By Biomass Digest correspondent Tim Sklar
In 2007 the Congress passed legislation that mandated producing 16 billion gallons per year of 2nd generation biofuels from cellulosic biomass by Year 2022. Although many experts believe this “Renewable Fuels Standard” (RFS) could still be attained, progress toward that goal has bogged down. Processes under development for producing 2nd generation biofuels on a commercial scale have yet proved to be economic. In addition, 2nd generation biofuels processes are still evolving, adding to technology risk of those who have committed to projects using existing technologies. Further, potential investors in 2nd generation biofuels development projects are also being confused by claims being made for technological breakthroughs for producing biofuels from algae and other forms of bio materials, and in some instances, investors are delaying making additional investments in 2nd generation biofuels projects they had initially backed. And, funding for some of these “advanced” 3rd generation biofuels projects have had the effect of siphoned off venture capital that may have otherwise been used to support promising 2nd generation biofuels projects. When added to current overall shortages being experienced by project developers in obtaining investment capital, 2nd generation biofuels project development is now not keeping pace with increasing biorefinery capacity requirements that will be needed to attain the RPS mandated 2nd generation biofuels requirements by Year 2022.
Most of the reporting being done on recent biofuels developments is focused on liquid biofuels. Not being adequately covered is reporting on the potential that already exists for converting wood waste and other woody biomass into torrefied wood (“TW”) pellets that could then be used as a substitute for coal in coal fired boilers and gasifiers. This current article addresses this shortcoming. This article also highlight the opportunities that now exist for projects designed to produce TW for use as “clean coal”. And it also presents observations and conclusions reached, based on insights gained by the author over the last 18 months, through participation in the development of a torrefaction plant project.
The primary objective of this article is to stimulate interest for those who are involved in promoting the development of biofuels, to fully reconsider the opportunities that are believed to now exist for producing this form of “green” bio-energy.
Materials That Could Be Used in Making 2nd Generation Biofuels
By definition, 2nd Generation biofuel is a renewable form of fuel that is produced from cellulosic biomass. Cellulosic biomass includes woody biomass, cellulosic bio crops and other cellulosic wastes.
Woody biomass is the most abundant of these materials, and includes all forest products, such as harvested timber used as pulp wood or as raw material for finished lumber used in construction and in manufactured products, as well as forestry wastes.
Forestry wastes include forest residue, pre-commercial thinnings from tree harvesting operations, and in some instances, commercial thinnings. As subsequently explained, these materials are ideally suited for conversion into 2nd generation biofuels.
Other wood wastes are most often, those wood wastes generated in saw mill and lumber mill operations. Wood waste from saw mills and lumber mills are often gathered and chipped and used as a supplementary boiler fuel. Included in this mix would be bark stripped from saw logs, un-merchantable trimmings, and sawdust. In certain instances, these materials can supplement forestry wastes as a feedstock for making biofuels.
Other cellulosic wastes include construction site lumber waste, and paper and paperboard recovered from landfills. Waste paper recovered from landfills is often used in manufacture of recycled paper and not as a biofuel. In certain instances, these materials can supplement forestry wastes as a feedstock for making biofuels.
Cellulosic Bio Crops include plants such as switch grass, miscanthus and jathropa that are specifically grown for conversion into ethanol and other biofuels. They do not have a dual use as food or animal feed, such as corn, sorghum, wheat, hay, and sugar cane. Nor are they a by-product of a food crop, such as corn stover, sorghum, wheat straw, and bagasse. Cellulosic bio crops are often grown on idle acreage or as rotational crops. But as with all crops, they require water and fertilizer. They also cannot be stored indefinitely and have to be pre-processed to facilitate transport to biorefineries. Bio crops have not achieved their full biofuel feedstock potentials, as a market for these crops has yet to develop to support their cultivation.
Materials Most Suitable for Use In Manufacture of Biofuel
Woody biomass is ideally suited not only to the manufacture of 2nd generation liquid biofuels. Woody biomass is even better suited to producing TW. When woody biomass undergoes torrefaction all of the components are used, without having to separate out each component for separate treatment, prior to producing biofuels. In other words, cellulose, hemi-cellulose and lignin, the components of woody biomass are processed in chip form to produce synthetic gas and carbonaceous residuals that end up as TW.
