Desulfurization plant - Gazpack

Sulfur as an element is ubiquitous. In the universe, it ranks 10th inprevalence; on the earth, it comes in fifth. It is found in vitamins, amino acids, the earth&#;s crust, and virtually every plant and animal. There are myriad uses of sulfur in industry and agriculture, including herbicides, pesticides and fertilizers. Needless to say, it is a critical element in nature and in the economy. Nevertheless, when present in certain chemical compounds and mixtures, sulfur does not play well with others. These compounds can be hazardous to health; damaging to the ecology; and corrosive to many materials utilized for industrial purposes. The organisms and matter where these dangerous compounds are found must be subject to the desulfurization process.

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What Does That Mean?

The desulfurization meaning is actually two-fold: it could refer to removing elemental sulfur at the molecular level or, alternatively, extracting sulfur compounds from chemical mixtures. On its own, sulfur can be extricated from subterranean deposits by melting it through the injection of water heated above the boiling point. Once melted, the sulfur is piped out to the surface. Simple enough. What, though, about those dangerous compounds that find themselves in diverse compositions like natural gas, biomass and petroleum, for instance? What does desulfurization look like when the element is bonded to others?

Natural Gas

Natural gas formations exist all over the world. The largest deposits are located in the Middle East, the Russian Federation and Europe. In North America, natural gas is most abundant in Texas, Oklahoma, New Mexico, Wyoming, and Louisiana. In the United States, over 50 percent of homes are heated by natural gas and this fuel accounts for 24 percent of energy use overall. Natural gas is also a constituent ingredient of many paints, plastics and even medicines. Propane gas &#; popular with avid grillers and barbecue enthusiasts &#; is also derived from natural gas.

Natural gas is often retrieved from its earthen deposits in the same manner as petroleum &#; through drilling or hydraulic fracturing. Sometimes a &#;horse head&#; pump is employed to draw both natural gas and oil to the surface. This resource is found in shale formations, coal beds and sandstone. Once the gas is drawn from its source, it is transported via 300,000 miles of pipeline to locations throughout the U.S., eventually available to customers. Before this lengthy trip, however, the gas must be treated to remove the hydrogen sulfide (H2S) and carbon dioxide (CO2).

The desulfurization of natural gas is necessary for two very important reasons. The H2S that is present is not only toxic to workers and consumers but it is also detrimental to the very pipeline networks that deliver the fuel. At its worst, it can induce respiratory failure, coma and even death to those exposed. Meanwhile, it can quickly oxidize the metal interiors of natural gas pipelines, thereby damaging their safety and overall efficiency. Therefore, natural gas needs treatment at a desulfurization plant soon after its retrieval from wherever it is deposited.

If hydrogen sulfide presence surpasses 5.7 milligrams per cubic meter of natural gas, the gas passes a threshold from user-grade to &#;sour&#; gas. Under such a circumstance the gas is ordinarily run through a tower or column that contains amine, i.e. a derivative of ammonia. The amine solution absorbs the H2S compound as the gas stream passes through, leaving a purer effluent natural gas in its wake. The amine solution is available for further use over many cycles of H2S removal. This procedure for H2S removal is performed in the vast majority of occasions when hydrogen sulfide absorbtion is necessary.

Biogas

Essentially, biogas is the methane (and CO2) that gets released from organic matter under anaerobic &#; or oxygen-deprived &#; conditions. This can take place naturally, as at the core of a compost heap or landfill, or it can be induced by human technology, i.e. engineered anaerobic digesters. Absent oxygen, bacterial micro-organisms operate on the organic material, breaking it down so that biogas is released from the solids that had retained it. The material is as diverse as sewage, food waste, compost, wastewater, rotting vegetation and animal manure. Once captured, the biogas powers combustion engines which, subsequently, charge electrical generators. Biogas can provide energy to vehicles, farms, neighborhoods and even public transit systems.

Still, biogas, too, has an H2S problem, the very same problem evident with natural gas. The public health threat and infrastructure impairment are equally real with biogas production. Fortunately, biogas desulfurization is available by means of two distinct methods.

