Coffee Roaster Filtration System
|Copyright Diedrich Roasters|
The goal of the project is design, fabricate, and test a low-cost air filtration system for an industrial batch coffee roaster. The system will reduce visible smoke output by 95% and mitigate odor transmission. The system is adaptable to Diedrich Roaster's IR-5 and IR-12 models, and serves as a more affordable alternative to their catalytic oxidizer.
- 1 Problem Definition
- 2 Previous Design Considerations
- 3 Project Learning
- 4 Current Design Process
- 5 Validation
- 6 Meet the Team
- 7 Documentation Archive
- 8 References
The unique chemical compositions of coffee beans result in diverse emissions during the roasting process. These emissions can be generalized into two categories: Gaseous emissions and particulate matter (PM).
In batch roasting, the primary gaseous release of concern is carbon dioxide, factoring for approximately 10% of gaseous emissions up to as high as 25% in roasters which utilize thermal oxidizers in post-treatment. A small amount of carbon monoxide is released as well, but at near-negligible rates.
Both carbon monoxide and carbon dioxide are relatively color- and odorless. The main cause of smell and smoke in roasting comes from various classes of volatile organic compounds (VOC). VOCs are any number of organic (containing carbon) compounds that have a relatively low boiling point, near room temperature (volatile). Methane is a common VOC, but the are myriad compounds which fall within the category. Due to coffee's complex chemical nature, identifying the specific VOC's emitted is a challenge, as each bean will release a slightly different combination.
Particulate matter (PM) refers to solid matter, typically waste carbon, ejected during oxidation. In coffee roasting, this primarily appears in the form of chaff.
In commercial applications, local governments often have emissions codes dictating an upper limit on allowable emissions from coffee roasters. These emissions typically aim at mitigating greenhouse gases, which are primarily generated by the natural gas burners that serve as the main emissions source.
Most smaller companies and private roasters are not bounded by regulations due to smaller batch sizes. Many standards, while very possible to achieve, are expensive and prohibitive towards smaller companies or individuals early in their roasting career, and few solutions exist on the market that are both emissions-compliant and affordable. Rather than emissions reduction, filtration systems for smaller outfits tend to focus on reducing more noticeable effects, such as smell and smoke.
Because roasting happens below the point of combustion, the exhaust from a roaster is largely unburned matter. This is evidenced in the amount of VOC's reached, as many of the compounds are flammable. Thermal oxidisers exploit this issue by providing an afterburn to the exhaust of a roaster. By raising the exhaust temperature significantly and exposing the outgoing material to open flame, a thermal oxidizer is able to convert many of the harmful VOCs into CO2 and H20, the products of complete combustion in hydrocarbons.
Thermal oxidisers are one of the most common aftertreatment systems in the coffee roasting industry due to their effectiveness, and used in many other commercial settings, such as air control in factories. A common enhancement in these systems is the catalytic oxidizer, wherein the exhaust stream is forced through a catalytic converter. This also helps to mitigate some of the emissions from the natural gas burners.
The filter must be able to accept flow rates typical of the IR-5 and IR-12 and maintain acceptable filtration levels. The filter must be self-supported and capable of existing as a standalone system. The design must be low-cost, constructed primarily of sheet metal, and low-maintenance. Finally, the system should be innovative, setting itself apart from other filtration systems on the market.
|Flow Rate||1500 cfm <cn>|
|Backpressure||-.15 - .25 inH20|
|Dimensions||4ftL x 4ftW x 6ftH|
|Maintenance Interval||> 8 hrs (1 workday)/service|
|Operable Temperatures||Up to 500 °F|
Previous Design Considerations
Electrostatic Precipitator (ESP)
An electrostatic precipitator uses electric charge to attract pollutants as flow passes through. ESP's have low flow obstructions, which makes them ideal for many scenarios such as HVAC. They are also energy efficient, as they apply energy to particulate matter as opposed to the entire flow. However, their collection efficiency is determined by resistivity, an electrical property highly dependent on temperature. This means that ESP's are designed for specific temperature ranges, and higher temperatures result in both higher material and operating costs.
An electrostatic filter operates on the same principles as ESPs, but instead of having an external power source, generates static charge from particulate as it passes through. Electrostatic filters are common in household and smaller commercial use due to their ability to filter fine particulate. They are also have a low cost associated with them and an essentially infinite operational lifespan.
Nonetheless, electrostatic have a number of shortcomings. Of the filtration systems researched, they have the lowest operational efficiency, especially as particulate size increases. They also require consistent cleaning, and have a tendency to become a haven for mold in high-humidity environments, a dangerous habit for systems dealing with organic matter. Finally, they too suffer from efficiency losses with increases in temperature.
Wet filters are another passive filter system, like the electrostatic filter. Unlike the electrostatic filter, though, wet filters do not rely on electrical principles. Instead they use a trapping fluid, typically water, that collects particulate matter from a flow passing through it. Wet filters are among the most efficient and versatile of passive filters, and, when properly utilized, can filter gaseous pollutants or particulate matter. They also enjoy the low costs common in passive filter design. However, they do not have the infinite lifespan of electrostatic filters and suffer from the inherently high-humidity environment.