Forestry waste is the form of woody biomass that is most suited to making biofuels, especially TW, as much of this waste is either not used or at best, is used as a marginal fuel. Forestry wastes can be supplemented by other wood wastes when they can be obtained on a reliable basis at competitive prices. The following is a more precise description of the forestry wastes that are most suitable for this use.
Forest residue is the unused portion of growing-stock trees cut or killed by logging and left in the woods. In certain instances, forest residue is cleaned, chipped and used as a boiler fuel. But for the most part, forest residue goes unused.
Pre-commercial thinnings (a.k.a., “first thinnings”) are un-merchantable timber that consists of smaller and less desirable trees that are removed from timberlands in order to enhance residual tree growth. These first thinnings have stems that are less than 5” in diameter at breast height (“dbh”) and are currently cleaned chipped and used as an inexpensive boiler fuel.
Commercial thinnings have stems that are 5’’ to 8.9” in dbh are primarily used in making pulpwood used by pulp and paper mills. Commercial thinnings are often not available or command too high a price for use as a biofuels feedstock.
Forestry Waste Availability
In the US, forestry waste is the most readily available and abundant material that can be used to produce biofuels. Tour country has significant forests in a variety of regions and there are millions of acres of timberland that are already being harvested. Estimates provided by a number of nationwide studies indicate that in addition to timber being harvested, the total nationwide forestry waste potential is in excess of 317 million green tons per year (gtpy). It has also been estimated that on average, the annual yield of timber is 6 green tons per acre and the annual yield of forest residue and pre-commercial thinnings estimated at 1.2 green tons per acre.
As part of a pre-feasibility study undertaken for a torrefaction plant to be located in South Carolina, an assessment was made of the availability of forest waste that was strategic to the proposed plant location. Data obtained from recent studies suggest that there are in excess of 12 million acres being harvested in South Carolina and the amount of forest residue and pre-commercial thinnings is estimated to exceed 12.9 million gtpy. A more detailed county-by-county survey was obtained for sixteen counties that comprise the “Northern Coastal Plain” region of South Carolina, the harvestable timberland region in which the planned torrefaction plant project is to be located. This survey indicated that 4.6 million acres of timberland are being harvested in this region and this regional “wood basket” now produces in excess of 40.5 million gtpy of timber and has the potential for producing ~1.6 million gtpy of forest residue and 3.4 million gtpy of pre-commercial thinnings. This is 20 times the amount needed to operate one torrefaction plant with an annual input requirement of 250,000 gtpy.
Converting Wood Waste to Biofuels
Forestry waste can be converted into a number of biofuels in a variety of forms. Forestry waste can be chipped and cleaned and dried. Chips can be compressed into pellets or briquettes. Wood waste can undergo fast pyrolysis to produce charcoal. Wood waste can also undergo torrefication and TW pellets can then be produced to facilitate handling, storage and transport. And wood waste can also be preprocessed for use in biorefineries capable of producing a range of 2nd generation liquid fuels.
Cost and Value Added Attributes of Various Cellulosic Biofuels
At the high end of the cellulosic biofuels spectrum are liquid transportation fuel blend stocks, such as cellulosic ethanol, butanol, and methanol, tank ready transportation fuels such as biodiesel and JP-8 jet fuel, DME, a gasoline substitutes and B5 “bioheat”, a clean distillate blend for home heating. All of these liquid fuels command high prices, but the processes used to produce them require substantial capital investments and many of the processes that can be used are still in the development stage and have yet to be proven on a commercial scale.
At the low end of the cellulosic biofuels spectrum are wood chips and charcoal. These biofuels have relatively low BTU value and have limited use as a supplemental low-cost boiler fuel, replacing coal or natural gas. But their use often requires modifications to material handling systems, furnaces, ash collection systems and emissions clean-up systems.
In the mid-range of the cellulosic biofuels spectrum is TW, that has a BTU value comparable to coal and can be burned in coal fired furnaces without modification. It is believed that TW is the cellulosic biofuel that offers the most immediate and significant opportunities.
Justification for Pursuing Wood Waste Torrefaction
There are three compelling reasons for undertaking torrefaction of wood waste at this point in time.
- First, the cost of producing TW is affordable.
- Second, TW can be readily sold at relatively high prices.
