CO and portable generators.
The magnitude of such reductions needed to reduce exposures to a specific level depends on the complex relationship between CO emissions from these generators and occupant exposure. To better understand the CO emissions from portable generators, the potential for reducing these emissions and the impacts on occupant exposure, a multi-year research effort was conducted involving both experimental and simulation studies. (2,3)
Measurements of CO Emissions From Portable Generators
To better understand CO emission rates from both stock (currently available) and reduced-emission prototype portable generators, experiments were conducted in a single zone shed and in a three-bedroom test house with an attached garage. This column summarizes the measurements conducted in the shed; the tests with the generator operating in the attached garage are described in Emmerich et al. (2013). As discussed in that report, previous measurements of CO emissions from generators have been conducted in ambient air or in chambers with very high air change rates. By conducting these tests in an enclosed space with more realistic ventilation conditions, the [O.sub.2] levels will decrease as the generator operates, providing more relevant measurement results.
The shed experiments were conducted in a 43 [m.sup.3] single-walled, uninsulated timber structure for the purpose of measuring the CO emission rates and [O.sub.2] consumption rates of the generators tested. Photo 1 shows a generator installed in the shed along with the load bank used to place an electric load on the generator. The shed had two operable windows at both sidewalls and an exhaust fan, which were used to vary the air change rate during the tests from about 0.5 [h.sup.-1] to 10 [h.sup.-1]. Tests were conducted with three different generators that were configured in multiple ways. Two unmodified "stock" (i.e., in their as-purchased condition) generators were tested. The first generator had a full-load power rating of 5.5 kW with a 10 hp, carbureted, single cylinder gasoline engine and no CO emission control technology.
The second generator was powered by a carbureted 11 hp single-cylinder gasoline and had a full-load power rating of 5.0 kW. This generator was tested in both its stock, unmodified condition and modified as a low-CO emission prototype. The modifications included an engine management system (EMS) with sensors and actuators for electronic fuel injection (replacing the carburetor) and a muffler with a small catalytic converter. The third generator was similar to the second, but with an output rating of 7 kW and a different EMS.
Figure 2 shows the measured CO and [O.sub.2] concentrations for two of the experiments with the first, unmodified generator (see Emmerich, et al. (2) for an explanation of the test conditions). For reference, a CO concentration of 1200 ppmv (which is equivalent to 1400 mg/[m.sup.3] in Figure 2) is considered immediately dangerous to life or health (http://tinyurl.com/22drnn). The patterns of CO concentrations in both tests are almost inverses of the [O.sub.2] levels for this unmodified generator. The CO level is low at the beginning of generator startup and increases steadily as the [O.sub.2] level drops. As the [O.sub.2] drops further, causing a very rich fuel mixture in the engine, CO generation reaches a maximum level. Test 13 shows an extreme case in which the generator eventually produces a zero electrical load after the [O.sub.2] drops to almost 16%, although it was set at a full load and the crankshaft was still rotating.
To generalize these test results to other conditions beyond these particular tests, it is necessary to convert the results into CO emission and [O.sub.2] consumption rates. Figure 3 shows 5-minute average CO emission rates as a function of [O.sub.2] levels in the 13 shed tests of the first, unmodified generator. For both full and half load settings, CO emission rates increase with decreasing [O.sub.2], reaching maximum values when [O.sub.2] drop to about 17% to 18%, and then decline at lower [O.sub.2] levels. Under the extreme case of Test 13 (5.0kw-CWLA), the CO rate decreases dramatically as the [O.sub.2] level reaches around 16.4% with an electrical output of zero. The solid points in Figure 3 are data points for a half-load setting (2.5 kW) and the hollow ones are for a full load setting (5.0 kW).
The second generator was tested in both unmodified and modified (low CO emission) configurations. Figure 4 presents the CO emission rates as a function of [O.sub.2] levels for the unmodified generator, while Figure 5 presents the CO emission rates as a function of [O.sub.2] levels for the modified generator. Although the modified generator was not tested as many times as the unmodified version, these figures show the dramatic reduction in CO emission rates due to the modifications included on the prototype. Most of the modified generator's emission rates were well below 500 g/h.
Simulations of CO Exposure from Portable Generators
To address the CO exposure associated with portable generators and to support potential control strategies such as reduced emissions, a better understanding of the relationship between CO emission rates and occupant exposure is needed.
This relationship involves the interaction between generator location and operation, house characteristics, occupant location and activities, and weather conditions.
