How Energy Efficient is Your Drying Operation?
By Peter Garrahan FPInnovations
Almost four years ago we ran an article in this series on energy considerations in lumber drying (see Energy Matters in Drying, September/October, 2006). Much of the material in that article is still relevant but, given that drying is such an energy intensive operation, it is worth devoting some further attention to this subject.
By Peter Garrahan FPInnovations
In 2009 the Athena Institute (www.athenasmi.org) and FPInnovations completed a report on “Status of Energy Use in the Canadian Wood Products Sector”. This report was prepared for the Canadian Industry Program for Energy Conservation (CIPEC) of Natural Resources Canada. The report details energy usage patterns for all of the major segments of the solid-wood products industries in Canada including softwood lumber.
The CIPEC survey and study re-confirms that drying is the major energy consuming operation in the manufacture of lumber. Two-thirds of the total energy consumed in converting logs to dry-dressed lumber is consumed in the drying operations. To quantify this, the report shows that, on average, softwood lumber production consumes 1,514 Megajoules of energy per cubic metre of lumber produced (MJ/m3). Of this total, drying consumes 1,000 MJ/m3. These numbers are the total of all forms of energy including biomass, fossil fuels and electricity.
Another positive note highlighted in the CIPEC report is that over 50% of the industry’s current energy use is derived from renewable biomass fuel. This is an increase of about 10% over the previous 10 years and this has been associated with a 10% decrease in fossil fuel consumption (mostly natural gas). In addition, our industry has become more energy efficient, with the net energy used per unit of production decreasing by about 10% over the past 10 years.
This report also goes on to show why wood is such a good building material to use from an environmental standpoint. The net carbon balance in producing softwood lumber shows that by using wood we sequester far more carbon than is emitted to the environment due to the manufacturing process.
Despite all these positive notes, we can make our manufacturing process even more energy efficient and lumber drying is the logical place to look for significant gains.
There are many areas in the drying process that will have an impact on energy consumption. Rather than trying to list all of them, I have chosen what I believe are the top five in terms of their impact on total energy consumption and/or energy costs.
Heat Exchangers – Heat exchangers have been around for many years. When drying softwoods, 60% to 70% of the energy consumed is used to evaporate moisture. If your kilns are in good condition, all of this evaporated moisture exits the kilns through the vents. Heat exchangers have consistently been shown to be able to preheat the outside air to within 10 to 20 F of the operating temperature of the kiln. This improves energy efficiency and helps achieve a more uniform temperature inside the kiln.
Fan Efficiency – Electrical energy consumed by the air circulation system may only represent 5% to 10% of the total energy used to dry lumber but, due to the higher cost of electricity, it can represent up to 20% of your energy costs. Our experience has shown that through a combination of improving the fan efficiency and implementing an adjustable speed drive, the electrical energy consumption can be reduced by 30% to 40%.
Uniformity of Drying Environment – The objective of every kiln operator should be to operate and maintain the equipment in a manner that ensures that every board is exposed to the same drying conditions. Although some variations are inevitable (for example those due to temperature drop across the load), most of the temperature variations we see in dry kilns are due to poor maintenance and/or poor design of the kiln. All of the major components need to be addressed including the systems for heating, venting, airflow, and humidification (if the kiln is equipped with one), as well as the tightness and insulation of the kiln structure.
Since the duration of the drying process is often determined by the slowest drying material, any variations in the process will needlessly extend the drying time.
Uniformity of Raw Material – For the same reasons as listed above, a uniform supply of lumber to the kilns will also help improve energy efficiency as well as kiln productivity. There are many simple things that can be done such as not mixing dimensions or material with vastly different log storage or air drying histories. There are more complex things that can be done such as pre-sorting by species, weight, or moisture content (MC).
Improved Process Control – One of the weak links in drying has always been the control of the drying process. Limited choices in tools that accurately monitor intermediate and final moisture content have made it difficult to apply the most appropriate drying conditions and to know when to terminate the drying process. This situation is changing with the advent of new in-kiln MC detection systems. Considering the implications of over-drying or under-drying lumber and the need to achieve the best productivity possible, every kiln should be equipped with these new technologies.
