Boilers: Economic Change From Coal to Biomass
Clearly there are significant economic, environmental and political reasons for changing boiler fuel from fossil fuels to renewable biomass materials. Up to now the conversion of coal- or oil-fired boilers to biomass has been a costly proposition, with the boiler needing to be significantly modified or even replaced to allow the burning of biomass materials. Hundreds of millions of dollars have been spent in the past few years in the conversion from coal or oil to one or another biomass material. Many boilers have been modified to burn natural gas, but this is another fossil fuel with finite reserves.
Development in the past few years has produced technology that could completely change the situation. One of the technologies is the combined use of two conventional processes, which have been used for many years and are currently in use in other industries. Biomass can now be converted to a material that can be fed to the boiler in place of the coal with no or minor modifications to the boiler. The two processes are torrefaction and pelletization.
Another technology, equally known and proven, and showing equal or greater promise, is gasification. The gas produced can replace fossil fuels whether it is coal, oil or natural gas. Both of these technologies will enable “green” credits to be obtained, which will enhance the economics of using them.
Torrefaction and Pelletization
Most of us are familiar with torrefaction as the process that roasts the coffee beans to give the product we grind to make our coffee. In the process, volatile material is driven off and the remaining solid matter of the bean, which is biomass that is largely carbohydrate, is roasted to improve the flavor and make the beans ready for grinding. The same process applied under carefully controlled conditions to biomass, such as wood waste from the forestry industry, reduces the total dry weight of the biomass by some 30 percent while retaining about 90 percent of the thermal capacity. In effect, the process increases the energy density of the biomass by 20 to 30 percent depending on the feedstock and process conditions. The resulting energy density is similar to that of sub-bituminous coal and about twice that of pelletized sawdust.
The torrefied biomass, which results from the process, can now be ground and pelletized also by a conventional and well-known process often used to produce animal feed as well as fuel for residential stoves. The final product is easy to handle and transport. The bulk material receiving and handling systems as well as the feed equipment of most coal-fired boilers will handle it without modification, either mixed with coal or alone.
The torrefied pellets are delivered to the power plant using the same equipment that is used to handle coal. The pellets can be stored with the coal because torrefaction changes the biomass to a hydrophobic material, so the pellets will not absorb water in storage. The transport cost per unit of energy produced is much lower for torrefied and pelletized biomass than for bulk unprocessed biomass, due to the energy density and physical compaction.
The pellets can be mixed with the coal before pulverizing for a pulverized coal boiler, or fired mixed, or separately in other boiler types. Emission controls will need to be checked, but current controls in place would normally be adequate for firing torrefied material.
The greatest barrier to conversion to torrefied/pelletized biomass is availability of sufficient product. Currently, there are no operating plants in the United States, although there are demonstration plants running in the Netherlands and Sweden. Considerable work in developing the technology and equipment has been done in these countries as well as in Ukraine and Australia. Various machine manufacturers now have integrated torrefaction units and pelletizers have been available in a wide range of production rates for decades.
So far there is only one commercial torrefaction plant operating in the world, located in the Netherlands. It supplies torrefied biomass pellets to large coal-fired power plants, which receive a green credit for each ton of biomass burned. Integro Earthfuels and Agri-Tech among others that are now planning similar plants in the United States.
Torrefaction and pelletization of biomass provide a viable and economic alternative to the modification of coal-fired boilers. Considerable savings in the cost of conversion in the United States can be realized once commercial plants are installed and running. Capital spent in installing plants using torrefaction and pelletization will be much less than that necessary for the sum of the costs of conversion of each individual boiler.
Another technology that shows obvious promise in the conversion of coal, oil or natural gas-fired boilers to alternate renewable fuels is gasification. Again, this is not new technology, having been used extensively in Germany during WWII to substitute coal and biomass for scarce liquid fuels.
In the recent past, gasification has gotten a bad name for poor reliability andoperational sensitivity, but new technology has resulted in gasifiers that are both simple and robust.
One equipment supplier has developed a family of gasifiers that can process 150 to 600 bone dry tons per day per unit. These units have proven to be extremely flexible and reliable on biomass. Even more interesting is that they work equally well on refuse-derived fuel (RDF). During a trial burn, the fuel—landfill material (garbage, trash, plastic and paper) with no prior processing—was gasified and the gas burned without any problems.
The conversion from coal to syngas (synthesis gas) is simple. The required number of gasifiers are installed, normally outside. Each unit has a loading/metering system feeding the gasifier. The gasifier itself is a skid-mounted, pre-erected unit requiring only a foundation, electric power and igniter fuel. The discharge from the gasifier(s) is piped into the furnace. For burning in a boiler, gas cleaning is not required. Ash is minimal out of the boiler. Most of the ash is discharged at the exit of the gasifier and consists of inorganic material and ungasified biomass, which has proven to be a valuable byproduct as a soil conditioner/fertilizer.
Emission controls for burning syngas normally consists of a wet electrostatic precipitator. The production unit for the syngas should quickly pay for itself by avoiding continuously purchasing coal, oil or natural gas, even when using relatively pure biomass as raw material. If RDF is used the payback will be much faster.
These two processes, whereas not fully proven commercially in the U.S., have reached a level of development, which enables a confident prediction that they will be able to significantly reduce the investment needed to convert boilers from fossil fuels to biomass, especially when factoring in the effect of green credits.
Where biomass is readily available, torrefaction/pelletization can provide a viable and economic means of converting boilers to biomass fuel. Gasification, especially of RDF, can do likewise in areas where biomass is less available.
“A review of torrefaction for bioenergy feedstock production,” Ciolkosz and Wallace, October 2010.
“A Study of Torrefaction for the Production of High Quality Wood Pellets,” Jianghong Peng August 2007.
"Torrefaction for biomass upgrading," Bergman and Kiel. Available at: www.ecn.nl/biomass
“Biomass Torrefaction as a Preprocessing Step for Thermal Conversion,” August 2009, © EverGreen Renewable LLC Handbook Biomass Gasification, edited by H.A.M. Knoef and published by the Biomass Technology Group, The Netherlands in 2005
Authors: James L. Wise
P.E., Process Engineer, Baisch Engineering Inc.
Executive Consultant, Baisch Engineering Inc.