To support life-safety based analyses of potential CO emission limits for generators, a computer simulation study was conducted to evaluate indoor CO exposures as a function of generator source location and CO emission rate. Simulations were performed using the multizone airflow and contaminant transport model CONTAM, (4) which was applied to 87 single-family, detached dwellings that are representative of the U.S. housing stock. Using these homes, indoor CO concentrations were calculated over a range of generator locations, CO emission rates, and weather conditions. More information on these simulations and the results obtained are available in Persily et al. (3)
These simulations yielded CO concentrations in the rooms of each house as a function of time during the 24-hour analysis interval. To compare the results for different cases, the concentrations from each simulation were used to calculate carboxyhemoglobin (COHb) values for an exposed occupant in each occupied room. COHb forms in red blood cells upon contact with CO and hinders the ability of hemoglobin to deliver [O.sub.2] to the body. The maximum COHb value among the occupied rooms was used as a metric of CO exposure for each combination of house, source, and weather. For reference, COHb levels of 70% or greater are associated with death in less than 3 minutes, levels of 50% are associated with headache, dizziness and nausea in 5 minutes to 10 minutes and death within 30 minutes, levels of 30% with dizziness, nausea and convulsions within 45 minutes and becoming insensible within 2 hours, and levels of 20% with a slight headache in 2 hours to 3 hours and a loss of judgment. (4)
The results of the simulations constitute a large amount of data, which can be interpreted by considering the percentage of cases simulated that meet a specific criterion for the target value of maxCOHb. Determination of such criteria was beyond the scope of this project but for comparison purposes, the maximum source strength was estimated for which 80% of the cases simulated are below 30% maxCOHb for each of the source locations considered. The values of 80% below 30% maxCOHb are used only for illustrative purposes and are not presented as life-safety based limits to support any policy or regulatory decisions. Considering all the constant source results, the maximum source strength corresponding to 80% of the cases having a value of maxCOHb below 30% is 27 g/h. Note that the CO emission rates measured in unmodified generators mentioned earlier tended to be well above this value, but that the modified generators tested were in this range. Therefore, the CO reduction technologies considered in these tests show promise for reducing the risk of death and injury due to inappropriately used generators.
In 2006, CPSC issued an Advance Notice of Proposed Rulemaking; Request for Comments and Information describing its strategy to reduce generator engine CO emission rates. Additionally, Underwriters Laboratories, Inc. has formed a working group to develop a specific proposal for requirements for portable engine-generator sets that fall under the scope of UL 2201, Portable Engine-Generator Assemblies to reduce the risk of death and injury due to CO poisoning.
This research was supported by the U.S. Consumer Product Safety Commission (CPSC) under interagency agreement CPSC-I-06-0012.
(1.) Hnatov M.V. 2013. "Incidents, Deaths, and In-Depth Investigations Associated with Non-Fire Carbon Monoxide from Engine-Driven Generators and Other Engine-Driven Tools, 1999-2012." U.S. Consumer Product Safety Commission.
(2.) Emmerich S. J., Persily A. K., and Wang L. 2013. "Modeling and Measuring the Effects of Portable Gasoline Powered Generator Exhaust on Indoor Carbon Monoxide Level." NIST Technical Note 1781. National Institute of Standards and Technology.
(3.) Persily, A. K., Wang, Y., Polidoro, B. and Emmerich, S. J. 2013. "Residential Carbon Monoxide Exposure due to Indoor Generator Operation: Effects of Source Location and Emission Rate." NIST Technical Note 1782. National Institute of Standards and Technology.
(4.) Goldstein, M. 2008. "Carbon monoxide poisoning." Journal of Emergency Nursing. 34(6):538-542.
(5.) Walton, G. N., W. S. Dols. 2005. "CONTAMW 2.4 User Guide and Program Documentation." NISTIR 7251, National Institute of Standards and Technology. I
Steven J. Emmerich is a mechanical engineer, and Andrew K. Persily, Ph.D., is the group leader of the Indoor Air Quality and Ventilation Group at the National Institute of Standards and Technology (NIST), Gaithersburg, Md. Liangzhu (Leon) Wang, Ph.D., is assistant professor in the Department of Building, Civil and Environmental Engineering at Concordia University in Montreal.
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|Title Annotation:||COLUMN: IAQ APPLICATIONS|
|Author:||Emmerich, Steven J.; Persily, Andrew K.; Wang, Liangzhu "Leon"|
|Date:||Sep 1, 2014|
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