Air Pollution Control Innovations

One of the most difficult value engineering challenges I encounter is the selection of instrumentation.  Instrumentation within air pollution control systems may be subject to high salinity, high halide concentrations, pH swings, extreme temperatures, extreme ambient conditions, intrinsically safe environments, and abrasive particulate, just to name a few.  Selecting the proper instrumentation for each environment is a chore and can dramatically effect the cost.  It is a topic that both end-users and integrators should attempt to resolve before a system is built.

Extreme temperatures

Most know of the obvious differences between outside environments as opposed to inside environments.  Rain, snow, and sun all batter instruments outside; equipment inside rarely see any of the three.  Extreme heat though is not the exclusive domain of outside environments - I am currently working on a project where inside temperatures can reach 140F.  For that project, I have moved all of the temperature sensitive transmitters inside an air conditioned control cabinet to meet the temperature requirements of the analyzer.  When dealing with extreme temperatures, it is best to find ways to reduce the effect.  For extreme cold environments, use insulation or heat tracing.  For the extreme heat, look at moving sensitive electrical equipment into a temperature controlled environment.  Remember, plan for the extremes!

Be a copycat

A lot of the projects that I see are retrofits - the addition of new air pollution control equipment on existing processes or equipment.  In these cases, it is usually best to select instrumentation that is already working well within the plant.  First, copying instrumentation removes most doubts of instrument failures.  Second, operators have a familiarity with the instrumentation, and are more apt to use the instrumentation properly.  Finally, copying instrumentation reduces the number of spares that a plant needs to keep available, thus reducing the net cost of the instrument.  Whether you are buying the equipment for a customer, or buying equipment with prepackaged instrumentation, always find out what instrumentation is already working! 

Preventive Maintenance

Even after selecting the right equipment, there is still the need to properly maintain the equipment.  Acid gas scrubbers almost always require a pH probe for dosing basic reagents; those pH probes require regular calibration and often have lifetimes of six months.  A probe off by 1.5 pH units can cause a drop in acid gas removal efficiency from 99% to 75% - a significant problem if your stack tester is visiting! Conductivity sensors and hardness analyzer and ion selective electrodes also require regular maintenance.

For instruments that do require calibration, it is important to use bypasses to allow for calibration during operation.  A bypass loop with an instrument hold function allows an operator to carefully calibrate the instrument while the system continues to operate in a safe fashion.

Recommendations

Above all else, work with the end-user to identify instrumentation requirements.  A customer running a 24/7 operation has dramatically different requirements for instrumentation than one that is on a single shift operation or batch process.  Operators should be thought of as well.  Are the instruments accessible?  Can operators view indicators?

Remember, selecting the right instrumentation minimizes downtime; downtime that may cost you ten times the price of the instrument.

Back in July I wrote a blog post for gasification syngas cleaning where I discussed two general approaches, 1) thermal tar destruction, and 2) tar removal scrubbers.  Both approaches require particulate removal. This blog post discusses several design considerations related to particulate control for syngas wet scrubber systems, including:

1. Performance
2. Capital Cost
3. Operating Cost
4. Safety

These considerations will be discussed in the context of two wet scrubber approaches for particulate control:

1. Venturi Scrubber
2. Wet Electrostatic Precipitator

 Performance - The distinguishing feature between a Venturi scrubberand a wet electrostatic precipitator (WESP) is the removal efficiency for sub-micron particulate.  This is shown in the above figure which compares the particle removal efficiency for a wet electrostatic precipitator (WESP) and a 50 inch water column (W.C.) pressure drop Venturi scrubber.   The figure illustrates that both the WESP and Venturi are highly efficient for removing particles greater than 1 micron.  The removal efficiency of a Venturi, however, begins to degrade for particles smaller than 1 micron.  The Venturi performance can be enhanced by sub-cooling the gas and taking advantage of condensation effects to grow the size of the particulate.  The effects of sub-cooling to improve Venturi performance is discussed in greater detail in the Envitech paper, "Wet Scrubbing Technology for controlling biomass gasification emissions" presented at the 2008 Joint Conference: International Thermal Treatment Technologies (IT3) & Hazardous Waste Combustors (HWC). 

