Generating power with waste wood
By Richard S. Atkins, Ph.D., P.E., Environmental Risk Ltd.
Among the biomass renewables, waste wood has great potential with environmental and economic benefits
highlighting its resum?
In the United States and throughout the world, large quantities of waste wood are available to fuel power plants. Unfortunately, most of this material is not being used for fuel and, to a large degree, this abundant source of available energy is being wasted.
Approximately a decade ago, a number of waste wood combustion independent power producing (IPP) facilities were built in California. These units were established because of prevailing regulations that favored their development and provided premium electricity pricing for biomass power generation. These biomass power projects were based upon wood fuel streams of virgin wood materials from land clearing, improved forestry, utility line maintenance operations and other sources. Further-more, a number of these facilities also were permitted on the basis that they burn agricultural waste as fuel. Because these plants improved overall regional environmental conditions by reducing the burning of agricultural byproducts in the field, operators received particulate emissions as well.
Unfortunately, supply shortages of virgin wood fuel are increasingly hampering the operation of these California plants. Why? For one reason, too many facilities were built and now demand is overstepping supply. Moreover, plants permitted to burn agricultural waste discovered that these materials have poor combustion properties that adversely affect boiler operations. Consequently, agricultural wastes no longer supplement the wood fuel supply.
Alternate waste wood fuel streams
To continue operating, operators at some of these facilities have begun to examine alternate waste wood sources and other materials to fuel their boilers. For example, construction/demolition (C/D) wood, telephone poles, railroad ties, paper sludge and tires are now being tried.
Most recently, a number of waste wood combustion facilities have been built on the East Coast and in the Midwest. These facilities were, for the most part, developed by IPPs who have also received the same sort of premium electric contracts that plants on the West Coast operate under.
In an effort to cut costs, utilities with coal-fired power plants also are exploring the economic benefits of supplementing their primary fuel with waste wood. Several utilities are testing the co-burning of waste wood fuels with coal and other fuels as a means of reducing SO2 and NO emissions.
As a fuel, waste wood offers both environmental and economic benefits. Some of the most important reasons for power generators to consider using waste wood as a fuel source include:
Y Waste wood emits less SO2 and NO emissions than most other fossil fuels.
Y Firing boilers with waste wood provides an economic outlet for this material
and reduces its subsequent demand on landfill space.
Y Waste wood is a renewable fuel resource and using
it decreases the economic risk associated with fluctuating fossil fuel prices and lessens the dependence on imported oil.
Y Because biomass is created through the absorption of atmospheric CO2, its combustion helps alleviate greenhouse gases and, perhaps, global warming because there would be no net increase in CO2. Also, the disposal of waste wood in landfills results in emissions of methane gas which is known to have 15 to 20 times higher greenhouse gas effects than the equivalent molar concentration of CO2.
Y Many public utility commissions have recommended that the utilities put in place renewable energy sources within the state.
Y Co-firing waste wood with coal in a stoker or pulverized coal-fired boiler may result in lower SO2 emissions and could allow power generators to avoid costly SO2 controls.
Y The U.S. Department of Energy is seeking to establish a renewable energy base and has established tax credits for using biomass-based fuel.
Waste wood comparisons
Waste wood can be obtained from forestry, secondary wood processing, utilities, C/D, manufacturers, municipal solid waste sites and other disposal/recycling activities. Table 1 presents typical ultimate/proximate analyses of various OcleanO and OtreatedO wood fuels. OCleanO waste wood fuels are materials that have not been treated with paints, binders, glues, chemical additives, etc., and include virgin sources such as wood chips. OTreatedO waste wood materials have had one or more of the previously mentioned chemical treatments applied during any number of processes used to develop the wood?s commercial or technological value.
For example, railroad ties and some telephone poles are treated with creosote. Telephone poles also are treated with pentachlorophenol and chromated copper arsenate. Consequently, these types of wood wastes have a higher sulfur, heat, metals and/or chlorine content (on a dry basis) than the clean virgin wood types. Due to higher concentrations of bark and vegetative matter, yard waste and forest residuals also tend to have higher levels of chlorine and SO2 emissions than wood chips.
