Air Pollution Control Innovations

As the EPA continues to tighten the emissions belt, I am seeing new industries with air emissions issues. One such industry is pharmaceuticals, who are now more commonly regulated for acid gases on post-combustion devices.

Pharmaceutical air emissions are typically a result of an organic fume from a solvent. The fume, containing vaporized solvent, is captured either within a fume hood or central ventilation system. When regulated, the most effective way of removing a fume is to combust it in a regenerative thermal oxidizer (RTO) or some other combustion device.

The combustion of a solvent such as methyl chloride in an RTO leaves three compounds: carbon dioxide, water vapor, and hydrochloric acid. The last of the three - hydrochloric acid - is often treated as an emission and if so must be removed from the outlet exhaust.

Pharmaceutical Scrubber 

The most best method for removing hydrochloric acid from a gas is the use of a pharmaceutical scrubber. A scrubber offers extremely high efficiencies (greater than 99%, or as required) at a low pressure drop. Recirculating neutralized water across a packed tower, the capital and operating cost of a scrubber is minimal. Further, the effluent from a HCl scrubber contains only sodium chloride - table salt - and can easily be disposed of through a wastewater sewer with little to no further treatment. Using FRP for the scrubber provides a low cost building material highly resistant to acid attack.

Hydrochloric Acid Corrosion

The removal of hydrochloric acid from a combustion exhaust does offer one particular difficulty over other common acid gases, of which designers and operators in the pharmaceutical industry need to be wary. Hydrochloric acid and neutralized chlorides are very aggressive towards most metals, especially so at elevated temperatures typically seen on the outlet of a combustion process. Since the HCl is contained in the exhaust of a combustion process, the inlet gas temperature to the scrubber is high. In turn, the recirculation water temperature is also high, usually well above 100F. Standard metallic materials such as stainless steel will quickly corrode in this environment.

In the past, I have used both AL6XN and hastelloy for metallic materials in HCl scrubber systems. Common metallic items in a pharmaceutical scrubber include the quencher, instrumentation, and downstream devices.  AL6XN is a duplex material that provides very good corrosion resistance to around 1000F. It also has about an order of magnitude greater chloride pitting resistance than stainless steel at neutral pH, and over two magnitudes resistance at low pH.  AL6XN is ideal for quenchers on the exhaust of an RTO, where the outlet temperature is usually around 500F. Hastelloy is more expensive, but it offers heat resistance to 2500F as well as a further order of magnitude resistance to chlorides over AL6XN.

Hydrochloric Acid Mist

The other issue provided by hydrochloric acid in a gas stream is the formation of hydrochloric acid mist, which I have previously touched upon in my acid gas dewpoint post.

Hydrochloric acid mist usually requires a high efficiency mesh pad for removal of any HCl aerosols that may form in the scrubber.  A mesh pad is more expensive than a standard wave form mist eliminator, and is also much more prone to particulate plugging.  If hydrochloric acid is in your gas stream, make sure you consider a mesh pad and beware of particulate!

If you would like to learn more about corrosive acid gas scrubbers, download the free case study below.

Venturi scrubber performance hinges on four key design factors: pressure drop, particle size, water flow, and entrainment separation.  When a Venturi scrubber is not performing properly, it is best to review these design factors one-by-one.

Pressure Drop

Perhaps the most important aspect of a Venturi scrubber is its pressure drop.  The pressure drop of the Venturi is directly correlated to the velocity of the gas passing through the throat.  The higher the pressure drop, the faster the gas.  The speed of the gas is important, as the success of a Venturi is due primarily to inertial impaction.  In inertial impaction, a fast moving particle in the gas strikes a relatively slower moving water drop.  The higher the velocity difference, the greater chance that particle is unable to "duck" into a slipstream around the water drop.

When the pressure drop of a Venturi decreases, the performance of the Venturi decreases.  A drop in pressure can occur for a variety of reasons, but by far the most common is a drop in overall air flow.  In a fixed throat system, the drop in air flow may be a result of a new or upset condition.  In a reflux system, it may be to due to a pressure drop increase in downstream equipment, or a failure with a reflux damper.  For a variable throat Venturi, that is designed to handle changes in air flow, a drop in pressure across the Venturi can signal a problem with the Venturi damper.  In any of these cases, the problem must be fixed to regain the Venturi performance.

Particle Size

In a Venturi scrubber, the particle size actually refers to the aerodynamic size of the particle, which is much more influenced by the mass of the particle than by the diameter of the particle.  Again, the reason is that the Venturi scrubber is an inertial impaction device, and thus the mass of the particle directly influences removal.

The best example of the effect of inertial impaction is a car windshield.  Large, heavy particles like rocks and insects slam into a car's windshield at high speed.  Plastic bags, though much larger, contain very little mass, and "slip" over the windshield.  Particles in a Venturi are captured similarly.  A denser particle with the same volume will be captured more efficiently in a Venturi scrubber than a corresponding lighter particle.

When the performance of a Venturi scrubber varies, it is important to look at upstream equipment to ensure that the particles themselves have not changed.  Smaller, lighter particles will reduce performance of the Venturi scrubber, and necessitate either a greater pressure drop or downstream particle removal equipment.

Water Flow

In inertial impaction, the particles must collide with water (or some other liquid medium) to be collected.  If there is not enough water to collect the particles, performance will degrade.

Water flow can be impacted by a host of common issues.  Since the water is pumped, a problem with a pump can lead to performance issue with the Venturi scrubber.  Plugging of the nozzles or of valves can also occur.  Both of these issues can be solved by performing regular preventive maintenance on the pump and the nozzles.