In general, WESP's are used in applications where the sub-micron particulate concentration exceeds the capability of a Venturi to meet the performance requirements. It is therefore important to understand the following:

• Inlet particulate & tar concentration
• Inlet particle size distribution (PSD)
• Particulate tolerance of the downstream internal combustion engine (ICE)

A Venturi scrubber will give syngas cleaning performance similar to a WESP. The removal efficiency for particles greater than 1 um diameter will be equal to or greater than a WESP.  For particles smaller than 1 um diameter, a Venturi scrubber will be less efficient that a WESP.  However, many ICE engines will most likely tolerate these particles.  Understanding the tolerance of the engine is therefore a key aspect of deciding which approach is best suited for your application.

Capital Cost - It is broadly understood that a Venturi scrubber is much lower capital cost than a wet electrostatic precipitator.  Under most process conditions this cost difference can be as much as 3 to 4 times.  The trade-off for a lower capital cost Venturi scrubber is higher operating cost to provide the pressure drop. 

The Venturi scrubber capital cost is determined predominately by the size of the gas flow.  The WESP capital cost, however, is determined by both the size of the gas flow and the desired removal efficiency.  The desired removal efficiency can dramatically affect the size and cost of the system. The higher the removal efficiency, the higher the collection area, and consequently, the greater the number of collection tubes required.   The cost of a WESP is approximately exponentially related to the required removal efficiency.  It is important to define the performance requirements before budgeting for a WESP.

In addition to metal fabrication, there are other items contributing to the higher capital cost of a WESP, including the T/R set to provide a high voltage, electronics for a more sophisticated control system, and safety interlock system.

Operating Cost - Although a wet electrostatic precipitator is higher capital than a Venturi scrubber, part of that cost is offset by lower operating cost. The pressure drop of a WESP is in the range of a couple of inches W.C. compared to 30 to 50 inches W.C. for a Venturi scrubber.  The electricity cost for the fan horse power requirements is therefore considerably lower for a WESP than for a Venturi.  There are other WESP operating costs that need to be accounted for including the electricity for the T/R sets and for the heater and blowers for the insulator compartments.

Safety - The last design consideration discussed here for a syngas cleaning system is safety.  A key aspect for a syngas cleaning system is that it contains a combustible gas.  This carries a greater risk of fire than for other types of scrubber system.  If the system is located in a confined space, it is often required for instrumentation and motors to meet division I, class II (explosion proof) requirements.  A WESP can operate in sparking mode which can be an ignition source for the gas.  Care must be taken to ensure the WESP operates in a safe condition at all times.  A WESP has additional safety interlock requirements because it operates at a high voltage.  For these reasons, a WESP it is more costly to ensure safety in a WESP than a Venturi scrubber.

Summary

• A Venturi scrubber is lower capital cost than a WESP and in most cases is preferred if it can meet the performance requirements.
• A WESP is generally used in cases where the concentration of sub-micron particulate exceeds the capability of a Venturi scrubber to meet the peformance limits.
• Although a higher capital cost, a WESP has the advantage of lower operating cost. It will also achieve greater overall removal efficiency because it is more efficient for particles smaller than 1 micron.
• Because syngas is a combustible gas, there are safety considerations for both a Venturi scrubber and a WESP. Because a WESP uses a high voltage and can act as an ignition source, the cost to mitigate safety risks is generally considered to be higher than for a WESP.

 To learn more, please download our presentation on tar removal.

