfide Oxidation
HS + 2Fe+++ S° + 2Fe++ + H+ (3)
Absorption of Oxygen
1/2 O2 (Gas) + H2O (Liquid) 1/2 O2 (Aq) + H2O (Aq) (4)
Iron Oxidation
1/2 O2 (Aq) + H2O + 2Fe++ 2 OH + 2Fe+++ (5)

Equations 1 and 2 represents the absorption of H2S into the aqueous, chelated iron solution and its subsequent ionization, while equation 3 represents the oxidation of hydrosulfide ions to elemental sulfur and the accompanying reduction of the ferric (active) iron to the ferrous (inactive) state. Equations 4 and 5 represent the absorption of oxygen (from ambient air) into the aqueous solution followed by oxidation of the ferrous iron back to the ferric state.
Equations 3 and 5 are very rapid. Consequently, iron-based systems generally produce relatively small amounts of byproduct thiosulfate ions. However, equations 1 and 4 are relatively slow and are the rate controlling steps in all chelated iron processes.

It is interesting to note that the chelating agents do not appear in the process chemistry, and in the overall chemical reaction, the iron cancels out. So the obvious question is why is chelated iron required at all, if it doesn’t take part in the overall reaction.

The iron serves two purposes in the process chemistry. First, it serves as an electron donor and acceptor, or in other words, a reagent. Secondly, it serves as a catalyst in accelerating the overall reaction. Because of this dual purpose, the iron is often called a “catalytic reagent”. Although there are many metals which can perform these functions, iron (Fe) was chosen for the LO-CAT process because it is inexpensive and non-toxic.

The chelating agent(s) do not take part at all in the process chemistry. Their role is simply to hold the iron ions in solution. Neither ferrous (Fe++) nor ferric (Fe+++) ions are very soluble or very stable in aqueous solutions. Iron will ordinarily precipitate at low concentrations as either ferric hydroxide Fe(OH)3 or ferrous sulfide (FeS). The chelating agents are organic compounds that wrap around the iron in a claw-like fashion, preventing the iron ions from forming precipitates. The LO-CAT process uses a proprietary system of chelating agents to hold the iron in solution over a very wide pH range.

LO-CAT has developed into a very versatile processing scheme for treating gas streams containing moderate amounts of H2S. Advantages of these systems include the ability to treat both aerobic and non-aerobic gas streams, removal efficiencies in excess of 99.9%, essentially 100% turndown on H2S concentration and quantity, and the production of innocuous products and byproducts.

Process Flowschemes

In applying this chemistry to a wide range of gas streams in diverse industrial processes, many different flowschemes have been successfully employed. The two most common processing schemes utilized in LO-CAT across this range of applications, as well as in gasification, are illustrated in Figures 1 and 2. Figure 1 shows a “Conventional” unit, which is employed for processing gas streams which are either combustible or cannot be contaminated with air. This flowscheme is utilized to direct treat syngas streams without first using a solvent system to separate acid gas from the syngas. Figure 2 illustrates an “AutoCirculation” unit, which is used for processing non-combustible streams which can be contaminated with air. When downstream processing requires removal of CO2 from the raw synthesis gas, it is usually preferable to remove the acid gases (CO2 and H2S) using a solvent based system, and then recover the sulfur from the acid gas using this indirect LO-CAT configuration. The direct treatment of syngas by the LO-CAT process can achieve reductions in H2S that are comparable to those achieved by solvent based acid gas removal units, so the choice of LO-CAT configuration is normally determined by considering the economics of the complete project flowscheme.


Figure 1. Direct treatment of SynGas using Conventional LO-CAT scheme

When using the Conventional LO-CAT scheme, the raw syngas is cooled and cleansed of particulates and entrained liquids in pretreatment equipment upstream of the absorber(s). In this scheme, equations 1 through 3 are performed in the absorber(s) while equations 4 and 5 are performed in the oxidizer.

Several types of absorber are routinely used to direct treat syngas. Since solid sulfur will form in solution in the absorber, only non-fouling devices may be used. Figure 1 shows active LO-CAT solution contacting the pretreated syngas in a venturi absorber in series with a Mobile Bed absorber. These devices are used alone or in combination in low pressure applications requiring minimal syngas pressure drop. Other devices are used as the absorber for high pressure operations as well as for schemes requiring ultra high removal efficiencies. The long history of designing and servicing LO-CAT units that satisfy a wide range of processing conditions and objectives has resulted in a portfolio of contacting devices that provide robust absorber operation for every circumstance. “Sweet” syngas exits the absorber(s), and typically passes through a knock-out pot with mist eliminator for removal of entrained LO-CAT solution, and is then available for downstream use.

The LO-CAT solution from the absorber is directed to the oxidizer for regeneration. In the oxidizer, air is sparged uniformly through the solution, converting the iron back to the active state. The oxidizer consists of a cone bottom vessel containing air spargers and a series of baffles and weirs. Directed air injection creates an “air lift” which promotes circulation and thorough contacting of the solution with air in the upper section of the vessel. The regenerated solution is pumped back to the absorber to complete the cycle.

Sulfur settles in the cone bottom section of the oxidizer and is concentrated into a slurry of 10 to 15 weight % sulfur. The concentrated sulfur slurry is pumped from this cone to the sulfur filter. The cone bottom not only provides a relatively calm area for promoting the concentration of sulfur particles, but also provides space for sulfur inventory, allowing the filter to be removed from service for maintenance without affecting the continuing operation of the LO-CAT cycle.

