Strong regulations are being put in place to mitigate the raw wood PM release from grinders ie: California requires grinders to meet strict PM regulations in their Title V permits. In some areas like Martin County, Florida, buildings were constructed at great taxpayer expense to house the grinding operations in an effort to control the airborne particles. In many cases the Air Curtain FireBox is a better solution as it meets a much tighter PM standard than Grinders and does not emit raw wood particles.

With today's grinder technology it is very difficult to control PM emissions. The state of grinder technology today will only allow US authorities to regulate grinders to a maximum opacity of 20% whereas the limit for PM emissions from the air curtain burner is a maximum of 10% opacity. Opacity is a measurement of the density or thickness of a PM emission plume rising up from a stationary source. This means that it is recognized by the authorities that a FireBox will release much fewer particulate emissions when operated properly than is practical and achievable for the grinder.

The grindings also pose an extreme environmental hazard. There is a very limited market to recycle the wood grindings, first, for the grindings to be useful for gardening and landscaping they must be sorted to eliminate the spread and contamination of invasive species, second, the grinding will not eliminate any molds, fungi or diseases from destructive insect larvae, like longhorn beetle and third, with almost no recycling market the grindings are being piled at landfills and transfer stations across the country significantly increasing the occurrence of spontaneous combustion and the devastating environmental impact that follows.

PM - Mulch Fire in Florida caused by spontaneous combustion of wood grinding piles

CO₂

The gaseous emission of concern emitted from both air curtain burners and grinders is the dangerous non-biogenic CO₂. We are using this as the standard for comparison as it is classified as the major undesirable Greenhouse Gas and it tends to be the common denominator when comparing emissions. Other releases from a process of incomplete combustion, such as organic gases (CH₄, etc.) are not addressed in this review, as their releases follow the same proportions as CO₂ with regard to this FireBox-to-Grinder comparison study. Biogenic CO₂ also does not enter the comparison, because Biogenic CO₂ from wood is considered carbon neutral by the IPCC and USEPA and the biogenic carbon release ultimately is the same for the air curtain burner and the grinder. The following sections compare and explain the CO₂ emissions.
 

C. Carbon Cycle

First we should understand “Biogenic CO₂ and the Carbon Cycle, based on which the IPCC and other agencies have determined that the burning of woody biomass is CARBON NEUTRAL, that is the release of biogenic CO₂ is not considered a bad Greenhouse Gas in contrast to the carbon dioxide that is released into the atmosphere from the combustion of hydrocarbons such as petroleum or Diesel fuel. That CO₂ is formerly sequestered “bad” Non-biogenic CO₂. The carbon that is released in this case was sequestered deep inside the Earth and it would have remained there forever, were it not harvested by man and combusted in Diesel engines of trucks, grinders, ocean ships, airplanes, etc.

The Biogenic CO₂ from Biomass burning represents carbon that was absorbed by trees and taken from the surrounding CO₂ in the air as the result of nature’s process of photo syntheses by which life is sustained on Earth. The carbon portion of the CO₂ remains sequestered in the woody tree or other vegetation and the oxygen is released into the surrounding atmosphere.

When the wood waste burns in the air curtain burner, as it would naturally in a wildfire, but at very high combustion efficiency in the ACB, the carbon from the woody biomass again combines with oxygen and forms CO₂. The cycle is complete. No additional CO₂ was added to the atmosphere. The carbon cycle is depicted in the image to the right. The sequestered CO₂ in the wood will eventually be released whether it is submitted to the ACB or the grinder or left to fall in the forest.

 

D. Overview of Emissions from Air Curtain Burner and Grinder

1. Air Curtain FireBox

The air curtain burner selected for this comparison is a mid-size Model S-220. It is equipped with a small Diesel engine (59 hp) that powers the air fan. Emissions from the engine’s exhaust will be considered.

The wood is burned inside the burn chamber where the wood waste is the only actual “fuel” that is combusted. PM emissions from the combustion process will be considered.

2. Wood Grinder

Grinders are powered by large Diesel engines, as large as 12 cylinders, 1000HP engines with high fuel consumption. Emissions from the engine exhaust will be considered. The wood waste grinding operations release PM in the form of wood dust (a) from the grinding process itself and (b) from the release of the chips or mulch from the machine via a conveyor belt or similar system. (c) Fugitive PM is released from the storage pile, the on-site staging by a machine (i.e., front-end loader) that pushes the material away from the conveyor belt outlet area, and also each time the material is loaded, transported and finally dumped at its ultimate disposal site, usually a landfill.

 

These charts graphically show the stark differences between two similar sized machines that can process 100 tons of wood and vegetative waste in one day. The quantitative details are defined below.

F. Brief Discussion of the Equipment and Operation

Air Curtain Burners are above ground refractory walled burn chambers that provide a high velocity curtain of air (“air curtain”) over the top of the burn chamber. The air curtain acts like a lid trapping the PM rising from the fire with the intense hot gases and re-burning the PM until the particles are so small that they escape as a gas through the air curtain. (www.aircurtaintechnology.com).

 
The high velocity air curtain is created by an air fan powered by a small Diesel engine. Air curtain burners reach very high temperatures of 800ºC -1200ºC thereby achieving virtually a complete combustion of the wood waste. The residue consists of 2%-3% or less of residual ash that has beneficial use and is usually applied to the surrounding land. (97% to 98% of the wood waste is being totally eliminated. A larger FireBox, such as model S-327, has been designed to eliminate 10-12 tons per hour.