Envitech is often asked to make recommendations for particulate removal on a wide range of industrial applications.  This might entail deciding if a Venturi scrubber can do the job or if a wet electrostatic precipitator (WESP) is required.  Envitech uses proprietary modeling that accurately predicts Venturi performance under various conditions of pressure drop and sub-cooling.  Because Venturi performance is highly dependent on particulate size for particles less than 1 micron diameter, an essential piece of data for making a performance guarantee is the particle size distribution (PSD) for the particles in the inlet gas.

There are several test methods for determining the particle size distribution, however the test method must determine the aerodynamic particle size to adequately predict the Venturi performance.  The most common method for determining the aerodynamic particle size is to use a cascade impactor.  This can be a challenging test because the inlet gas stream may be at high temperature and contain a high particulate loading and moisture content.  It is important to select a testing company that has experience with this type of environment.  Below is a references from the California Air Resource Board (CARB) that describe acceptable test methods for this purpose.

• CARB Method 501 - Determination of Size of Distribution of Particulate Matter Emissions from Stationary Sources

A method that some companies use to to determine the PSD is to collect particles on a filter and analyze the dust with a particle size analyzer.  This test method will give the physical particle size.  However it can give an inaccurate estimate of the aerodynamic particle size.

The problems commonly associated with this method are outlined below:

• You may not get a representative sample. Different size particles end up in different layers on the filter. Agglomerates of particles can come apart. Individual particles can agglomerate and show up as one particle.
• This is a measure of the physical size and not the aerodynamic size. Venturi scrubbers primarily collect particles according to their aerodynamic size through inertial mechanisms. The aerodynamic size is a function of the density of the particle and the size of the particle. The greater the density, the greater the aerodynamic size. In some cases, particles may not be solid particles of inorganic material, but may be hollow glassy spheres, which will make them act more like soap bubbles rather than hard balls. Some combustion processes at extremely high temperatures can lead to hollow spheres.
• The size of the particle affects the aerodynamic size because, the smaller the particle, the easier it is to slip between gas molecules. As the particle size approaches the mean free path of the gas molecules this becomes a significant component of the aerodynamic size.
• To use data from a particle size analyzer, it is necessary to generate a log normal PSD curve of the physical size. This curve then needs to be corrected to provide an aerodynamic PSD curve. However, this requires making two assumptions: 1) particle density, and 2) particle shape (usually assumed to be a solid spherical shape). These assumptions may or not be valid and can lead to inaccurate Venturi performance predictions.

Before settling on a design path for particulate removal, Envitech recommends testing the process to generate good particle size distribution data.  This will help ensure meeting the performance requirements in the most economical way possible.

To read more about Envitech's Venturi scrubbers, download the free case study on a Venturi scrubber treating the exhaust of a dryer.

photo credit: azredheadedbrat

Back in June I mentioned that I was getting ready to head to the 2009 International Fuel Ethanol Workshop and Expo, which was held June 15-18 in Denver, CO.  I had the opportunity to present a new ethanol scrubber design during the emissions abatement optimization session in the energy & environment track.  My presentation dove- tailed well with other topics of the co-presenters, given below.  

Track 2: Energy & Environment
Emissions Abatement Optimization

• Moderator: Monty McCoy, Technical Manager, US Water Services
• New Ethanol Scrubber Reduces Plant Capital and Operating Costs
  Andrew Bartocci, National Sales Director, Envitech Inc.
• Fail-Safe Scrubber Emissions Compliance for Ethanol Biorefineries
  Monty McCoy, Technical Manager, US Water Services; and Bob Elliott, Environmental Field Manager, American Engineering Testing Inc
• VOC, CO, and NOx Abatement and Optimization with RTO's
  Andy Rodger, Engineer, Pro-Environmental Inc.
• Air Emissions Permit Compliance and Pollution Control Device Optimization Using Advanced Measurement Techniques
  Thomas Dunder, GE Energy

I discussed a new ethanol scrubber used to recover ethanol from the fermentation and other vent streams.  These streams contain CO2, ethanol and low concentrations of various volatile organic compounds (VOC's).  Chilled water is commonly used to recover the ethanol, however, many of the VOC's (acetaldehyde, etheyl acetate, acrolein, and acetone) are highly insoluable in water and do not scrub out well.  Post processing is often required to meet emission limits, which adds costs.  I introduced a new, 2-stage scrubber design that uses re-circulated ethanol in the bottom stage and once-through chilled water in the top stage.  Ethanol is an excellent solvent for the residual VOC's and it is readily available at ethanol plants.  This approach eliminates the need for post processing and significantly reduces the plants capital and operating costs. 

