Green Machines

As the industry faces tough new emission control requirements, producers are turning to technology suppliers and design-engineers for new and improved solutions. Take a look at the ethanol industry's
By Tom Bryan | July 01, 2003
Under today's increasingly stringent emissions control technology plans, ethanol producers are utilizing a variety of control solutions intended to keep plants operating at, or below, parameters established by state and federal agencies.

To be permitted as minor emitters, ethanol producers are responsibly turning to abatement technologies that effectively reduce volatile organic compounds (VOCs), particulate matter (PM), hazardous air pollutants (HAPs), carbon monoxide (CO), NOx gas and dust.

Below, with help from a handful of industry suppliers, EPM takes a look at the latest ethanol plant emissions control technologies on the market.

Thermal Oxidizer-Waste Heat Recovery
Minnesota was the first state to negotiate consent decrees with operating fuel ethanol plants, primarily because the regulated emissions from distillers grains dryers was, in many cases, beyond permitted limits, according to Dennis Vander Griend, senior process engineer at ICM, Inc.

"Of primary concern were VOCs, PM and CO," Vander Griend tells EPM.

Last fall, ICM Engineering started its first distillers grains dryer thermal oxidizer (TO) plant in the Midwest; the company now has four TOs operating. According to Vander Griend, all four TO plants have demonstrated "very good VOC and PM destruction" as each plant has "no stack opacity or odor complaints." Stack emission tests, he says, confirm very high destruction efficiency for VOC and PM.

In a nutshell, here's how the technology works.

In an emission abatement system of this type - a thermal oxidizer coupled with a waste heat recovery boiler - the DDGS dryer exhaust passes through the TO, where dry exhaust is heated to 1500F for oxidation of the VOC, PM, and CO. The heat content of the TO exhaust is recovered using a "heat recovery steam generator" - or "HRSG" - to provide all the process steam requirements for cook, evaporation, and distillation. A "fin tube" water coil is used to heat process water and recover additional heat from the TO exhaust. The system design recovers 85 percent of the TO fuel's higher heating value, Vander Griend tells EPM.

What about the problems initially reported with first generation ethanol plant TO designs from suppliers that have now gone out of business?

"ICM Engineering is improving the existing TO design," Vander Griend says. "An additional air-to-air heat exchanger will be added to maintain an elevated TO oxidation temperature without having to burn more fuel in the TO. This is commonly called a recuperative TO. Also, an additional water coil is being added to recover more heat from the exhaust stack. The process heat recovered from the TO is expected to be 88 percent of the fuel's heating value.

ICM-designed DDGS dryers, thermal oxidizers, and heat recover equipment will likely prove an overall utility heat requirement of 30,000 Btu per anhydrous gallon of ethanol, Vander Griend asserts. "A heat requirement of 32,000 Btu per gallon has already been demonstrated," he tells EPM. "However, the TO temperature was too low to satisfy CO emission compliance."

Regenerative Thermal Oxidizer
Regenerative thermal oxidizers (RTOs) have been used since the early 1970's to abate high-volume, low-concentration air streams.

"RTOs are used because they effectively treat large air flows at reasonable fuel and electrical costs," says Howard Hohl, of Eisenmann Corporation. "Fuel requirement - from either the process or fuel gas - for the RTO is in the three to five Btu per square cubic foot range."

Hohl tells EPM the "dynamic heat transfer process" moves heat from an inlet (preheat mode) ceramic filled vessel to an outlet (heat recovery mode).

In general, this is how an RTO functions.

An RTO cycles from one heat exchange mode to another over time as a function of available preheat in inlet air. If the preheat temperature drops, the heat source or burner is fired harder to raise the bulk inlet gas temperature to the required temperature. Normally this temperature for volatile organic compounds (VOCs) destruction is 1500F. For carbon monoxide, the required operating temperature is closer to 1600F with the correct oxygen content and retention time, Hohl tells us.

In 1994 the Environmental Protection Agency (EPA) listed several manufacturing categories that would require abatement of VOCs and several of these manufacturing categories were not experienced in the use of RTOs or the need for any fume conditioning requirements necessary to insure smooth, reliable operation. "History has shown that these first installations were designed according to the best available data and knowledge at that time," Hohl tells EPM. "Unfortunately, some of the design decisions made at this time were not the best for insuring uninterrupted plant operations under the tougher operating conditions."

What were the problems of the past? And how have they been overcome?

The symptoms of what Hohl calls "abatement miscues" usually surfaced in the inability of the abatement systems to pull sufficient air from the process. Once this situation occurs, he says, the RTO operator recognizes that the pressure drop across his RTO is extremely high and fan amperages are at a maximum. The first step in fixing this situation is usually to do a "bake-out" or "burnout" of the ceramic bed.