- Third, the cost of building a commercial scale torrefaction plant is only a fraction of the investment needed for a comparably sized biorefinery.
What makes the conversion of wood waste into TW even more attractive at this point in time is that torrefaction technology has advanced far enough to be considered commercially viable, thereby making project risks manageable. This is supported by the fact that a number of torrefaction pilot plants are successfully operating and at least three commercial scale torrefaction plants are currently being built.
Torrefaction Process Defined
Torrefacation can generally be defined as a process that uses “mild pyrolysis” to separate water, VOCs and hemicellulose from the cellulose and lignin contained in woody biomass. The VOCs and hemicellulose fractions are combusted to generate process heat, leaving only the cellulose and lignin to produce TW, a charcoal like solid. And depending on the process time, the TW yield is quite high. varying between 66% and 75%.
The mild pyrolysis process is lucidly described in an article written by Robert Flanagan, titled “Torrefied Wood vs. Charcoal”. As Flanagan explained, in mild pyrolysis, green woody biomass with 50% moisture is subjected to temperatures in the 250oC to 300oC range in a closed torrefaction unit in which little or no oxygen is added. And depending on the process dwell time (i.e., residence time), the woody biomass is reduced to a char, with only 25% to 33% of the amount of input material used, being driven off as a gas.
The torrefaction units that are available differ primarily in how the woody biomass is fed into the torrefaction unit, how it moves through the unit and how ash is removed. All units need a backup heat source such as a natural gas burner to ignite the process and help control the process temperatures, but process heat is primarily provided by the synthetic gases produced from the woody biomass being torrefied. Although torrefaction is a sophisticated process it is not nearly as complex or as costly as processes used in biorefining.
Torrefied Wood Pellets vs. Wood Chips and Coal
Green Wood Chips
The advantages of using forest residue and thinnings in chipped form as an energy product are numerous. For instance: they are carbon neutral; and they are relatively low in VOCs, having a low sulfur and mercury content. But green wood chips have many disadvantages. For instance: they are bulky; they have a low BTU content; they are high in moisture content; they are perishable; they often are commingled with dirt and other debris; and they are more costly to transport. And if used as a fuel, green wood chips often causes glazing of boilers, particularly if soil and other contaminants are not removed.
Dried Wood Chips
To overcome some of these deficiencies, the most widely available best practice is to use air drying to reduce the moisture content of green wood chips from ~50% to ~35%. And where stack gases can economically be used in pre-treatment, moisture content can be economically reduced further to ~20%.
Torrefied Wood vs. Wood Chips
Torrefacation substantially reduces the moisture content of the wood chips used and increases the BTU value per ton. TW is dense, dry, water resistant, non-perishable, easily crushed and energy dense, reducing transportation cost per BTU. TW’s increased density and lower moisture content allows it to be stored for long periods. And if the TW is pelletized, its density is doubled, further reducing the cost of transport by ~ 1/3 of the cost/ton mile associated with transporting green chips.
Torrefied Wood Pellets vs. Coal
When TW pellets are used in lieu of coal as a boiler fuel, significantly less ash is produced, and sulfur emissions are low. Further, TW’s energy content as measured in BTUs/lb, is comparable to coal. If used as a gasifier feed, TW pellets can be ground to a particle size similar to pulverized coal and can be fed into gasifiers designed for coal gasification without further modification.
Metrics and Costs of Torrefaction of Wood Wastes
Torrefaction plants capable of processing between 1,500 gtpd and 2,000 gtpd day of wood waste or 750 tpd to 1,000 tpd of dry chips are estimated to cost between $10 million and $16 million, depending on the technology used. This compares favorably to the $180 million to $250 million a liquid biofuels refinery would cost that is capable of processing the same amount of wood waste.
Metrics and unit cost estimates being presented in this article were developed from data provided by several torrefaction technology providers. The following are some of the key estimates that had been derived:
- The costs of acquiring and harvesting and chipping forest residue and pre-commercial thinnings from softwood timberland acreage in South Carolina are expected to range between $22.50/gt and $26/gt and average ~$24.25/gt.
- The added cost to transport, clean and dry these green wood chips is expected to increase their value to ~$55/dt.
- Torrefaction of dry chips is expected to results in a 20% loss of weight thereby increasing the cost of wood waste used to ~$69/ short ton (st).