The presentations by Monty McCoy of US Water Services, Bob Elliot of American Engineering Testing, Inc., and Thomas Dunder of GE Energy showed test data from various ethanol scrubbers which illustrate how water flow rate, water temperature, and  bisulfate injection rates impacts scrubber performance.  The data also showed how the emission rate changes throughout the fermentation process.

Other issues of ethanol scrubber performance were discussed including channeling and fouled mist eliminators. Channeling is related to non uniform liquid to gas (L/G) ratio. When a limited amount of water is applied to the packing in a tall, narrow tower, uniform distribution of the water is critically important. If there are parts of the packed section where the L/G is lower than the average value, gas passing through those parts of the packing will not be scrubbed as efficiently.  If any water runs down the tower walls, it won't spend as much time in contact with the gas and will absorb less ethanol than water trickling over the packing.  A properly design scrubber should have internals that help maintain uniform distribution.

Fouled mist eliminators can result in excessive pressure drop.  This can be caused by improper selection of the type of mist eliminator or from not having a proper wash system to keep the mist eliminators clean during operation. It's important to not view the fermentation scrubber as a simple can with packing in it.  There are nuances to the scrubber design that enable it to operate at optimum performance with minimum maintenance.

To download the presentation click the icon below.

photo credit: stefanie says

The primary products of synthetic gas (syngas) production from biomass are hydrogen gas, carbon monoxide, and methane.  Unfortunately, those are not the only compounds formed.  Other compounds form depending on the elemental chemistry of the biomass.  One of the more common byproducts is ammonia, released from organically-bound nitrogen.

Why ammonia rather than NOx

Organically-bound nitrogen converts to ammonia, rather than NOx, in a gasifier.   Gasifiers are typically oxygen-starved environments.  Conversion of organic nitrogen to NOx requires oxygen.  Without oxygen, combustion thermodynamics favor the production ammonia.  In fact, ultra-rich combustion environments produce reduced compounds, like ammonia, rather than oxidized compounds.

Why it is still NOx

The main goal in the production of syngas is to eventually burn it!  Ammonia left in the syngas WILL convert to NOx in a rich combustion environment, like an internal combustion engine or a syngas-fired boiler.  NOx is a heavily controlled environmental pollutant and it is important to minimize its production.

How do I remove it

Fortunately, ammonia is significantly easier to remove than NOx.  Ammonia can be removed with an ammonia scrubber using water and sulfuric acid.  Ammonia reacts with the sulfuric acid to form ammonium sulfate.  Provided any particulate is removed upstream, an ammonia scrubber can produce a very high concentration of liquid ammonium sulfate that has commercial value as a fertilizer.  Since sulfuric acid is relatively cheap, the operating costs of an ammonia scrubber are minimal. 

Ammonia gas can also be formed using a scrubber/stripper approach, first removing the ammonia in an ammonia scrubber, and then liberating the ammonia as a gas in an ammonia stripper.

Envitech's experience with ammonia scrubbing spreads into other industries.  Please read our white paper on the reduction of ammonia emissions for a sludge dryer.

Photo Credit: ajturner

Quenchers are used in air pollution control equipment to rapidly cool a high temperature gas stream.  Rapid cooling prevents gas phase reactions that may occur at intermediate temperatures, such as dioxin and furan formation.  Rapid cooling also relaxes the material requirements of downstream equipment, allowing the use of materials such as FRP, CPVC, and duplex steels.