Construction/demolition wood has a low moisture and chlorine content and a slightly higher sulfur content, while waste wood from manufacturing operations such as pallets, spools, shipping crates, etc. offer power generators an ideal combination of low moisture, high heat content and low metals emissions. Generally, treated waste wood materials have better combustion properties (with the exception of those containing water-borne preservatives) and are a more efficient as a boiler fuel than the clean waste wood from virgin wood operations. Why? Virgin waste wood materials have a very high moisture content, whereas treated waste wood has a much lower moisture content and higher heat content.
Waste wood ash
Generally, the ash content of waste wood materials range from approximately 1 percent to 30 percent. This, in part, is due to the entrained dirt and other inert materials carried over during various collection and sizing operations. After combustion, yard waste and forest residuals tend to have high ash content while clean industrial waste has lower levels. However, homogeneous waste wood fuel that has been screened during processing typically has lower ash content than mixed waste. Table 2 presents data on flyash and combined ash (fly and bottom) from several types of waste fuel streams. It should be noted that the metal concentrations in flyash and combined ash from clean wood is lower than the concentrations of these elements in C/D and creosote-treated woods.
While metals are present in all wood fuel sources, the treated waste woods generally have higher metals concentrations than the clean and virgin waste wood streams. The metals come from the various chemicals and other contaminants contained within these wood fractions such as paints, glues, binders, etc. Table 2 also shows that most of the metals stay in the bottom ash fraction and very little remain contained in the flyash. But, because lead and zinc have a relatively low volatilization temperature and vapor pressure, treated waste wood streams have approximately 3 to 10 times higher concentrations of these metals in the fly ash fraction.
Pilot scale tests
In order to evaluate the emissions from burning various waste wood fuel types, Environmental Risk Ltd. (ERL) built a small pilot test combustion system. ERL has test-burned various wood waste fractions in this unit and samples of the flyash and bottom ash were collected and analyzed to predict metals emissions from full-scale facilities. These predictions, in turn, have been used for the permitting of a number of full-scale facilities. Several of these units are currently operating and are safely meeting their permit conditions.
Other pilot-scale tests conducted by ERL have clearly demonstrated that plywood, resinous particle board (binder impregnated), C/D and municipal sludge could be burned within a wood-fired boiler with low organics and particulate emissions. The resulting emissions from these tests were comparable to burning whole tree wood chips.
Full-scale test data
Several full-scale test burns recently completed by ERL and others on 20-MW to 50-MW wood-fired boilers demonstrated that treated or contaminated wood wastes could be burned in a well-operated boiler at lower pollution levels and with similar environmental impacts to that of whole tree chips. Also, tests at operating full-scale facilities burning various types of C/D waste wood indicated that metal and organic emissions are at relatively low levels and well below EPA and state air toxic requirements.
So, even though treated wood has higher contaminants levels the air emissions from its combustion is similar to, and in many cases lower than emissions from, burning clean wood fuels. This can be attributed to several factors. First, treated-wood combustion units are generally newer and equipped with higher efficiency particulate collection systems. Thus, these facilities are able to keep metal emissions very low, comparable to units permitted to burn clean wood.
Second, treated wood combustors burn a higher heat content fuel than clean wood combustors
and that results in lower levels of organic emissions. Table 3 compares the typical air emissions in pounds per million British thermal units (lb/mmBtu) for several waste wood combustion facilities.
Because some states have very stringent permitting policies in place, most treated wood fuel types are not allowed to be burned as boiler fuel. Current practices in these states are resulting in unnecessary permitting difficulties (cost and time) and are drastically curtailing the use of this valuable resource. So, in spite of the data confirming the benefits of treated waste wood fuels, why are potential plant operators having difficulty in getting the permits to burn this type of wood waste?
For one reason, some state?s environmental agencies narrowly define what is, and what is not, acceptable waste wood fuel based on little more than the notion that because treated wood waste has organic chemicals and metals in it, burning it will release harmful elements at levels greater than other more familiar fossil fuels. These states tend to exclude painted wood, particle board or plywood, wood that has been coated, treated or contaminated with oil, finishes, glue, chemicals, etc., and wood that is chemically treated with preservatives. In many parts of the country the only alternative for disposing of treated wood, besides the landfill, is burning it in units permitted as incinerators.
