COFFEE ROASTING OPERATIONS
Last adopted: May 15, 1998
The roasting of coffee beans is a common activity that occurs throughout the Bay Area at a wide variety of facilities ranging from small gourmet coffee shops to medium-sized commercial operations with locally distributed products to large facilities with national product distribution that operate 24 hours per day. The associated coffee roasting equipment ranges from small 25 pound per hour batch roasters located at gourmet coffee shops to industrial, 4 ton per hour, recirculating, continuous roasters located at large facilities. The air pollutant emissions resulting from coffee roasting operations include particulate matter, volatile organic compounds, organic acids, and natural gas combustion products. The odorous and visible emissions (smoke) resulting from the roasting process have the most obvious and direct impact on the public.
Sections I and II of this Permit Handbook chapter describe the processes involved in the roasting of coffee beans and the air emissions associated with those processes. Sections III and IV discuss Bay Area Air Quality Management District permit requirements for this industry and the information necessary for a permit application. Section V is an engineering evaluation template and includes typical equipment descriptions, sample emission calculations, applicable regulatory requirements, and sample permit conditions.
I. PROCESS DESCRIPTION
The roasting of coffee beans typically consists of the following processes. Throughout this document, coffee roasters will be classified by size according to the following approximate guidelines:
Small Roasters: < 300 pounds per hour
Medium Roasters: 300 to 2000 pounds per hour
Large Roasters: > 2000 pounds per hour
A. Roasting
After screening to remove dirt and other debris, green coffee beans are transferred to feed hoppers which charge beans to the roaster which may operate on a batch or continuous basis. Roasting temperature typically ranges from 700oF to 1000oF with roasting times ranging from 5 to 20 minutes depending on the desired coffee bean color and flavor.
B. Quenching
When the beans reach the desired color, they are immediately quenched with a water spray to halt the roasting process. Due to the elevated temperature of the roaster, the water applied is emitted as steam.
C. Cooling
The beans are then transferred to an enclosed cooler, where ambient air is blown over and/or through the beans as they are stirred or agitated to facilitate the cooling process. In the case of small, batch type roasters, this step is carried out in an open bin (referred to as a cooler car) equipped with a rotating stirring arm and blower to facilitate the cooling process.
D. Destoning
The cooled beans are then transferred to a destoner, which is essentially an air classifier which separates the beans from heavier material such as stones, metal fragments, and other waste materials that were not removed during the initial cleaning process. In the case of small, batch-type roasting operations, operators typically rely on the initial cleaning process and do not mechanically destone the beans after roasting.
E. Miscellaneous Operations
1. Green Bean Handling
a. Unloading/Receiving – Bags of green beans are opened and dumped into receiving hoppers.
b. Conveying – at large facilities, green beans are typically conveyed via pneumatic systems.
c. Cleaning/Screening – Green beans are screened to remove dirt and other debris.
d. Blending – the beans are weighed and blended according to product specifications prior to roasting.
2. Roasted Coffee Bean Processing
a. Grinding – roasted beans are pnuematically conveyed to hoppers that feed multi-stage grinders
b. Packaging – ground coffee and whole coffee beans are packaged into various sizes for consumer and commercial distribution.
F. Process Flow Diagram
A typical large coffee roasting operation is depicted in the process flow diagram below. In many cases, small coffee roasting operations do not include all of the processes shown.
INSERT FLOW DIAGRAM
II. AIR EMISSIONS
As stated earlier, the roasting of coffee beans results in the emission of particulate matter, volatile organic compounds, organic acids, and natural gas combustion products. Green coffee beans contain a wide variety of chemical compounds including proteins, fats, sugars, dextrine, cellulose, caffeine, and organic acids. Some of these compounds volatize, oxidize, or decompose as part of the roasting process. Consequently, toxic compounds such as aldehydes (as formaldehyde), organic acids (as acetic acid) and acrolein are emitted as a result of the coffee roasting process.
Sections A through E below discuss the air emissions potential for each process described in Section I. Emission factors and emission calculation methods are presented for any processes that may have significant organic, particulate matter, or toxic compound emissions. Section F below discusses the types of air pollution control equipment that are typically employed on coffee roasting operations.