In the video above, we take a look at a rectangular quencher.  We have used the rectangular quencher for regenerative thermal oxidizers.  A rectangular quencher provides sufficient quenching for relatively low temperature gas streams (200F to 600F).  Water is sprayed directly on to the gas, away from the upstream process.  Water drains from the quencher into the basin for recirculation.

In the picture to the right, the spray headers are installed using a double flange approach.  The two flange approach improves the accessibility of the nozzles in the quencher, which can be susceptible to plugging in some applications.  Water is connected to the first (header) flange.  The second flange connects to the quencher.  The spray header can then be removed like a lance.

The picture to the left shows an example of a rectangular quencher.  A rectangular quencher in low flow, low temperature quench applications are typically more cost-effective than round quenchers, especially if they are attaching to rectangular or horizontal ducts.

Download a 3D video of the spray lances using the link below.

Last week I discussed a recent presentation on biofuels focusing on algal biodiesel.  Algae is a big topic in the biomass industry at the moment, as it offers high growth rates as compared to other forms of biomass.  The problem is that, as a new technology receiving a high level of interest only in the past 3-5 years, the optimum method for cultivating the biomass has yet to be determined.  A recent article in Biomass Magazine looks at an alternative to biodiesel: catalyzed gasification.

What does it produce?

The technology, developed by the Pacific Northwest National Laboratory (PNNL), involves the catalytic conversion of aquatic biomass.  The authors state that the process transforms the algae into methane, carbon dioxide, ammonia, and water.  Methane, of course, is the chief component in natural gas, and the main product of the process.  The authors suggest that the water and carbon dioxide can be pumped back to the growth ponds as feed.  Ammonia and sulfur products come out in the water, and must be removed prior to reinjection.  Using a train of an ammonia stripper and an ammonia scrubber, the ammonia can be converted into ammonium sulfate; possibly using the sulfur byproducts.  So far, so good.

Is it efficient?

The real interesting part of the technology is its ability to create natural gas from the aquatic biomass without drying the biomass, which represents a huge drain on the thermal efficiency.  The article suggests temperatures of 350C (662F), but under pressure, so the water remains a liquid.  If true, rather than expending ~1115 BTU/lb to dry (evaporate) the water from the biomass, the process only uses ~670 BTU/lb to heat the water, a thermal savings of 40%.

Too good to be true?

I do have some reservations about the technology as presented.  The biggest drain on biomass is the amount of water contained in the biomass.  Water is an energy sink - it does not combust and reduces the combustion of heat of the biomass.  Any savings offered by this process is dependent on the ability to generate a "dry" biomass.  I typically see biomass sources containing 20% biomass and 80% water.  An algae stream with 10% biomass would have twice as much water, and would lose the gain in thermal efficiency as it has to heat twice as much water.  The math only works if the biomass is concentrated. 

And that may be only half the problem.  My experience with fluidized beds is that there is a limit to the concentration of solids in a fluidized bed.  The simplest way around that is to recycle the water produced in the process (which presumably is still hot and pressurized).  However, that is still a loss in the system, and the exact solids concentration limit will have an impact on the efficiency of the process.

Final thoughts

I believe, as stated in this article, that harvesting algal biomass is quite possibly the most critical step to its economic viability.  Water is an energy drain on all of biomass.  PNLL and Genifuel look to have found one way to possibly reduce the cost.  Further, I like their approach at looking at waste streams first, as solving a wastewater discharge problem improves the economics of the process; it is one of the reasons that waste to energy projects have succeeded.

To read more about ethanol recovery, please download our presentation at the 2009 FEW conference.

On Wednesday I discussed the first part of Professor Steven Briggs presentation on biofuels.  The second part of Professor Briggs presentation was specific to algal biodiesel.  Like Wednesday, I will provide a bit of commentary on some of his remarks (underlined, paraphrased).