Wood Grinders, such as horizontal or tub grinders were designed to facilitate the transportation of wood debris, not to eliminate it. It was the objective to grind or chip the wood debris into smaller pieces so more weight would fit onto a truck for hauling it to a location for final disposal or storage. The function of the grinders therefore is to provide an interim process, not a final disposal solution. Grinders are powered by large high horse power Diesel engines that drive massive mechanisms which cut or hammer the wood debris into small pieces commonly referred to as chips or mulch. The mechanism is quite susceptible to breakdowns, because objects, such as chunks of stones or metal which often are inadvertently loaded with the wood waste cannot be tolerated well. Rocks, stones and metal are of no concern for the ACB. This is actually the main reason why it is so preferred for getting rid of root balls.

For a comparison of the commercial or cost related aspects of the two methods (next section), the most important single distinction is the fact that the air curtain burner offers an end solution for wood waste disposal whereas the grinder offers only an interim process facilitating the transport of the woody debris. This process does not eliminate the waste, as the air curtain burner does. 

When evaluating different wood waste disposal alternatives, it is often overlooked that a comparison of air curtain burners to grinders is really quite flawed: the FireBox offers an end solution; the waste is eliminated. The grinder in contrast thereto is only an interim step, a tool to facilitate the transport of the wood waste to the ultimate disposal location. Whenever the two methods are compared, whether from the standpoint of economics or environmental friendliness, the cost and the pollution from the transport vehicles must be factored into the equations. Only then a true and useful comparison can be drawn. The air curtain FireBox will be superior every time by margins so wide, that those voices wanting to argue the finer points of the above comparisons will be swiftly muted.

A quick discussion regarding the residual products of the two disposal methods is worthwhile. Forest fires burning vegetative waste represent a natural process that has been going on since the beginning of time. A lightning bolt strikes the ground, a wildfire starts and the forest floor is cleared of dead limbs, pathogens, insects and nutrient-robbing undergrowth. The ash becomes a beneficial soil amendment. The burning of wood waste in the FireBox replicates the burning of woody material by natural forest fires, but without the smoke, the FireBox ash becomes a beneficial soil amendment. Because the ashes have a high pH, they are also useful as an additive to the daily landfill cover at a commercial landfill site further augmenting the overall cost savings enjoyed by the use of the FireBox.

The mulch from the grinder, on the other side, more often than not poses a real commercial and environmental problem. The grinding and hauling is expensive, tipping fees can be high and the mulch hardly ever has a beneficial (commercial) use. Typical mulch grindings may include, seeds of invasive species plants, insect larvae, inorganics, mold spores and termites. The mulch is still a waste product, and it costs money to dispose of waste. For the environmentalist charged to safeguard our environment there are several concerns. First, a major problem across the US is the safety and pollution issue from the spontaneous combustion of larger mulch piles. They tend to smolder for many months with no good way to extinguish the fire. Second, the mulch will go septic from natural decomposition. That causes discharges of leachate that are undesirable, as they enter the ground drinking water table. Finally, mulch piles can attract rodents and other pests and when used as landscape material, the mulch can alter the ground ecosystem or even bring pests to homes, if scattered near them. The US Forest Service has long recognized the detrimental effect of mulch deposited on the forest floor and heavily restricts the amount of mulch that can remain from cutting operations.

I. Conclusion

The Air Burners FireBox is a well proven and tested technology that delivers benefits to both the corporate budget and the environment. From an environmental standpoint we need to embrace technology that incentivizes the user to protect our environment. From a corporate budget standpoint you can purchase the FireBox for less than a competing grinder and you now have the “end solution”.

A comparison of two common methods for the disposal of wood debris has been: (1) Air Curtain FireBox versus (2) grinder and associated trucking option where the ground mulch is hauled to a remote location for ultimate disposal.

The air curtain option is superior both from the economics of the operation and the protection of the environment. In every instance, grinding the waste and hauling it away will be considerably more costly and will release a higher level of undesirable pollutants. Protecting the environment and saving money is a winning combination.

J. References

1. Diesel Engine Emissions Calculations:
US EPA AP42, Section 3.3 “Gasoline and Diesel Engines”, Los Alamos National Laboratory (LANL/US Energy Dept.): “Air Curtain Destructors, General Description of Source Category”.

2. Air Curtain Burner PM Emissions Calculations:
a. USDA-Forest Service: Reducing PM2.5 Emissions Through Technology, Results from a Recent Study Evaluating the Effectiveness of an Air Curtain Burner.
b. Fountainhead Engineering: Air Burners S-327 Emissions Test Report
c. US EPA Chalmette, Louisiana Air Curtain Burner Test (S-327)

4. Grinder PM Emissions Calculations:
State of California BAAQMD Regulation 6-301 and BAAQMD Condition #6385 part 4.
Ringelmann 1/20% opacity; BAAQMD Regulation 6-311:
E=0.026(P^0.67) where E=allowable emission rate (PM, lb/hr) and P=wood process rate (lb/hr)

5. Air curtain Burner Technical Data:
www.airburners.com

6. Grinder Technical Data
Generic literature accessible at www.google.com  

K. Links

1. Air Burners, Inc. FireBox Brochure

2. FireBox Specifications for S-327 and S-220

3. Air Burners, Inc. 2011 MSRP Price Sheet (US) Click Here and request a pass word.

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