1. Wet de-sulfurization &#; can be further broken down into three versions:

• Chemical absorption &#; is most akin to amine sweetening with natural gas. Besides amine, straight ammonia and carbonate serve as alternate solvents for the attraction and intake of hydrogen sulfide.
• Physical absorption &#; combines components of solvent with a drop in pressure to separate the H2S from the biogas.
• Wet oxidation &#; uses a weak basic solution to absorb and oxidize the H2S, thereby removing the elemental sulfur from the compound.

2. Dry de-sulfurization &#; rather than using a solvent or solution, dry de-sulfurization utilizes powder/particle agents to treat the biogas. Doing so minimizes the possibility of corrosion in the biogas tank. In general, this method works better when sulfur content is on the low side.

Once properly purified, biogas is an unsung hero of renewable fuels. Not only is it environmentally friendly and cost-effective, it answers a large part of the seemingly eternal waste management question.

Flue Gas

Hydrogen sulfide is not the only problematic sulfur compound in energy production. Sulfur dioxide (AO2) is emitted from exhaust flues at energy plants that burn fossil fuels and facilities that incinerate solid wastes. In nature, SO2 is released when volcanoes erupt. On a smaller scale, it is emitted whenever a match is lighted. Implicated in acid rain, SO2 is a major air pollutant that affects habitat livability and the survival of various plant and animal species.

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How does flue gas desulfurization work? As with the categories noted above, there are multiple ways to do this. One such procedure is wet scrubbing. This involves an alkaline absorbent slurry &#; limestone, lime or even seawater &#; that is sprayed directly on the gas stream or pooled to receive the stream. At other times the slurry is converted to powder by means of hot gas. When the flue gas makes contact with the particles, a similar reaction removes the sulfur molecules. This method is known as spray-dry scrubbing. Other means of flow gas desulfurization are employed depending on sulfur content and related factors. Like atmospheric CO2 scrubbers, these treatments isolate the SO2 from the flue gas stream before it is released into the air.

The atmospheric residue desulfurization process helps to keep toxic particulates from doing damage to ecosystems and the air we breathe. The various methods heretofore mentioned take place in an atmospheric residue desulfurization unit where not only sulfur dioxide, but nitrogen oxides and other particulates are captured and isolated.

How Is Sulfur Retrieved from Compounds?

Many of the agents responsible for neutralizing H2S can separate the sulfur from its compounds. However, research demonstrates that an iron oxide sold sponge &#; i.e. wood shavings infused with iron, or ferric, oxide &#; is effective in hydrogen desulfurization by first absorbing the H2S, and then converting ferric oxide into ferric sulfide. The ferric sulfide subsequently reverts to ferric oxide after releasing elemental sulfur.

Adsorptive Desulfurization

Adsorptive Desulfurization is an innovative technique that has transformed the way we address the presence of sulfur in gas streams. At Gazpack, we pride ourselves on leveraging cutting-edge technology such as Adsorptive Desulfurization to remove sulfur from a variety of gases. This process involves adsorbents like zeolites, activated carbons, or metal-oxides that selectively trap sulfur compounds, resulting in a cleaned gas stream. Importantly, Adsorptive Desulfurization provides a more efficient, economical, and environmentally friendly alternative to conventional desulfurization methods, particularly when dealing with gas streams containing low concentrations of sulfur. 

In Summary

Energy sources with less CO2 emission than traditional fossil fuels promise greater energy independence and a cleaner environment. Yet even these gases have contaminants that require removal before they are burned for electrical generation or combusted for vehicular performance. The good news is that technology (desulfurization plant) provides for the removal of sulfur compounds before the energy is consumed.

Lime plays a key role in many air pollution control applications.  Lime is used to remove acidic gases, particularly sulfur dioxide (SO2) and hydrogen chloride (HCl), from flue gases.  Lime-based technology is also being evaluated for the removal of mercury.  Lime is more reactive than limestone, and requires less capital equipment.  SO2 removal efficiencies using lime scrubbers range from 95 to 99 percent at electric generating plants.  HCl removal efficiencies using lime range from 95 to 99 percent at municipal waste-to-energy plants.  There are two main methods for cleaning flue gases from coal combustion at electric generating stations:  dry scrubbing and wet scrubbing.  Lime is used in both systems.  Dry scrubbing is also used at municipal waste-to-energy plants and other industrial facilities, primarily for HCl control.