Biomass Production

• Algal - 48 metric ton/acre/year
• Switchgrass - 12 metric ton/acre/year
• Sugarcane - 10 metric ton/acre/year
• Corn - 6 metric ton/acre/year

The information above was one of several slides that Professor Briggs used to make a point on the effieciency of algae in converting to biomass.  I believe Briggs was indirectly tackling the land use issue.  For those a bit removed from the biomass industry, there is a growing land use issue that both Andy B and I have heard about at recent ethanol conferences.  The theory is that land use for energy will replace acreage that is currently absorbing carbon dioxide (by way of trees and foliage), converting the land from a carbon sink to carbon neutral, and thus increasing the amount of global warming.  It is an interesting and complex topic, and given Briggs attention to land use in his presentation, I believe it is likely to be used as a counterargument to biomass collectively, whether it is true or not.  In any event, it is becoming important politically to maximize yield per acre, as there are other thorns on the land use issue (food production, fertilizer run off, genetically-modified plants) even if this one is resolved favorably for biomass.

We need to refine biodiesel to higher grade fuels 

Professor Briggs presented a wonderfully simple flow chart showing the steps from farming to fuel.  There were several steps in the process where Professor Briggs noted that algal biodiesel was not yet efficient enough for commercial production, but the most interesting step for me was the refining process.  Refining biodiesel is essentially maximizing the production of low molecular weight carbons and increasing the heat content of the fuel.  However, there is also a third goal of refining, and that is in minimizing the production of secondary pollutants (sulfur oxides, nitrogen oxides, and chlorides).  A great deal of time and money has been spent to minimize the secondary pollutants produced by hydrocarbons.  Will biofuels be able to capitalize on these technologies, or will there need to be new technologies developed to reduce secondary pollutants?

One final note - Professor Briggs displayed a colorful table at the end of his presentation showing a variety of fuel sources in rows (hydrocarbons, algal biodiesel, electricity, etc.), a variety of issues with those fuels in columns (cost, security, availability, etc.) and a green (good), yellow (fair), or red (bad) symbol in each box for their impact.  Teasingly simple, it also suggested that there is quite a fight amongst "green" energies - all of algal biodiesel boxes were green, while other green sources were noticeably multi-hued - despite the lack of an in-place commercial-scale techology (isn't that a bit important?).  I guess competition in the green industry is no longer just limited to beating up hydrocarbons.

For a presentation on ethanol scrubbing, download the presentation below.

 Photo credit: higetiger, octal

Yesterday I attended a presentation on biofuels by Steven Briggs, a biology professor at University of California, San Diego.  Professor Briggs possesses a wealth of academic and industrial experience with genomics and biofuels, including corn ethanol, biodiesel, and algal biodiesel.  Undoubtedly, Professor Briggs is an expert in bioenergy, and the overwhelming attendance indicated the importance of the industry to the greater San Diego area.  Today I would like to highlight some of the topics Briggs spoke on (underlined, paraphrased).

Liquid hydrocarbon fuels account for 41% of the energy consumed

Professor Briggs highlighted the tremendous consumption of liquid fuels, primarily oil.  Briggs later noted that there is an existing infrastructure for liquid fuels. 

What does this mean?  Biofuels that are compatible with existing hydrocarbon infrastructure have a tremendous financial advantage.  All of the biomass projects I have seen create energy or fuel that fits into existing infrastructure.  Ethanol, for instance, has found a market as a replacement for liquid hydrocarbons precisely because it can be used within the existing infrastructure (car engines).  As Professor Briggs noted though, ethanol is more corrosive than oil, which has prevented its use in the existing infrastructure that transports oil from refineries to gas stations.  Other projects I have seen include converting wastes (biomass) into electrical or steam energy, or into solid fuels for use in cement kilns.  The common thread for current projects: Apply renewable fuel sources to existing infrastructure.

Oil production in the United States peaked in 1970.  Using theories derived from oil production in the United States, and that of coal production in England, worldwide production of oil is expected to have peaked in 2005.  Early estimates indicate that this may be the case.