When acid gases are being removed from the syngas using solvent based processes, the only pretreatment of the acid gas required prior to the LO-CAT unit is a knock-out pot to remove entrained liquids. As can be seen in Figure 2, the acid gas and regeneration air are both directed to the AutoCirculation vessel, and the excess air and “cleansed” CO2 are combined in the single vent from this vessel.


Figure 2. Indirect treatment of SynGas derived H2S using AutoCirculation LO-CAT scheme

In the AutoCirculation scheme, equations 1 through 3 are performed in the “centerwell” which is nothing more than a piece of pipe open on each end. Equations 4 and 5 are performed in remainder of the vessel which serves as the oxidizer. The purpose of the centerwell is to separate the sulfide ions in the absorber from the air in the oxidizer to minimize byproduct formation. In these units, acid gas that has been separated out of the syngas is sparged into the centerwell, and air is uniformly sparged into the remainder of the vessel. The differential in “aerated” density between the solution on either side of the centerwell results in a natural circulation from the oxidizer into the absorber and back again. The resulting unique feature of the AutoCirculation scheme is, of course, that no pumps are required to circulate solution between the absorber (centerwell) and the oxidizer.

As in the oxidizer vessel in a conventional flowscheme, the AutoCirculation vessel is given a cone bottom, where sulfur is concentrated to 10 to 15 weight % before the slurry is pumped to filtration. This conical area once again serves as an operating inventory for sulfur product when maintenance is performed on the filter.

In either LO-CAT scheme, a chemical addition skid and sulfur filter round out the unit.

The chemical addition skid comprises tanks and metering pumps that allow controlled addition of the caustic, iron catalyst and chelate formulation to replace traces lost in the sulfur cake and losses due to chelate degradation and salt formation. Many of the improvements to LO-CAT technology over the past 30 years have successfully reduced these losses and the required make-up. Other proprietary chemicals are also made available from this skid to deal with foaming and biological activity on an as needed basis.

A range of filtration methods are employed in LO-CAT, with the choice primarily dependent on unit size. A slurry of sulfur and LO-CAT solution is pumped from the settling cone to a filter, where the sulfur is concentrated to a cake consisting of sulfur and water with traces of salts, iron, and chelates. The filtration method represented in Figures 1 and 2 is a vacuum belt filter which is the most common method utilized within LO-CAT units across all industries. This method typically produces a cake that is roughly 65 wt. % sulfur. The moist cake is easily handled after collection in dumpsters or supersacks fed from the filter belt, without problems that could result from free liquids. This moisture content also allows for sufficient removal of byproduct salts in the cake to balance their production in the process, thus eliminating the need for a liquid waste stream to be processed in a waste treatment facility.

Sulfur Product

Elemental sulfur recovered by the LO-CAT process has quite profound differences from sulfur produced by other processes. Recovered sulfur has a particle size ranging between 8–45 microns, much smaller than sulfur generated by other means. And since this solid sulfur is formed in a aqueous solution, it harbors no trapped H2S vapors that can evolve during subsequent handling. The LO-CAT process gives sulfur these distinct differences:

  • Softer particle texture,
  • Hydrophilic nature and water miscibility,
  • Faster soil absorption characteristic and degradation.

Benefits to the agricultural industry are attributed to this faster performance and ease of use. Specifically, agricultural users recognize the benefits of using LO-CAT sulfur for soil pH adjustments, plant nutrients and foliage fungicides.

LO-CAT sulfur is nontoxic and has been approved for use by the Organic Materials Review Institute. OMRI establishes standards for crops to receive “organic” designation. OMRI follows the guidelines required for compliant use of sulfur under the USDA National Organic Program Rule 7 CFR Part 205.

As SO2 emissions restrictions around the world are tightened, the reduction in acid rain is leading to increased demand for sulfur based soil amendments to provide optimum soil pH and restore the nutrient balance needed by many crops. LO-CAT units provide a source of fast acting sulfur for local agricultural needs.

The Future

The benefits of converting many liquid and solid materials formerly labeled as waste to useful synthesis gases will continue to expand the volume and variety of waste gasification projects around the world. By nature these projects will be limited in size, and the variety of waste materials utilized will result in a large number of projects that will generate low to moderate quantities of H2S contained in the raw syngas product. LO-CAT has been proven to be an excellent process choice for syngas purification and sulfur recovery for projects with this profile. And based on almost 30 years experience of adapting the process to a wide range of processing markets, it is likely that LO-CAT will play an increasing role in the successful use of gasification in this field.

Additionally, small coal gasification projects are expected to be built to provide fuel gas to replace conventional natural gas in selected alternative fuel projects. Coal gasification has also been proposed to generate power and/or valuable liquid products from coal at the mine sites to minimize the costs of transportation. When these projects utilize low sulfur coal, the low to moderate quantities of H2S produced will once again favor the LO-CAT process. As gasification extends to these and other as yet undefined projects, LO-CAT can provide a reliable low-cost path to clean syngas and useable sulfur.

Jess Karr, President of Hondo Chemical Inc., “Sulfur: At The Crossroads of Energy, The Environment, and Agriculture”, Fertilizer International May/June 2002