A Newer version of this study is available;
please, call Air Burners Sales Dept. at 772-220-7303.
July 2018



Air Curtain Burner vs. Wood Grinder

Disposal of Wood Waste

A Comparison of Critical Emissions and Basic Economic Parameters from Two Disposal Methods

A. Overview:

The purpose of this paper is to provide details concerning a better environmental and economic method for the disposal of wood and vegetative waste. Technical details will be provided for comparing two machines typically used for the disposal of wood and vegetative waste: the Air Curtain FireBox and the Diesel powered Wood Grinder. For the disposal of accumulated wood and vegetative waste, an Air Curtain Burner (ACB) “FireBox” is a better choice for protecting the environment and it is also the most economical choice.

1) The ACB FireBox is an “end solution” and actually eliminates the wood waste, while a grinder is just a “handling process” where 100 tons of wood is ground-up into 100 tons of mulch.

2) The FireBox uses the waste as its fuel, where the grinder uses hundreds of gallons of Diesel fuel to grind wood in to mulch.

3) The FireBox creates a small amount of natural clean ash that is good for the soil, the grinder creates a huge amount of mulch that is bad for the soil and must be transported off-site.

4) The FireBox has less impact on the environment; the grinder and associated transportation have a significantly negative impact on the environment.

5) The FireBox is significantly less costly to purchase and to operate than the grinder.

6) The FireBox can create energy from the waste, the grinder only consumes energy.

The Air Curtain FireBox follows the same natural process as has been happening on Earth for millions of years, but without the unwanted particulate matter or black carbon release. As we concern ourselves with the reduction of climate changing impacts on our environment, the FireBox is the best choice for immediate reduction in black carbon and non-biogenic CO2 emissions. This paper will provide more details in comparing the two machines and outlining the path to achieving significantly lower emissions today.


B. Comparison of Critical Pollutants


FireBox - No visible PM discharge


Grinder - Significant PM discharge

The following critical emission components will be compared: PM and CO2. These are emissions, either aerosol and gaseous, which are classified as harmful Greenhouse Gas Emissions within the framework of Global Warming and Climate Change discussions.



PM or Particulate Matter from a combustion process is Black Carbon (BC) consisting of clusters of carbon molecules measured in microns i.e. 10μ (PM10) which are strictly regulated. Black Carbon emissions constitute an aerosol and not a gas, because BC consists of pure carbon suspended in the atmosphere as tiny solids. BC tends to gravitate back to earth over time and is considered one of the most significant contributors to “Global Warming.” While suspended in the atmosphere, the carbon particles absorb sun rays and release infrared rays as heat that raise the temperature of the layers in the atmosphere where they reside thus contributing to the warming of the planet. As they fall back onto earth, the Black Carbon causes reduction of “albedo”, the ability of an object or particle, such as snow or ice crystals, to reflect sunlight (heat). When Black Carbon is deposited on snow and ice, it accelerates the melting of the snow, ice, or glaciers in areas where typically no appreciable reduction of frozen mass occurs naturally. It adversely affects “Global Warming” which is most noticeable on earth in the permafrost regions but more significantly, its residence time in the atmosphere is very short measured in weeks and months.

Reducing Black Carbon would have a near immediate positive effect on Global Warming in contrast to CO2 which remains in the atmosphere for 100 years or more, and any CO2 mitigation efforts would yield results only for future generations. While we should work diligently towards the reduction of CO2, immediate efforts should be made to mitigate or eliminate BC releases from combustion processes, whether from open burning or combustion of hydrocarbons in engines, due to the immediate positive impact on the effects of climate change. The Air Burners FireBox was designed specifically for the reduction of Black Carbon and it is a proven and effective tool for achieving the goal of Black Carbon reduction now.

In addition to PM from Diesel engine combustion, the grinding of wood waste and the subsequent handling, transporting and storage result in additional fugitive dust (PM) and these particles are predominantly raw wood aerosols. Raw wood particles are a known carcinogen with obvious health implications, especially to a population close to the source, such as the site operators. It is obvious that raw wood PM emissions from grinders must be minimized. 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


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.



E. Quantitative Comparison of Selected Two Pollutants from FireBox and Grinder for 100 Ton of Wood Waste Disposal/Processing

1. Air Curtain Burner (ACB) - 59HP S-220 by Air Burners, Inc.

a. PM from ACB Diesel engine
b. CO2 from ACB Diesel engine
c. PM from wood combustion (processing)


2. Grinder - 400HP Generic Grinder

a. PM from Grinder Diesel engine
b. CO2 from Grinder Diesel engine
c. PM from wood chipping/grinding (processing)
d. PM, fugitive releases from on-site handling
e. PM from Truck Diesel engines
f. CO2 from Truck Diesel engines
g. PM, fugitive releases from transporting and discharge
h. PM, fugitive releases from remote/landfill site handling


3. Basic Specifications of Air Curtain Burner (FireBox)

Model: Air Burners, Inc. S-220
Engine: Kubota Model V2403-TE, 59 HP max.
Diesel fuel consumption: 8.5 L/hr average
FireBox through-put: 8 tons/hr (8000 kg/hr)


4. Basic Specifications of Wood Grinder

Model: Generic Model
Engine: Generic Diesel, 6 Cylinder, 400 HP max.