Installing New Equipment?
If you are looking at building a new drying facility, there are other decisions that can be made at the equipment specification stage that will have a major impact on energy consumption and/or energy costs associated with drying. Here is a short list of technologies that should at least be considered as part of your equipment review and specification process:
Kiln Selection – Some drying technologies are inherently more fuel efficient than others. Here are a few things to consider:
Dehumidification (DH) drying uses less energy (up to 50% less) than conventional, heat-and-vent kilns. Most of the energy used in this process, however, is electric and in most cases electricity is still more expensive than thermal energy. For smaller-scale operations and/or installations where the energy alternatives are expensive, this type of drying system makes sense. This is especially true if some of the thermal energy requirements for the system can be provided by a residue-burning system.
Direct-fired kilns use less energy because of fewer heat exchange operations. Compared to a steam-heated kiln there are no boiler efficiency and transmission losses to consider. The anticipated future cost of fuel (natural gas, bunker C, biomass, etc) has to be considered in making the final decision.
High-temperature kilns dry lumber in one half or less of the time required to dry in a conventional-temperature kiln. Although we are still using roughly the same amount of energy to dry the wood, we are doing it in a shorter time period, which means fewer heat losses. In addition, less venting is required when operating at higher temperatures due to the increased moisture holding capacity of the air. A study on southern pine showed a 13% reduction in energy when drying at high temperatures.
Energy System Choices – The obvious source of energy in our industry is biomass but the economics do not always make this choice the most economically viable. In the past, low fossil fuel prices and lower initial capital cost often made the choice of a biomass-fuelled system less attractive. Today, however, there is no expectation of long-term low prices for fossil fuels. In addition, there are many new technologies on the market that make burning biomass more attractive, including small-scale green and dry residue burning boiler systems that are readily available and operate more efficiently and cleanly than in the past.
For direct-fired kilns there are a number of new-generation green-residue burning systems available.
Gasification systems are available for larger operations that now make it possible to use lower-grade wood residue and still produce a clean fuel for use at the kilns.
Co-generation is worth considering. Producing steam for electricity production and a thermal heat source for the kilns results in a greater overall energy efficiency. In many jurisdictions it is now becoming more attractive to feed electricity into the grid and get a credit for the power produced.
Whether they intend to or not, most mills air dry lumber. Just a few days in the yard between the sawmill and kilns can significantly reduce the MC of the lumber. One FPInnovations study showed that during the spring and summer months, mills in Eastern Canada could expect to lose almost 1% MC per day when air drying balsam fir. For every 1% reduction in the initial MC, the energy load at the kiln will be reduced by approximately 40,000 BTUs. On a kiln of 200 Mbf capacity, this would be a reduction of 8 million BTUs for every 1% MC drop you achieve in the air drying yard. Just remember, if you are going to air dry, do it properly. Take the time to look up the proper practices and procedures to achieve a fast and uniform drying rate.
How do you Measure Up?
From a theoretical standpoint it takes roughly 1.6 kWh of energy for every kilogram of water removed in a conventional (heat-and-vent) kiln. This is taking into account energy system efficiencies and expected kiln losses. What it doesn’t take into account is the impact of the items listed previously in this article such as poor kiln maintenance and the impact of poor decisions before and during the drying process. A good energy monitoring program will help you determine if your kilns are operating at or close to the optimum level.
I heard a story a while back of a company that had energy bills of over $1 million dollars a month and had a manager that was assigned to monitoring cell phone use but no manager assigned to energy issues. Do you have someone in your organization looking at your energy use patterns and where opportunities exist for improvements? It’s a common belief that drying is an inevitable consequence of producing lumber and that there is not much than can be done to influence the outcome. That situation has changed drastically due to new technologies and changes to our economic structure. Can you afford to overlook the opportunities in this area?
Peter Garrahan is a drying expert at FPInnovations.