Since the EPA decided many years ago that volatile organic compounds (VOC) emissions must be controlled, state agencies and pellet plant operations, along with many others, have gone about the task of making it happen.
By Malcolm Swanson | February 27, 2015
To people outside the forest products industries, it is surprising to learn that the nice fresh pine fragrance our noses detect around a wood pellet plant is actually evidence of emissions of volatile organic compounds (VOCs). Since the EPA decided many years ago that VOC emissions must be controlled, state agencies and pellet plant operations, along with many others, have gone about the task of making it happen. Wood naturally releases VOCs whether it is still standing in the forest, being harvested, or processed into a product such as wood pellets. The natural VOC release is accelerated when heat is applied, as in a drying process, or when more fiber surface is exposed, as in size reduction via hammer milling. Also, forming the dried wood dust into pellets tends to release more VOCs, because of the application of a significant amount of energy to the material and the resulting temperature rise.
Accelerated VOC release in the pellet making process is not only an emissions issue but it also represents a loss of energy from the pellets. VOCs are natural chemical compounds that have energy content. Obviously then, the better job we can do of minimizing the release of VOCs from the wood, the higher the energy content of the pellets. So, the best means of VOC emission control is to, as much as possible, avoid releasing these valuable compounds from the wood in the first place.
Of course, regardless of how we process the wood into pellets, some VOCs will be released from the wood. To keep the plant operation in environmental compliance, the VOCs released from the wood must be contained and controlled within the plant system. There are two fundamental approaches to VOC control. The most widely used approach is to incinerate the VOCs in a regenerative thermal oxidizers (RTO) or regenerative catalytic oxidizer (RCO) at the plant’s exhaust points. A wet Electrostatic Precipitator (WESP) is typically used upstream to reduce the contamination that occurs in the RTOs and RCOs. This method is necessary when using the conventional convection type rotary dryers. A newer approach involves using a hot oil tube dryer, operating with a hot oil temperature of about 500 degrees Fahrenheit (vs. the 1,100 F hot gas temperature of the convection dryers), to remove water while removing less of the hemicellulose. The various compounds that make up hemicellulose are those that, when evaporated in the drying process, become VOCs. These compounds have relatively low boiling points, so, the drying temperature makes a difference in the mass of VOCs liberated.
There is a significant difference in the economics of the pellet-making operation between these two different approaches to the VOC emissions issue. The WESPs, RTOs or RCOs required with the conventional dryer systems are major pieces of capital equipment. They must be maintained and require fuel and electricity to operate. By contrast, VOC control is incidental with the hot oil tube dryer. No additional special equipment or capital equipment is needed. Also, no purchased fossil fuel is needed. In fact, the VOCs that are evaporated from the wood in drying, although not a lot, are used as part of the fuel to run the drying process. So, instead of VOC destruction adding expense, it actually reduces operating cost.
One large plant has a natural gas cost of $1.25 per metric ton of pellets or $625,000 per year for 500,000 tons. By contrast, the hot oil tube dryer system burns no fuel at all for destruction of VOCs.
In addition to the avoiding the cost of fuel and maintenance for WESP or RTO type systems, a benefit is found simply keeping in the pellets what would, in the convection dryer systems, become VOCs. Hemicellulose has a higher heating value of 13.6 megajoules per kilogram, so, it has an energy content that is worth keeping. To determine the quantity of the VOCs being released from the wood in the pellet-making process, consider what EPA’s document, AP42, shows as the uncontrolled emission factor for condensables from the drying process of particle board making. (EPA considers this the process most similar to pellet making.) AP42 shows 1.1 pounds per ton. (See AP42, 10.6.2.) For a pellet plant producing 500,000 tons of pellets per year, the uncontrolled dryer emissions of condensables would be 550,000 pounds per year. Although experience shows that the pellets actually have a little more energy per ton when the material is dried in a hot oil tube dryer, the simplest way to show some value to this is just to consider that the 550,000 pounds per year of mass largely ends up in the pellets. That means, in effect, that a producer would have an extra 250 tons per year to sell. Since the cost of the overhead, material, and production are there either way, the entire sales price of $45,000 goes straight to the bottom line as net profit.
Going back to EPA’s AP42 emission factor for condensable VOCs, we see that it is just that, condensable VOCs. Also, it shows only the VOCs from the drying process. The total mass is easily twice this amount when we include noncondensables and emissions from other steps of the process such as dry hammer milling, pelletizing, and cooling. Because the hot oil tube dryer has a hot oil heater combustion chamber available in which to burn VOCs, noncondensables are scavenged from their various sources and used almost entirely as fuel for the drying process. This means that either some of the incoming green wood that would otherwise be used as fuel isn’t used that way and is, therefore, available to become pellets, or some purchased fossil fuel is not needed. In either case, a conservative way to apply a value to this is to say it is about the same as the additional pellet value associated with the condensables. So, the total additional pellet sales will be about $90,000. Again, this total goes straight to the bottom line because all costs are already there.
As briefly mentioned above, retaining as much as possible in the pellets rather than evaporating it results in a higher energy content in the pellets. The jury is still out on exactly how much advantage, since it is virtually impossible to get a valid direct comparison but it appears to be on the order of 0.5 gigajoules per ton. Since this is anecdotal information at this point, the value is not included in the following totals.
The maintenance cost for the WESPs and RTOs and associated equipment is around $2.25 per ton or about $1,125,000 per year. The capital cost of the equipment is about $13 million.
This rather simplistic analysis does not attempt to address the cost of downtime associated with regularly washing the ceramic elements of the RTOs, etc. So, there is considerably more money that could be added to this bottom-line advantage calculation, if one wanted to take the time to work it out. Obviously, the assumption is made here that the capital cost and maintenance cost of the two different dryer islands (not the back end pollution controls) are at least similar. That may or may not be true, but there is no doubt that WESPs and RTOs or RCOs are required with the convection type dryers and are not required for the hot oil tube dryer systems.
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