Pellet Offgassing: Simple Problem, Simple Solution?

Wood pellet-derived carbon monoxide has been a cause of concern for indoor storage scenarios, but research suggests it’s an easy fix.
By Katie Fletcher | January 25, 2016

Serious and fatal accidents in a range of scenarios in Europe have sounded the alarm when it comes to pellet carbon monoxide (CO) offgassing and its potential hazards. As a result of these instances, pellet offgassing has become an issue of contention in the U.S., where the pellet heating industry is still in its infancy and research has brought forth varying conclusions regarding offgassing and its dangers.

In May 2002, during the discharge of pellets from British Columbia onboard MV Weaver Arrow in the Port of Rotterdam, one stevedore died and several other workers were injured. Another fatal case occurred in November 2006 onboard the MS Saga Spray in the Port of Helsingborg, Sweden, while the vessel was, again, discharging wood pellets from British Columbia. One seaman was killed, a stevedore was seriously injured and several rescue workers were slightly injured after entering an unventilated stairway next to a cargo hold.

Other incidents caused by the release of CO from wood pellets have occurred beyond ship’s cargo holds, including three deaths in Europe since 2010, caused by entry into wood pellet storage facilities on domestic sites. The most recent occurred in 2011, when a 28-year-old pregnant woman died in Switzerland after entering a pellet storeroom that supplied 60 households.

CO, coined the “silent killer,” is an odorless, colorless, tasteless and highly toxic gas. CO becomes hazardous at elevated concentrations and exposure times, and because of this, various organizations have set exposure guidelines. For example, the Occupational Safety and Health Administration’s guideline is any enclosed space shall be maintained at no more than 50 parts per million (ppm) as an eight-hour average area level. The National Institute for Occupational Safety and Health guidelines are more stringent, at a maximum of 35 ppm CO concentration per eight hours of exposure time. The World Health Organization’s guideline is 9 ppm per eight hours of exposure and 25 ppm for one hour.
In response to the fatalities in Europe, the U.K. Health and Safety Executive issued a warning to raise awareness of certain issues involved in using wood pellets as fuel. The North American pellet industry is learning from its European counterparts, and conducting its own research in response to these incidents.

Well-versed on the issue, Europe maintains considerable research and dedicates more funding toward establishing standards and guidelines for bulk pellet storage. Meanwhile, the U.S. recognizes the concern and is working on establishing common industry understanding. One example of this is Biomass Thermal Energy Council’s document of best practices regarding quality and safety assurance for the storage of loose bulk wood pellets for small- and medium-scale central heating systems.

The New York State Energy Research and Development Authority is pushing research dollars into studying pellet offgassing. One study conducted by Clarkson University in Potsdam, New York, observed the presence of CO in wood pellet storage facilities and in the laboratory due to offgassing, and investigated methods to improve air quality in pellet storage areas. CO monitors were positioned in or near pellet bins, and CO concentrations were found to be as high as 44 ppm in a residential basement, which exceeds NIOSH exposure guidelines of 35 ppm in occupational settings and 9 ppm in residential and commercial buildings. On the other hand, a study released by the University of New Hampshire in August found that indoor storage of pellets in homes does not pose a risk of generating CO levels above recommended thresholds. Both studies unveiled varying emission measurements, but also differed in where CO monitors were placed and the variables studied. Adam Sherman, manager of the Vermont-based Biomass Energy Resource Center, says this research takes a shallow dive, but because of this, everything else is conjecture as to whether pellet offgassing is a problem.

Although the reason behind these studies creates an ominous outlook for indoor pellet storage, research conducted thus far demonstrates that pellet offgassing is a simple problem that should not invoke panic.

Similar Studies, Different Results
While the research conducted at Clarkson University offered elevated levels of CO, the study didn’t measure any values close to lethal levels experienced in the European incidents. “It’s not a hazard we need to panic over, but it’s certainly a hazard we need to take seriously and work on resolving,” says Philip Hopke, director of the Center for Air Resources Engineering and Science at Clarkson University.

The concern is that as biomass boilers become more widely used, a broad array of homes with varying levels of natural ventilation will install pellet heating systems with inside-the-structure storage bins. According to the study, in energy efficient (low air exchange rates) homes, this situation could produce unacceptable CO concentrations. The results raise a safety question regarding how pellet storage bins are designed and sited. The study found that active and natural ventilation clearly reduces the average concentrations, although higher values were still observed. Also, pellet aging clearly reduced the amount of observed CO. In the study, there was a clear, positive relationship between the offgassing of CO and CO2 and temperature where higher temperatures produce higher offgassing production. Thus, bin temperatures need to be considered when designing or choosing pellet storage bins.

Another variable studied by Clarkson University considered hardwood pellets versus softwood. The study suggested northeastern U.S. hardwood pellets have less CO offgassing, but caution should be taken because hardwood pellets had the tendency to absorb more moisture than softwood pellets. Moisture can be concerning, the study suggests, because it found that dried wood pellets were less active in CO offgassing, and pellets stored under high relative humidity had a faster and higher CO emission rate. Pellets should be stored under low-humidity, low-temperature conditions.

Hopke and others at Clarkson are furthering their research. One project he mentions began last summer and aims to understand the CO formation mechanism and to determine whether there are ways that, before leaving the factory, a pellet can be processed to potentially lower CO offgassing. The university is doing a series of basic mechanistic studies to understand what kind of controls they should have and what it is that’s oxidizing, an issue that’s puzzled over, according to Hopke.

