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Barbecue is all about the smoke. Without the smoke, you might as well be grilling.
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May 2014

Every fuel source and smoker will emit a characteristic mix of combustion gases, and reach a different maximum temperature. The "flue" gases were measured with a TPI 714 combustion analyzer or a hand held Universal Enterprises CO71A carbon monoxide meter. Temperatures were measured with either a 1/16" diameter bare NiCr/NiAl thermocouple (cantilevered inside a gas flame, but limited to temperatures below 2100F), a stainless steel sheathed flexible NiCr/NiAl thermocouple, or a Pt/PtRh thermocouple encased in a 1/4" diameter ceramic tube (accurate to 3000F, best inserted in-between coals or in a large flame).

In these experiments, infrared thermometers proved unreliable, as they rarely work above 1000F, average temperatures over a wide viewing angle (often larger than the flame, and thus include colder regions), and should be calibrated against the emissivity of each surface.

Specific details of each measurement are listed in each table.

 

Electric Smoker Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

Wood hunk- small thermocouple inserted in center of block.

900F-1000F

gray smoke:
<10,000 ppm

  <3 ppm  
900F-1000F

light smoke:
4000 ppm

0.15 <1 ppm 17%
900F no wood
CO 10 ppm
0.15 0 ppm 21%

Some CO is emitted in the absence of wood, probably due to combustion of residual drippings

Wood block rises in temperature til 212F, then stalls til water evaporates, then rises to 900F-1000F.

Smoke is initially gray and sooty in appearance, then as wood burns more uniformly, the color turns to light white or blue. Nitric oxide levels are always low with wood hunks in an electric smoker, because temperature must be above 1100F and with turbulent oxygen flow to convert ammonia to nitric oxide.

Thus electric smokers, even with wood hunks, rarely creates a smoke ring. However, the combustion temperatures are high enough to produce significant smoke flavor.

Partly burned wood hunks

 

 

Pellet Smoker Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2(%)

Pellet grill burner on low. Mostly embers. Ceramic thermocouple in embers

800-1000F 1000-2000 0.1 to 0.5 50 ppm 16-18%
 

pellet smoker burner cup on HIGH. Ceramic sheathed thermocouple in flame.

1800F-1900F 5000 0.3 10 ppm 19%
When the fan speeds up, the flame dramatically increases in size, and the CO content rises

 

Charcoal Briquette in Chimney Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

70 Kingsford blue briquettes. Ceramic probe inserted in fire center

Early flame:
1700F

>10,000 ppm   50 ppm  
Red Coals:
1950F
1000-3000 ppm   170 ppm  
White Coals:
2150F
700-1000 ppm   50-100 ppm  
Gases measured 6" above chimney, where temperature is ~1000F-1400F. Higher temperatures with white coals also generated greater airflow, thus dropping ppms

 

Lump Charcoal in Chimney Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

Royal Oak lump charcoal, ceramic sleeved thermocouple

1400F 7000 .14 40 14%
1800F >9000 .06 90 6%

Temperature measured in brightest section of coals. Gases measured just below chimney lip. The 1400F temperature reflects dull red coals, the 1800F brighter red. But still cooler than charcoal briquettes. Some ember spitting.

Lump can burn hotter than briquette, but often small chips fill the gaps between the large, blocking air flow. So in actuality somewhat cooler.

Lump charcoal is often sold as lower polluting, especially lower NO by a factor of 3. These measurements are consistent with that rough air pollution advantage.

 

Charcoal Cabinet Smoker Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

Kingsford Blue charcoal briquettes in minion arrangement. Two oak wood hunks shown on coals. With door closed, embers hardly glow.

NO Wood hunk
900F- 1100F
9000 0.2 20 15%
Wood hunk
900F- 1100F
>10,000 0.17 10 15%

Temperature controlled to 250F with PartyQ Guru and fan. Important to assure only ~10% of the briquettes are lit at any one time, or the fan cycles infrequently, causing dark gray smoke and high CO levels

NO levels are low due to poor oxygen supply and low coal temperatures. But will result in a smoke ring on most days.

 

Smokenator in Weber Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

Kingsford Blue charcoal briquettes. Two oak wood hunks shown on coals. With lid and temperature 250F, embers hardly glow.

Avg temp
500F-700F

Max temp on coal
1400F

>>10,000 0.7 <10 5%

NO levels are low due to poor oxygen supply and low coal temperatures. But will result in a smoke ring on most days.

 

 

 

Wood Fire Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

temperature measured in orange flames with ceramic sleeved probe

1400F to 1600F 4000-10,000 0.15 to 0.3 20-75 14% -18%

The fire is not uniform, so the measurements vary wildly depending on the probe location and winds.

While most of the nitrogen compounds were in the form of nitric oxide, there were 2-5 ppm of other nitrogen oxides

 

Gas Smoker Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

Charbroil vertical cabinet smoker. Temperature measured in orange flames with bulb sleeved probe

2170F no wood
3 ppm
0.0001 20 ppm 17.5%
2170F with wood
500 ppm
0.15 20 ppm 17%

The wood tray does not turn dull red though heated directly by the flames- actual temperature around 700-800F. So the wood chars slowly and with too little oxygen to produce much NO.

Gases measured in center of smoker

 

 

Natural Gas Kitchen Burner Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

kitchen burner with small thermocouple inserted in flame

Ceramic TC
1950F

Bulb TC:
2300F
15-30 ppm .0005-.001 <10 ppm 17%

Gases measured 6" above center of burner (850F)

Note ceramic TC is larger than flame, so underestimates temperature.

Clearly, almost all the natural gas is consumed and turned into carbon dioxide- very little becomes CO or NO. But the temperature is high enough to produce a little NO from the nitrogen in air. Which is why some gas smokers "pink" cooked turkey, where the light color is visible against the white meat.

 

Propane Gas Grill Temp (probe) CO (ppm) CO/CO2 ratio NO (ppm) O2 (ppm)

ceramic sheathed thermocouple in grill propane flame

Ceramic TC
1600F

Bulb TC:
2000F
25-50 ppm .003 - .004 <3 ppm 19.5-20%
Below grates, lid open 400 ppm .005

10-20
ppm

15%
above grates, lid closed 50-100 ppm .002 10-20 18%

With 100% combustion there would be no oxygen left in the exhaust. These oxygen levels are close to 21%, the amount in free air. Which implies there is 2-3 time more (e.g. "excess") air than required for combustion in a gas grill.

Clearly, almost all the natural gas is consumed and turned into carbon dioxide- very little becomes CO or NO.

With 4 hamburgers cooking on grill and lid open, drippings combust on the flame diverter and CO>1000 ppm and NO~5 ppm

Note the ceramic TC is larger than flame, so it underestimates temperature.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Additional articles on kitchen science can be found HERE.
Or follow me on twitter for (very occasional) alerts of new food science postings at @KitSci

In the practice of all-things barbecue, we appreciate the support and conversations with Meathead at AmazingRibs.com, Sterling at BigPoppaSmokers, along with numerous competition pitmasters and backyard chefs.


 

 

 


Contact Greg Blonder by email here - Modified - Copyright Genuine Ideas, LLC.