How is the performance of wood-fired pizza ovens measured?

The performance and pollution of wood-fired (and coal-fired) pizza ovens have become a “hot topic” with recent regulations promulgated in New York. This article briefly reviews the United Nations Education, Scientific and Cultural Organization (UNESCO) status of Neapolitan pizza and the importance of wood-fired ovens.

It then presents a high-level overview of the test and measurement equipment used to monitor the pollutants and particles emitted by wood-fired ovens and closes with a brief glance at the importance of selecting between seasoned and kiln-dried firewood.

The use of wood-fired pizza ovens began with the development of Neapolitan pizza in Italy. European Commission Regulation No. 97/2010 recognizes Neapolitan pizza as one of the traditional specialties guaranteed (TSG). The skill of the Pizzaiuolo (Neapolitan pizza maker) has been included in the Representative List of the Intangible Cultural Heritage of Humanity by UNESCO. Four basic recipes are recognized for Neapolitan pizza (Table 1). To fully qualify as a Neapolitan pizza, it must be baked in a wood‐fired oven for about 90 seconds.

Measuring wood-fired pollution

While initially developed for baking Neapolitan pizzas, today, wood-fired pizza ovens are used for a wide variety of pizza recipes. As a result of the popularity of wood-fired ovens, specific techniques have been developed for measuring their emissions, including carbon monoxide (CO), carbon dioxide (CO2), various nitrogen oxides (NOx), organic gaseous carbon (OGC), and particulate matter.

In one instance, a measurement system was built using a traditional fixed-top wood-fired pizza oven. Two types of fuel were used: beech wood logs like those commonly used in Italian pizzerias with a moisture content below 20% and compressed beech wood briquettes, certified for food use with a moisture content between 5.4% and 6.6%.

The gas sampling system was built based on the UNI EN 16510-1 technical standard for Residential solid fuel burning appliances from the Ente Italiano di Normazione (UNI) standards organization, which describes the test methods for sampling and measuring emissions of CO, NOx, OGC and particulate matter from solid biomass residential heating appliances (Figure 1). To isolate the oven performance, the tests were completed with and without pizza baking.

The exhaust gas temperature coming out of the oven was continuously monitored, and all the gaseous pollutants were measured continuously at multiple locations (small boxes with red bars in Figure 1) and fed into gas analyzers and a data acquisition system for analysis.

Smoke sampling to measure total particulate matter (TPM) was implemented at the entrance of the exhaust duct above the oven (PM) to establish a reference standard. After that, the exhaust gases were combined with gasses from a damper and diluted as is normal in this type of system (PM/VELOCITY).

In addition, the TMP concentration was measured in the diluted flue gases using an opacimeter (about one-third up the exhaust flue on the right-hand side of Figure 1). Opacimeters are highly dynamic partial-flow measuring instruments for the continuous measurement of exhaust gas opacity. Opacimeter measurements were used to identify possible changes in TPM concentrations during various parts of the combustion cycle.

Testing using beech wood logs and compressed beech wood briquettes helped identify the importance of the fuel selection.

Seasoned vs kiln-dried wood

In addition to choosing wood logs versus briquettes, many pizzeria operators must choose between seasoned and kiln-dried logs. Air-seasoned logs are air-dried for 6 months or longer. Achieving a moisture content below 20% can take multiple years using air drying. While it’s less expensive than kiln drying, the high moisture content of air-seasoned wood results in the production of more smoke (Figure 2).

The kiln drying process uses large propane — or natural gas-fired kilns to speed up the drying process. These kilns also include fans to circulate the heated air for a more uniform drying process. Many states in the U.S. require certification of kiln-dried wood.

Kiln drying can consistently produce wood with a moisture content well below 20%. The low moisture content means that the logs burn hotter and longer in addition to producing less smoke. The hotter temperatures can be especially beneficial when baking pizzas since they cook the ingredients more evenly.

Summary

Wood-fired ovens are a popular way to cook pizzas. Their use started with the development of Neapolitan pizza in Italy, and today, they are used to bake a wide variety of pizza recipes. There are pollution challenges associated with the use of these ovens and standards like UNI EN 16510-1 have been developed for quantifying the contents of oven exhaust. The selection of wood is an important factor determining the performance of wood-fired pizza ovens.

References

Determination of Pollutant Emissions from Wood-Fired Pizza Ovens, Chemical Engineering Transactions
Everything you need to know about regulations on wood-fired ovens, Alfa Refrattari
Performance characterization of a traditional wood-fired pizza oven, Journal of Food Science
The Ultimate Guide to the Best Wood for Pizza Ovens, Natural Timber Firewood