Dry Lime Scrubbing

In dry scrubbing, lime is injected directly into flue gas to remove SO2 and HCl.  There are two major dry processes: &#;dry injection&#; systems inject dry hydrated lime into the flue gas duct and &#;spray dryers&#; inject an atomized lime slurry into a separate vessel.  A spray dryer is typically shaped like a silo, with a cylindrical top and a cone bottom.  Hot flue gas flows into the top.  Lime slurry is sprayed through an atomizer (e.g., nozzles) into the cylinder near the top, where it absorbs SO2 and HCl.  The water in the lime slurry is then evaporated by the hot gas.  The scrubbed flue gas flows from the bottom of the cylindrical section through a horizontal duct.  A portion of the dried, unreacted lime and its reaction products fall to the bottom of the cone and are removed.  The flue gas then flows to a particulate control device (e.g., a baghouse) to remove the remainder of the lime and reaction products.  Both dry injection and spray dryers yield a dry final product, collected in particulate control devices.  At electric generating plants, dry scrubbing is used primarily for low-sulfur fuels.  At municipal waste-to-energy plants, dry scrubbing is used for removal of SO2 and HCl.  Dry scrubbing is used at other industrial facilities for HCl control.  Dry scrubbing methods have improved significantly in recent years, resulting in excellent removal efficiencies.

Wet Lime Scrubbing

In wet lime scrubbing, lime is added to water and the resulting slurry is sprayed into a flue gas scrubber.  In a typical system, the gas to be cleaned enters the bottom of a cylinder-like tower and flows upward through a shower of lime slurry.  The sulfur dioxide is absorbed into the spray and then precipitated as wet calcium sulfite.  The sulfite can be converted to gypsum, a salable by-product.  Wet scrubbing treats high-sulfur fuels and some low-sulfur fuels where high-efficiency sulfur dioxide removal is required.  Wet scrubbing primarily uses magnesium-enhanced lime (containing 3-8% magnesium oxide) because it provides high alkalinity to increase SO2 removal capacity and reduce scaling potential.

Comparing Lime and Limestone SO2 Wet Scrubbing Processes

More than ninety percent of U.S. flue gas desulfurization (FGD) system capacity uses lime or limestone.  This trend will likely continue into the next phase of federally mandated SO2 reduction from coal burning power plants.  In , the National Lime Association sponsored a study by Sargent and Lundy to compare the costs of leading lime and limestone-based FGD processes utilized by power generating plants in the United States.  The study included developing conceptual designs with capital and O&M cost requirements using up-to-date performance criteria for the processes.  The results of the study are summarized in two reports: Wet FGD Technology Evaluation and Dry FGD Technology Evaluation. The reports present the competitive position of wet and dry limestone and lime-based processes relative to reagent cost, auxiliary power cost, coal sulfur content, dispatch, capital cost, and by-product production (gypsum and SO3 aerosol mitigation chemicals).  The reports were updated in a report comparing wet limestone and dry lime based systems.

HCl Removal

Because lime also reacts readily with other acid gases such as HCl, lime scrubbing is used to control HCl at municipal and industrial facilities.  For example, at municipal waste-to-energy plants, dry lime scrubbing is used to control emissions from about 70 percent of the total U.S. capacity (as of ).  HCl removal efficiencies using lime range from 95 to 99 percent.  At secondary aluminum plants, for example, EPA identifies lime scrubbing as a maximum achievable control technology for HCl.   EPA tests demonstrate removal efficiencies greater than 99 percent.

Mercury Removal

Different methods for controlling mercury emissions are being evaluated in the United States.  One control technology being evaluated combines hydrated lime with activated carbon.  The reagent, a registered product, consists of 95-97 percent lime and 3-5 percent activated carbon.  Other calcium-based sorbents are also being evaluated as cost-effective alternatives for combined SO2 and mercury removal.

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