Entrainment Separator

The final step in a Venturi scrubber is to remove the particle-laden water drops from the gas stream.  Whichever way is selected to remove the water drops, it is important that it does perform well, or the drops will continue into downstream equipment (or even exit the stack).

Waveform entrainment separators, the predominant water separation method, work by causing a change in flow, forcing the water drops to hit the entrainment separator while letting the gas pass through.  The waveforms work as long as the gas travels through the waveforms at the right velocity.  At too high a velocity, the water drops re-entrain, and pass through the entrainment separator.  Particles can also stick to the entrainment separator, changing the waveform shape and reducing the area (thus increasing the velocity).  Waveforms must be designed properly upfront to ensure that flow is uniform through the waveforms, and waveforms must be regularly cleaned to ensure success.

To read more about a specific Venturi scrubber application, please download the white paper below.

Acid gases can be found in the exhaust of a large number of combustion processes.  As a gas, the acid compounds usually are not particularly corrosive and are relatively easy to remove.  However, when the temperature of the gas drops below the acid gas dewpoint, an acid mist can form.  The acid mist can turn into a fine aerosol or it can condense on a cold surface.  Acid mist poses a number of design problems, due to the small size of the mist particles and the corrosivity of the liquid form of the acid.

Aerosol Formation

Aerosol formation occurs when the bulk temperature of the gas drops below the acid dewpoint of the gas.  Much like the formation of fog, the acid gas condenses into tiny liquid droplets.  The size of these droplets can vary widely depending on the acid, the amount of condensation nuclei present in the gas, and degree of supersaturation.

The most common problem that occurs with acid aerosol formation is the inability to capture the aerosol.  Many acid gas exhaust treatment systems utilize a packed bed scrubber to remove the acid.  Packed bed scrubbers are extremely efficient at removing acid in the gas, but unfortunately are ineffective at removing acid aerosol.

The solution to removing acidic aerosol mist is to use a high efficiency entrainment separator or Venturi scrubber, which can effectively capture particles to 1-micron or 0.5-micron, respectively.  If the acidic aerosol mist is primarily sub-micron in nature, a wet electrostatic precipitator also provides a useful solution.

Wall Condensation

Wall condensation occurs when a cold surface is in contact with a hot gas.  If the wall is cooler than the acid dewpoint, acid can condense onto the surface of the wall.  There are several dangers with wall condensation.

First, the material selection for an acid is dependent on its form.  Many acids are not corrosive as a gas, but are very corrosive as acids.  Engineers selecting materials under the assumption that the acid remains a gas often choose materials that are not compatible with the acid in its liquid form.

Second, when condensed, the acid is much more concentrated than it is in the bulk medium.  Instead of selecting a material for a gas containing 10 ppm of SO3 gas, the engineer now has to worry about a nearly pure sulfuric acid droplet.

Finally, the acid can condense in non-ideal locations, leading to pooling and further corrosion concerns.

The solution to preventing the effects of wall condensation is to insulate walls to prevent cold surfaces and select materials for the concentrated, liquid form of the acid in locations where wall condensation is unavoidable.

Acid Dew Point

All gases have a dew point that is dependent on the temperature, pressure, and concentration of the acid in the gas.  This article provides acid gas dewpoint equations for a number of acids.  Below are the formulae for a few of the more common acids in exhaust gases.

Tdp = Dewpoint Temperature, K

Pw = Partial Pressure of water, mmHg

Pa = Partial Pressure of acid, mmHg

Hydrochloric acid (HCl)

1000/Tdp = 3.7368 - 0.1591 * ln (Pw) - 0.0326 ln (Pa) + 0.00269 * ln (Pa) * ln (Pw)

Sulfur Dioxide (SO2)

1000/Tdp = 3.9526 - 0.1863 * ln (Pw) + 0.000867 ln (Pa) - 0.000913 * ln (Pa) * ln (Pw)

Sulfuric Acid (H2SO4)

1000/Tdp = 2.276 - 0.0294 * ln (Pw) - 0.0858 ln (Pa) + 0.0062 * ln (Pa) * ln (Pw)

Click on the button below to down load an Envitech packed bed absorber cut sheet for acid gas removal.

Photo Credit: tinyfroglet

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.

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

Venturi scrubbers are used for the removal of fine particulate.  Gas is accelerated at a high speed through a Venturi throat.  Water is injected perpendicular to the gas flow.  The large water drops injected into the gas stream collide with the fine particulate through a process called impaction. 

The efficiency of this process is dependent primarily on the size and velocity of the particulate.  Superfine, sub-micron particulate are able to follow a stream line around the water drops and are not collected.  Micron-size and larger particulate are not able to slip around the water drops fast enough due to inertial effects.  The exact "cut" of the Venturi depends on the velocity; smaller particles are captured at higher gas velocities.  Venturi scrubbers are excellent particulate control devices for particulate at or above a micron in size.

The performance of a Venturi though is dependent on maintaining the gas velocity at design conditions.  The above video details a Venturi scrubber with an adjustable throat.  In most industrial applications, the gas flow rate varies and with a fixed opening, the velocity of the gas through the Venturi would vary as well, impacting performance.  An adjustable throat offers one method for ensuring the Venturi works over a wide range of operating conditions.

The adjustable throat is a damper blade controlled by a positioner.  The blade is positioned to maintain a constant pressure drop across the Venturi.  The pressure drop across the Venturi is directly related to the gas velocity.  Essentially, the damper blade maintains the gas velocity in the Venturi even at much lower gas flow rates.  As stated above, gas velocity is critical to particulate removal in a Venturi.  An adjustable throat ensures that the gas velocity remains constant, so that particulate removal is unaffected by operation in a "real" environment.