NYSERDA has taken action due to the absence of widespread industry knowledge concerning indoor bulk pellet storage. “Currently, in consultation with the New York State Department of Health, NYSERDA is using caution by not subsidizing in-building bins until we get a better handle on just what’s going on, and whether that would really represent a significant health hazard to the occupants of the building,” Hopke says.

Indoor pellet storage is considered a confined space, so there has to be a permit program established where the employer only gives permits to trained personnel to work in those spaces. This is an OSHA regulation. Clarkson’s faculty has provided confined space training to ensure that maintenance workers understand how they can work safely, as well as know the employer program requirements to limit access and provide continuing training to keep the workers safe. “We just want to find ways to do it efficiently, effectively and safely so we don’t wind up with incidents like they saw in Europe, because that would seriously hurt the pellet industry very quickly,” Hopke says.

According to results from the University of New Hampshire study, indoor pellet storage does not pose a risk of generating CO levels above recommended thresholds. Over a period of seven months, the CO concentration in the air of 25 residences in New Hampshire and Massachusetts were monitored. The fact that homes built in different eras possess a range of air tightness and dampness tendencies was taken into consideration, so construction varied from residences built in 1774 to 2013. Out of the 25, 16 of the homes use wood pellet boilers with indoor pellet storage containers at a capacity of at least 3 tons, four of the homes use outdoor pellet storage, four use other heating fuels and the remainder are at a university laboratory site. The study was designed to obtain preliminary survey data of residential CO concentrations at the level of ppm of ambient indoor air in the immediate vicinity of wood pellet storage and heating systems compared to that of the homes using fossil fuel systems. Using a threshold of 9 ppm for its average, hourly CO measurements, data was acquired every five minutes to determine concentrations that might adversely affect indoor air quality.

There are various factors not controlled for in the study, but of the data obtained, no evidence suggests there is an reasonable risk when having pellet storage inside, a conclusion that differs from Clarkson University’s study. Backing New Hampshire’s findings, Sherman says some installations throughout the northeastern U.S. are voluntarily putting CO monitoring equipment in boiler rooms next to pellet storage, and finding no evidence of emissions above an acceptable threshold. “I think that what the researchers are trying to understand is what happened with those early accidents and whether those cases are applicable to the most common situations where you’ve got—especially residential, but also commercial and industrial—pellet storage and for a lot of that, the answer is no,” says Ben Bell-Walker, technical affairs manager with BTEC. Bell-Walker says this is mainly because pellet offgassing is a volume issue. “If you’re talking about a residential, small commercial or even a larger commercial system, you’re really not talking about concentrations of pellets that are going to give you that amount of carbon monoxide.”

Industry Work
Dilution is the solution to pollution, Sherman references as the remedy to pellet offgassing. “CO only becomes a threat to human health if concentrated in an enclosed space, so if you vent it outside, it becomes a nonissue—simple solutions for simple problems,” he says.

On Sept. 24, BTEC released its 1.0 version of quality and safety assurance best management practice for the storage of loose bulk wood pellets for small- and medium-scale central heating systems. “This whole document is what we consider industry best practices, so the reason this was prepared is to provide manufacturers, deliverers, pellet suppliers a way to make sure they’re checking all of the boxes in terms of any kind of possible dangers,” Bell-Walker says. “Especially with residential pellet delivery, you get a whole variety of ways people have configured their space for storage. This is just a way for the company to do some due diligence.”

The guidelines address a the size of ventilation ports and the configuration of venting, not just for potential CO offgassing, but for minimizing dust and fines. Other areas addressed include storage volume recommendations, delivery preconditions, and explosion management. Bin-specific requirements are a big part of the best practices BTEC put together. Sherman says the question to ask when considering indoor pellet storage is what sort of monitoring and what sort of ventilation systems can be easily and cost-effectively installed, keeping in mind that if there is CO offgassing while the fuel is stored indoors, it can be passively vented, making it a nonissue. There are also active ventilation options such as fans that can be implemented.

Other Research and Future Studies
A number of studies have evaluated components of pellet storage at the University of British Columbia. One study assessed the rate and peak concentrations of offgas emissions in stored wood pellets such as sensitivities to temperature, relative humidity and headspace volume. The study found that increased headspace volume ratio increases peak offgas emissions because of the availability of oxygen associated with pellet decomposition.

A report providing similar data on offgassing and self-heating was prepared in 2011 by the Wood Pellet Association of Canada, in collaboration with UBC Department of Chemical and Biological Engineering with funding from Ethanol BC. The research focused on self-heating as a chemical process and kinetic modeling of offgassing to establish a prediction model for industry use. The research played a significant role in the new ISO quality classification and testing standards for solid biofuels. 
In March of 2013, the European Pellet Council held its first International Workshop on Pellet Safety in Fügen, Austria, where participants offered feedback on existing pellet storage guidance, opportunities to improve research and communication to industry stakeholders.

Sherman says he’d like to see research delve deeper into finding what variables are driving CO offgassing, rather than focusing on whether it is or isn’t an issue. “I think what would be interesting, in terms of future studies, is really looking into the critical variables,” he says. Sherman adds that when it comes to pellet offgassing, it’s particularly important to understand more about three variables,  including the age of pellets, the volume of pellets, and their physical and chemical properties.

Whether there is truly an issue, Sherman says, the industry must be proactive, establishing best practices and promulgating them. “This isn’t a problem unique to pellets that should warn undue concern if there are simple solutions for simple problems.”

Author: Katie Fletcher
Associate Editor, Biomass Magazine