Hohl explains, "If the PM was organic, the offline bake-out was fairly effective in decreasing the pressure drop across the RTO. If the PM was primarily inorganic ash, a bake-out was useless. Under this condition the owner would try to use water to washout the ceramic bed."

To overcome the issues of process downtime due to RTO maintenance needs, Eisenmann, for example, developed a procedure for online ceramic maintenance. A new bake-out process was developed over the last eight years to provide an effective on line bed maintenance program.

According to Dave Chiles, president of Pro-Environmental, another manufacturer of RTOs for the ethanol industry, the special designs used for the heat recovery ceramic beds, burner/combustion system and the RTO flow control valve systems are critical to ensure excellent gas distribution, uniform temperature profiles and gas-phase mixing in the oxidizer.

"This is essential to achieve high DREs for CO and organic combustibles emissions, as well as to provide high thermal energy recoveries to minimize operating costs," Chiles tells EPM.

Ultra-Low NOx Burner
In order to discuss NOx reduction techniques, it is first necessary to briefly review the basic theory behind NOx formation.

Burning natural gas, and other fuel gases with no bound nitrogen, produces NOx through two main routes. The first is a thermal route where high flame temperatures cause nitrogen molecules from the combustion air to break apart and combine with oxygen to form nitric oxide. The other method by which NOx is formed is called the prompt mechanism. As the fuel pyrolyzes it generates fuel radicals, which combine with available nitrogen to produce carbon radicals. These carbon radicals oxidize promptly at the flame front to generate NOx and other species.

To achieve NOx levels in the sub-30 ppm range, the relationship between temperature, stoichiometry and NOx formation must all be factored into the design of the burner. It was determined that the most direct method of achieving low NOx emissions from a natural gas flame is to avoid fuel-rich regions, with their corresponding potential for prompt NOx, and lower the flame temperature to reduce thermal NOx to the desired level.

To accomplish this goal, a burner design that rapidly mixes gaseous fuel and air near the burner exit was developed. The "rapid mixing" results in a nearly uniform fuel/air mixture at the ignition point, which virtually eliminates prompt NOx formation. This rapid and complete combustion is also what results in the extremely low CO and VOC formation by the burner. In effect, the burner performs like a pre-mix burner with one important distinction: because the fuel is added inside the burner, just upstream of the refractory throat, the extremely small pre-mixed volume eliminates the possibility of flashback inherent in pre-mix burner designs.

Ethanol Load-out Flare Systems
Most ethanol terminals must control evaporative hydrocarbon emissions from truck loading operations. The Minnesota consent decree, for example, calls for a 95-percent reduction of VOCs emitted to the atmosphere that occur during loading.

Vapor combustion technology, such as the enclosed flares made by John Zink Vapor Combustion Systems, are ideally suited to meet these emission requirements.

"John Zink brand Vapor Combustion Systems have been used for the control of terminal VOC emissions for over 20 years," says Bill Matthes. "Over 500 systems are in operation around the world, safely and effectively combusting vapors from truck, rail and marine loading operations."

Matthes tells EPM that "superior safety design" is included in all Zink Vapor Combustion Systems. The reason for the added safety features, he adds, is that vapors displaced from ethanol transports consist of a mixture of air and hydrocarbon vapor.

"As a result, the naturally occurring mixtures can be explosive," Matthes warns. "Thus, it is imperative that the system design be well thought out and proven to be safe in the application."

Improperly designed, vapor combustion systems may cause flashbacks from the combustion unit to the loading terminal via the vapor header, according to Matthes. And he says, "Terminal vapors should never be injected into standard process flares designed for low oxygen content streams."

The typical vapor combustion system consists of a combustion chamber, anti-flashback burners, automatic pilot, vapor line block valve, detonation arrestor, air-assist blower, piping, instrumentation and master control panel packaged as an assembled skid-mounted unit ready for field installation.

One of the primary safety devices included for flashback prevention are Zink proprietary anti-flashback burners. These burners are designed specifically for the safe combustion of explosive mixtures that are unsuitable and unsafe for standard burners.

Another safety device is a properly designed detonation arrestor. The detonation arrestor is an advanced technology flame arrestor used to stop the high pressures and velocities associated with detonations. Zink systems also utilize burner control valves to enhance system safety. The valves control vapor flow in order to optimize vapor velocities. The burner safety valve will not open unless the pilot has been proved.

Operational reliability is also important, Matthes tells EPM.

"One of the key components important to system reliability is a pilot that works in all weather conditions," he says. "Our pilots have been proven in wind speeds over 100 mph and are equipped with a continuous flame monitor. Another important reliability and operability issue is proper design of the combustion stack.

Since the trucks to be loaded may have contained gasoline from the previous load, the combustion stack must be sized to handle the heat release associated with those vapors. "Our designs always consider the potential for gasoline vapors in the sizing of the combustion device, he says. "Undersized systems will experience undesirable characteristics such as high temperatures, excessive radiation, flames out of enclosed stacks and potential smoking."