Diesel fuel consumption: 113.5 L/hr average
Grinder through-put: 16 tons/hr (16000 kg/hr)

5. Notes

a. The loading equipment, such as an excavator, is not included in the comparison, because both the FireBox and the Grinder are assumed to use the same or a similar loading machine, albeit the FireBox for 10 hours and the Grinder for 6¼ hours. This is balanced by the use of the equipment for removing the beneficial ash from the FireBox and for pushing the chips away from the grinder’s conveyor belt discharge area, where the mulch has to be cleared out over a period of 6 hours, however, the ash can be removed from the FireBox in less than 15 minutes.

b. Calculation of number of dump trucks required to transport 100 tons of wood waste ground into mulch:

  1. Weight: 100t fairly dry woody debris

  2. Typical grinding and mulch handling/spillage losses (wood dust and moisture) by weight 2% or ≈2,000kg (2t)

  3. As 1m3 of mulch has a weight of 290kg, 100t–2t= 98t (198000kg) of wood material ground has a volume of 198000/290 = 337m3

  4. 1 dump truck can hold (legally) heaped 17m3 (no weight consideration needed, load weighs only ≈5t)

  5. 337/17 trucks ≈20 dump trucks are required

c. Assumption of travel distance to mulch disposal site is 40km one way, total travel distance for 20 loads would be 20x80km (round-trip) or 1600km; at an average speed of 48km/hr, truck engines would release emissions for 1600/48=33 hrs for every 100 tons (100000kg) of ground wood waste hauled.

d. The fugitive PM emissions from road dust caused by the trucks are ignored, but could be significant in rural areas with unpaved roadways.

e. Emissions from the frequently needed Diesel fuel tanker to refuel the grinder and fleet of trucks are also ignored for this comparison.

PM & CO2 Data for 100 Tons of Wood Waste Processed




Typical Emissions
or Data

Air Burners


Wood Grinder



100t Wood Waste

Diesel Engine -Processing







Wood Processing





Fed Limit




Handling of Chips on-site*







Diesel Engine - Trucking 300 HP







Handling of Chips Trucking*







Handling of Chips Remote Discharge*







Total PM10



Diesel Engine - Processing








Diesel Engine - Trucking 300 HP








Total Non-biogenic CO2



Diesel Engine – Processing
Fuel Consumption







Truck -300HP
Fuel Consumption





Total Diesel Fuel



Wood Processing – Particulate EPA Limit








Summary of Totals (rounded)


25 kg

141 kg


283 kg

3656 kg

TOTAL Diesel Fuel Consumed

106 L

1369 L


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. 

G. Economics

One of the major benefits of the Air Curtain FireBox: it is a preferred environmental solution that provides an economic benefit to the operator. It is not often that you can have a machine that will benefit the environment and also provides an economic incentive for its use. The Air Burners FireBox is just that machine as demonstrated in the chart below.

Cost Comparison Example
for the Disposal of 100 Tons of Wood Waste
Air Curtain Burning vs. Grinding vs. Landfill Dumping

Air Curtain Burning

Grinding &
Trucking & Landfill Disposal

Direct Landfill Hauling
& Disposal

Operating Costs

Fuel Cost:
ACB: 10 hours=39 gals (106L)
Grinder: 6 hours=138 gals (500L)




S-220 $0.75/hr
Grinder $16.00/hr




Hauling of Residual to Landfill
(40km one way/80km round trip @ $2.50/km
with 20-ton Dump Truck


(20 Loads)

(35 Loads)

Tipping Fees at Landfill or other Mulch Disposal Cost ($10 per Ton)


(Note 4)


Total Disposal Cost for 100 Tons
or approx. 400m3 (1 Day)




1) Individual results may vary
2) HP to fuel consumption approx. hp × 0.05=FC in UD Gals/hr
3) Grinder: HP is 450, i.e. Vermeer HG4000. Diesel Fuel cost $3.75/USgal
4) It is assumed that chips cannot be reasonably sold for beneficial re-use.
5) Grinding process and mulch handling and transporting produces significant amounts of particulate matter (PM) in the form of wood dust (also PM) that can cause serious illness in exposed workers. Wood dust is a proven carcinogenic (cancer-causing). Mulch losses, in form of wood dust and spillage are approx. 2% or 2 tons in this example. Only 98 tons of wood chips are transported and dumped.
6) Air Curtain Burner residual is 2-3 tons of ash, is usually land-applied on site; Grinder output yields no weight reduction, but does provide significant volume reduction. In this example, waste is hauled by at least 20 trucks each carrying approximately 17m3 or 5-6 tons.
7) Direct hauling of land clearing wood waste to landfill will require approximately 35 trucks.
8) Loading cost is not considered, as the same machine would be used for any of the options.
9) The S-220 is a medium size above ground refractory walled firebox. Larger and smaller models are also available. See www.AirBurners.com



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.

Clean Burning at the Memphis Landfill


H. Energy Recovery from the FireBox

The Air Burners FireBox has an optional heat recovery system that allows the operator to utilize waste heat for other purposes, such as kiln drying and heating facilities. In late 2012 the PGFireBox FireBox will be available. Currently in testing this self-contained Biomass power generating system will produce electricity from the waste wood. In addition, the system is portable allowing users to “follow the waste”, again reducing the environmentally damaging and costly transportation component. You can read more about this exciting new product on our website at www.PGFireBox.com.


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:

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


K. Links (Updated 2018)

1. Air Burners, Inc. FireBox Brochure

2. FireBox Specifications for S-327 and S-220 (California Versions available -2018)

3. Air Burners, Inc. 2017/2018 MSRP Pricing Overview (US) Click Here and request a pass word.

Printer Friendly Version of this Site (Metric Data) PDF



4390 SW Cargo Way
Palm City, FL 34990

Vers. 06-JUNE-2012/2018 HG3