A. Roasting
Gaseous and particulate matter emissions occur during the roasting process. As stated earlier, the gaseous emissions result primarily from the breakdown of the sugars and oils present in the green coffee beans. These gaseous emissions include aldehydes, organic acids, phenols and other hydrocarbons. In addition, nitrogen oxides and carbon monoxide emissions occur as a result of the combustion of natural gas which is typically used to fuel the roaster. The particulate matter emissions are composed of coffee chaff (outer skin of coffee cherry) which is released when the coffee beans swell during the roasting process. Most of the chaff released during the roasting process is light enough to be carried off with the roaster exhaust. Although some chaff is emitted during the cooling process, the majority of chaff emissions occur at the roaster.
The following emission factors can be applied to coffee roasters. In the case of large continuous roasters, manufacturer’s guaranteed emission rates should be utilized if available.
Emission Factors for Coffee Roasters
Pollutant |
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Source |
Particulate Matter (lb/ton) |
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Nitrogen Oxides (lb/ton) |
Formaldehyde (lb/ton) d |
Carbon Monoxide (lb/ton) |
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| Batch Roaster | 4.2c |
0.86b |
0.1c |
0.054 |
N/Db |
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| Batch Roaster abated by Thermal Oxidizer |
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| Continuous Roaster |
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| Continuous Roaster abated by Thermal Oxidizer |
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a Will vary depending upon destruction efficiency of abatement device. Assume destruction efficiency of 90% by weight unless otherwise specified by manufacturer.
b Taken from EPA AP-42, Chapter 9.13, 9/95.
c Taken from EPA AP-42, Chapter 6.2-1, 2/72.
d The factor used to calculate this formaldehyde emission factor (0.063 lb formaldehyde/lb total organics) was taken from the Toxic Air Contaminant Emission Inventory for the San Francisco Bay Area Status Report, April 2, 1990. Emissions of the toxic compounds acrolein, acetaldehyde, and organic acids may be present in coffee roaster exhaust streams, however the amounts of these emissions have not been substantiated to warrant their inclusion in this Permit Handbook chapter.
B. Quenching
The primary emission resulting from the quenching process is steam. The steam contains primarily particulate matter and may contain trace amounts of the pollutants emitted during the roasting process.
C. Cooling/Destoning
The primary pollutant emitted during the cooling and destoning processes is particulate matter as coffee chaff. The vast majority of coffee chaff is composed of large flaky particles exceeding 100 microns in diameter. Cyclone collectors abating cooler/destoner emissions typically achieve a particulate matter collection efficiency ranging from 70% to 90% by weight. If the coffee roasting operation under evaluation includes a separate, independently operated cooler and or destoner, the following emission factor may be applied:
Particulate matter = 1.4 lb/ton coffee beans
This emission factor is without control equipment such as a cyclone. If no other data is available from the applicant or cyclone manufacturer, a conservative collection of efficiency of 70% by weight may be applied. It should be noted that particulate matter emission rates based upon a guaranteed grain loading number are not accurate due to fluctuating exhaust gas flow rates characteristic of quenching and roasting operations.
D. Miscellaneous Operations
1. Green Bean Handling
Unloading/Receiving, pneumatic conveying, Cleaning/Screening, and Blending:
These processes result in the emission of large particulate matter (>100 microns) that are composed of coffee chaff, dirt, dust, fibers, and other debris associated with the green bean harvesting, packing, and shipping process. No emission data or emission factors are currently available for these operations. Because these processes are typically abated by cyclones and/or fabric filter abatement devices which are very effective at collecting such large particles, particulate matter emissions are assumed to be negligible.
2. Roasted Coffee Bean Processing
Grinding and Packaging:
Because the particulate emissions resulting from grinding and packaging are not typically vented to the atmosphere, they are assumed to be negligible.
E. Abatement Equipment
Typically, particulate matter emissions from the roaster, cooler, destoner, and green bean handling equipment are abated by high-efficiency cyclones. Because the majority of particulate matter emitted is larger than 100 microns in diameter, these cyclones are very effective in capturing the particulate matter emitted. Collection efficiencies range from 70% to 90% by weight.
Gaseous emissions from roasters are typically abated by catalytic or thermal oxidizers downstream of cyclones. Recirculating roasters have reduced emissions since they redirect a portion of the roaster exhaust back through the burners, resulting in the oxidation of some of the pollutants.