Power plants proposing to burn any of these treated types of waste wood must demonstrate on a case-by-case basis that no adverse pollutant problems will occur and they must overcome all the negative public perceptions allowing an incinerator into the neighborhood implies. It is little wonder investors and utilities are less than enthusiastic about entering into the permitting process for these kinds of plants.
Furthermore, to eliminate the treated or contaminated wood from most waste wood streams involves expensive hand separation and processing. This makes it a much more costly proposition and still would not provide any significant environmental benefits. Unfortunately, there have been many suitable projects proposed but ultimately the developers did not have the financial and technical resources needed to comply with incinerator permitting requirements.
To overcome these hurdles, a list of waste wood types that state regulatory agencies and regional Environmental Protection Agencys (EPA) will readily accept as fuel (with proper emission controls) and without requiring specialized testing programs needs to be developed. Similarly, a database needs to be established to help potential biomass power generators evaluate the feasibility of burning a wide range of wood types and understand their effects on the environment.
There needs to be a rapid and inexpensive way of determining the approximate level of contaminants and its effects on a given facility?s emissions. Acceptable wood contamination levels that do not have a negative environmental effect need to be established and verified by a certified test program. One of the major issues raised during the waste wood facility permitting process is: How does a state agency regulate the quality of waste wood received at the burning facility? One approach is to routinely take wood samples, have them analyzed and then, if the sample meets the required criteria, burn the material which was initially sampled. On a day-to-day operational level this monitoring approach is not considered practical because it costs too much and takes too long.
Another major permitting issue is how clean is Oclean.O Some permits state that no treated wood will be burned at the permitted facility. This type of statement indicates that some regulators have a general lack of understanding of what waste wood is and how it is processed into a suitable fuel. Most waste wood streams are far from being homogenous and any unwanted materials must be sorted out by hand. Obviously, these workers, no matter how hard they try, would not be 100 percent reliable, and some unwanted material will inevitably be missed and passed on to the boiler.
A sampling and inspection method that may offer a solution was developed by ERL. Consisting of an inspection
grid with a square metal frame subdivided by wires, the device is used to visually assess the wood within each subdivision. Samples representative of the wood being delivered to the site, pulled from the wood pile or from the waste wood preparation facility, are viewed through the test screen. The viewer records the fraction of each box containing unacceptable wood. This surface area determination method is a reproducible, low-cost method for rapidly identifying and quantifying waste wood quality.
Finally, there needs to be a readily available database of emissions and emission factors for state agencies to use so they can evaluate wood combustors and related equipment. Regulators unfamiliar with waste wood combustion consistently raise concerns regarding the emissions of metals and products of incomplete combustion.
Tests performed by ERL and others have demonstrated that with high efficiency combustion and particulate collection systems, particulate and gaseous emissions do not represent a health problem and are well below the allowable air toxic emissions permitted by EPA and all state emissions-regulating agencies.
Editors note: For more information, please contact the author at Environmental Risk Ltd., 120 Mountain Avenue, Bloosfield, Conn., 06002, call (203) 242-993 or Fax (203) 243-9055. END
Richard S. Atkins is president of ERL and has a doctorate in chemical engineering from New York University and a bachelor?s degree in chemical engineering from the University of Rochester. Under his direction, ERL is actively permitting and managing environmental compliance work on waste wood to energy projects throughout the United States and South America. During the past nine years, ERL has handled the environmental permitting and compliance activities for more than 40 waste-to-energy facilities.
Construction/demolition wood offers power generators high heat content and low moisture for efficient combustion, but has been shunned by some regulators because it tends to be contaminated with too many additives.
Even though railroad ties and telephone poles are usually treated with creosote or similar preservatives, emissions are manageable with modern controls.
Whole wood chips have relatively low emissions compared to treated wood waste, but have poor combustion characteristics because of their high moisture content.