Fermentation Wet Scrubbers
The Minnesota consent decrees required little to no change for fermenter carbon dioxide (CO2) scrubbing, Vander Griend asserts.

Here's how a scrubber typically works.

The CO2 vapors exiting the top of a fermentation tank are typically sent to a water scrubber. The down flowing water mixes with the up flowing CO2 vapors to remove ethanol and other water soluble volatile organic compounds (VOCs). This water is recovered and used in the process to slurry the ground corn.

"The amount - and temperature - of down-flowing scrubber water usually determines the scrubber recovery efficiency," Vander Griend says. "When the CO2 scrubber receives five to six pounds of water per gallon of ethanol fermented, emission tests typically show 98 percent recovery of VOC, which meets the Minnesota consent degree requirements."

Most plants make the CO2 scrubber the primary source of fresh water coming into the cook-fermentation process. Usually, the only means of getting rid of water from the process is through the drying process of the distillers dried grains (DDGS), or when selling distillers wet grains. Most plants have between 10 and 12 pounds of water per gallon going to the distillers grains dryers or wet grain sales. The CO2 scrubber water represents about half the water coming into the process. Other sources of water are: raw corn, process steam injection, pump seals, floor wash, etc.

Dust Control
Today, dust control is necessary not only due to state emission regulation, but also worker safety, environment and overall plant cleanliness, according to Jim Litle, of Alanco Environmental.

Dust collection is generally utilized in grain handling areas such as truck dump pits, conveyors, elevator legs, hammer mills and DDGS transfer areas. Regulations and limits on the emission of dry particulate matter vary widely from state to state.

"Alanco not only manufactures the dust collection equipment needed for compliance, but will also work with the client to provide filtration efficiency information as required by their state for proper permitting," Litle tells EPM.

Alanco dust collectors with standard filter media normally provide 99.9 percent efficiency on particles two microns or larger, and usually should not exceed an emission rate of 0.02 grains of dry particulate matter per standard cubic foot of air volume handled. When required, even greater filtration efficiencies can be achieved by using more exotic filter media.

There are two dust collector filter styles generally used in this type of dust collection:

1. Pulse Jet Style - (See figure 1)
This style uses 90-100 psi compressed air to continuously clean the filter media while on line.

2. Reverse Air Style - (See figure 2)
This style utilizes a high volume fan to produce the air required to continuously clean the filter media while on line. This style filter is generally more economical in high air volume systems, because they do not need the large amount of compressed air required with a Pulse Jet type filter. This style is frequently used in Ethanol facilities.
The dust collection pick-up hoods, duct and duct sizing all hinge on the size of the ethanol plant equipment.

After air requirements are calculated and equipment is located in the plant layout, ductwork can be sized and routed to the dust collector, Litle says.

According to Mike Althouse with MAC Equipment, reducing particulate or dust emissions requires evaluating and possibly modifying several areas of the process. "First," he says, "is the source of the dust. Are the pickup hoods designed correctly and do they have enough airflow to capture the dust? If not, and the hood is designed correctly, a blast gate may be out of position or the duct may be plugged."

Plugging can be caused by a number of things, Althouse says, from belts slipping on the fan, to open, unused takeoff on the main trunkline, or operators making adjustments to blast gates elsewhere in the system.

"If the dust system worked well at one time and is not working now, it's good idea to have someone qualified to come into the facility to evaluate and re-balance the complete ductwork system," he tells EPM.

If the hooding and ductwork are in good condition and capturing the dust, the dust collector has the greatest potential for reducing emissions. Dust collectors in ethanol and other grain handling facilities are equipped with various types of cleaning systems. They may be high pressure, pulse jet (bags are pulse cleaned with 90-100 PSI compressed air); reverse air (bags are back flushed by a reverse air cleaning fan); and medium pressure pulse cleaning (bags are pulse cleaned with 6-8 PSI compressed air generated by a positive displacement blower). Most of these have 12 or 16 oz. polyester media. Some facilities may have centrifugal cyclones.

Although these types of collectors have been sufficient for many years there are media available today that can decrease the outlet emission of these collectors drastically. Pleated cartridges and P84/Polyester composite bags can be as much as four times more efficient than 12 or 16 oz. polyester bags.

Furthermore, utilization of "on-demand" cleaning will also reduce emissions. This type of cleaning method initiates the bag cleaning system only when the differential pressure in the baghouse reaches a predetermined high set point and terminates it once the differential pressure is reduced to predetermined low set point.

"Approximately 80 to 90 percent of the emissions from a baghouse occurs in just a few seconds after a cleaning pulse," Althouse says. "Minimize the pulsing and you can minimize the emissions. EP