Has the world hit the downslope of oil production?  There are far-ranging impacts to a decrease in oil production, both economic and political.  For the biofuels market in particular, it may lead to higher prices for hydrocarbon fuels and more cost-competitiveness, one of, if not the key obstacle that biofuels must overcome.  A further sign of a drop in oil production might be in the flurry of activity in biofuel research by traditional hydrocarbon-based companies.  Exxon-Mobil, Chevron, and British Petroleum have all made recent announcements of significant investment in biofuels.

The United States has set a target of 24% of all energy consumption from renewable fuels by 2022.

Briggs provided a table with mandates for renewable energy consumption for a number of countries.  Briggs was quick to point out that the mix of renewable fuels to meet this target is yet to be determined.  Obviously though, consumption will be driven to the lowest cost fuel that meets the definition of renewable fuels.  Hydrocarbon liquids have such a huge share of the energy market because they are precisely the lowest cost fuel.  We can reasonably expect that the lowest cost renewable energy fuel will corner the market in the same fashion that hydrocarbons have for the past 200 years.

I will have more on the second part of Professor's Briggs presentation, algal biodiesel, in my next post.

For a presentation on ethanol scrubbing, download the presentation below.

Photo Credit: Binary Ape

Concern for global climate change coupled with high oil prices has generated new interest in renewable energy sources.  One of these sources is waste to energy using gasification.  Gasification is a thermal destruction process which produces synthetic gas (syngas) as an end-result.  In one form, the syngas is then used as fuel in an internal combustion engine (ICE) to drive a generator, producing electricity.  Waste heat is recovered from the system to improve the overall plant efficiency.

During gasification, various pollutants may be produced depending on the make-up of the waste feedstock. The feedstock can vary by plant from biomass, municipal solid waste (MSW), or even hazardous waste.  The pollutants involved with these processes include sub-micron particulate matter, tars, ammonia, metals, dioxins and furans, and acid gases.  One of the primary challenges is cleaning the pollutants in the syngas to a level that is tolerated by the ICE.  There are many innovative companies working to commercialize waste-to-energy production using gasification.  Each application is unique and depends on the type of gasification process and feedstock material.  We've seen two general approaches regarding syngas cleaning:

1. Thermal Tar Destruction  
2. Tar Removal Scrubber

Thermal Tar Destruction - In this approach, the syngas passes out of the gasifier and through thermal process that destroys the tars at a high temperature.   This greatly simplifies the gas clean-up as it eliminates the need for a tar removal clean-up system.  The trade-off, however, is a lower energy content of the syngas.  The gas clean-up can be achieved with proven, reliable scrubbing technologies, similar to systems that have been used in conventional incineration scrubbing systems.

Tar Removal Scrubber - The tar removal scrubber approach has a lower outlet temperature and a higher energy content, but it contains tars that are more difficult to remove.  The main challenge of tar removal relates to the fouling that can occur in the initial stages of condensing and collecting the tars.  The source of the challenge is the formation of "tar balls" which are long-chained hydrocarbons that have a tendency to agglomerate and stick together, fouling equipment.  Tar removal processes also produce liquid wastes with higher organic compound concentrations, which increases the complexity of water treatment.

Although more complex, these problems can be overcome.  Envitech has developed a second generation syngas tar removal system that uses a clean liquid stream for condensing and collecting tars. The system utilizes an arrangement of conventional process equipment for solids/oil water separation that results in a clean discharge stream and return liquid to the scrubber. By returning a clean liquid stream to the cooling circuit and condensing section, problems associated with tar ball fouling is eliminated. In addition, the process mitigates the impact of organics in the liquid discharge.

In a future blog post I will discuss considerations involved with selecting a wet electrostatic precipitator versus a Venturi scrubber for particulate control for syngas cleaning systems.

Click on the icon below to download a white paper written about gasification emissions using wet scrubber technologies.

Photo - PRM Energy Gasifier