Wet scrubber refers to a whole class of filtration systems that use injection of a filtering fluid into the exhaust flow. Wet scrubbers can be optimized for gaseous pollutants and particulate matter of various sizes. They are becoming increasingly popular in industries that make use of boilers and furnaces due to their effectiveness against ashes and their ability to lower exhaust temperatures.
The wet scrubber we researched and ultimately decided to incorporate into our final design is a Venturi wet scrubber. Finer particulate trapping requires higher energy in the spray to decrease droplet size. A Venturi wet scrubber uses the Venturi effect, wherein flow is directed through a converging cone into a smaller diameter tube, increasing velocity. This passive design takes energy from the airflow, reducing the necessary energy input from the spray.
Wet scrubbers have a number of advantages over other filtration systems. They are the only researched active filter that can handle gaseous emissions, which are assumed to be the primary culprit of odor transmission. They are able to handle far higher temperatures than electrostatic systems without compromising trapping efficiency. Most importantly, because they significantly lower temperatures of the exhaust flow, they can reduce the amount of ducting necessary, decrease costs of fans and tubing that no longer requires high-temperature durability, and act as a suitable first-stage filter in a multistage design.
There are, however, known issues with wet scrubbers. The primary issue, which is partially rectified with a Venturi tube, is a high power requirement for fine particulate trapping. The issue of using the Venturi tube is an increased pressure drop, a consideration of high importance, as backpressure has a known pronounced effect on roasting quality. The secondary issue is of disposal of the waste: Trapped pollutants are often corrosive and require further treatment to meet wastewater regulations.
The Coffee Community
Especially being located in the PNW, hobby roasters are very common. Many have created their own ingenious and inexpensive ways to address the problem of affordable filtration. Coffee roaster forums have played an invaluable role in the design process by suggesting possible solutions and testing methods, pointing towards helpful research, and answering any questions that the team could think of.
Current Design Process
Main Filter - Cyclonic Wet Scrubber
The cyclonic wet scrubber operates on the same principles as a standard wet scrubber with some modifications. (More explanation will be added soon).
The current development on the cyclonic wet scrubber is a proof-of-concept venture tube, water injector, and cyclone chamber system.
Secondary Filter - Electrostatic Precipitator
Determination of the post-scrubber filter has not been determined yet.
A large focus of this project has been shifted to developing repeatable tests for smoke generation/filtration, as well as quantitative measurements that can be used to effectively benchmark filtration systems. Such tests and measurements allow various tests and design choices to be validated, demonstrating marked improvement over a control series.
- NOTE: This section is out of date. It will be updated soon.*
Opacity meters are used to determine the level to which content in air affects visibility, typically through use of a light transmitter that pulses a signal and compares either the reflected light or the transmission loss against a control level. Opacity meters are common in diesel fuel research and soot mitigation, and the UI used one for its Biodiesel program, but the meter has since been lost. A fabricated opacity meter design is in the works for use on the project.
The opacity meter is designed to be an inline meter. The transmitter is a simple white LED, and the receiver is a TSL2561 Lux sensor. The lux sensor was chosen due to its high range, relative sensitivity, and gain settings. The receiver and transmitter are placed opposite one another inside a matte black pipe, with prescribed diameter to fit the outlet piping. The matte black surface keeps light from bouncing within the sensor, allowing for higher accuracy.
The sensor outputs to an Arduino Uno, which also supplies power for the transmitter and receiver. When attached to a computer, the Arduino can either write data to a text file or pipe to the serial monitor. The lux sensor outputs a voltage response that can be mapped on a 0-100 scale; additionally, a lux library can be installed to convert the output to lumens using Bouguer's Law.
A challenge with testing any filtration system is being able to properly simulate the conditions the filtration system will encounter. Because the UI does not have the proper infrastructure for housing a temporary IR-12 roaster, a substitute system had to be improvised.
The smoke generator is a simple combustion cave. A heating element is placed in the bottom of a heat-proof rigid container (in our case a 30 gallon oil drum) with a mesh bed slightly above it, where the combustion material rests. An inlet fan controls incoming air and outgoing flow rate. To ease production efforts, a barbecue grill is used for generating smoke on the proof-of-concept model. The modified barbecue handles smaller volumes than will be seen in the IR-5 and IR-12, but provides adequate smoke and temperature control to be suitable for an initial testing and analysis.
As the test equipment and instrumentation have not yet been finished, specifics of the test procedure can not yet be determined. however, the general procedure is as such:
- 1) Cover bed with combustion material
- 2) Secure smoke generator seal and engage fan on low power
- 3) Activate heating element and wait until outflow reaches operational temperatures
- 4) Vary fan power to operational flowrate
- 5) Measure intake and exhaust opacities at 3, 5, 8, and 11 minutes
Testing has not been conducted; it is scheduled to begin at the start of the spring semester. Once results of the proof-of-concept are tested, they will be posted here.
Meet the Team
Major: Mechanical Engineering
Major: Mechanical Engineering
Major: Biological Engineering
Major: Mechanical Engineering