- The cost of torrefying woody biomass is expected to add ~$5/st.
- The BTU content of TW is estimated to be 11,000/st compared to coal that has ~12,000 BTUs/st.
- TW is expected to cost ~$80.27, after adjusting to reflect these BTU differentials.
This cost compares favorably to the ~$100/st price of coal now being delivered to industrial plants in South Carolina.
The prices that can now be obtained for TW exported to coal users in countries in Europe in which a carbon tax is imposed is much higher and it is expected that TW can command an average price of ~$262/mt. Because the cost to deliver TW to EU ports is estimated to be ~$80/st, the resulting delivered cost of TW produced in the US is only ~$148/st. This could result in US TW producers earning an average gross margin of ~43% and a potential Return on Investment of ~39%.
Domestic Market Opportunities for TW
TW Sold into Local and Regional Markets as Clean Coal
TW produced in South Carolina is easily transported at reasonable cost to coal users throughout the US east coast. TW can be used in industrial boilers without any modification or added investment. TW is clean as VOCs are removed as a result of torrefaction. Further TW is considered as a form of renewable energy and will not be subject to carbon taxes on fossil fuels if and when they are imposed by Federal, State and Local governments.
TW as a Replacement for Natural Gas Used in Power Generation
TW makes an ideal fuel for producing synthetic gas that can replace natural gas when generating power. This opens up an important market among utilities and industrial users who generate electric power from gas turbines or from industrial users that buy electric power from the grid. As previously explained, the TW process produces synthetic gas that is used to provide heat to the torrefaction unit. Additional clean syngas can be produced by gasification of some of the TW produced. This syngas could then be used to power gas turbines in lieu of natural gas they now are designed to use.
Renewable Energy Plant Grant Opportunities
Section 1603 of the American Recovery and Reinvestment Act of 2009 (“ARRA”) offers developers of U.S. renewable energy plants the opportunity to receive U.S. Treasury grants for up to 30% of the cost of “specific energy property” used in these plants. To qualify as a specific energy property the plant must use open-loop biomass, (such as wood waste) to produce electricity, and the project development must have commenced prior to the end of year 2010.
It appears that a renewable energy plant could consist of a torrefaction plant with TW gasifier capability and a gas turbine power plant that is part of one project. But in order to qualify for this grant, the torrefaction plant must make its TW out of wood waste.
Based on recent estimates, it is expected that the equipment used in the torrefaction plant would costs ~$14 million. It was also estimated that a GE Frame-7 combined cycle gas turbine would be needed to use all of the TW produced and generate ~600,000 mWh per year of electricity, and that its cost would be ~$13 million. If the Section 1603 grant was awarded this project, 30% of the $27 million in hard costs, or ~$8.1 million, could be obtained as a grant.
Preliminary calculations were then made of the potential annual savings that could be obtained from cogeneration if TW derived synthetic gas were used. These savings were estimated to be ~$5.2 million per year. Further calculations indicated that if a Section 1603 grant was obtained for the GE Frame-7 combined cycle gas turbine , its net cost of a would only be ~$9.1 million, and the payback from power savings would be less than 2 years.
And under the assumption that the industrial user of the power being generated cannot always use all this power, the excess power could be sold back to the grid, thereby assuring the industrial user that most of the estimated cogeneration savings could still be realized.
This grant program should provide incentives for companies that purchase sizeable amounts of electric power in their operations, to consider participating in a renewable energy plant project with firms capable of producing TW from wood waste.
When compared to cellulosic bio crops, harvested timber, saw mill wastes, construction lumber wastes and waste paper, forestry wastes are currently, the most readily available source of cellulosic biomass for production of 2nd generation biofuels on a continuous basis.
Of the three primary sources of forestry waste, forest residue is the easiest to obtain, as it does not have viable alternative uses as do pre-commercial thinnings or commercial thinnings.
Forestry wastes are in sufficient supply in many regions of the US and if torrefaction plants are located in these regions, these plants could be supplied on a continuous basis at reasonable cost.
When processed into TW, forestry wastes and other woody biomass need not undergo separate treatment for its various components in order to obtain high yields. In comparison, when make liquid 2nd generation liquid biofuels from woody biomass separate treatments are required for cellulose, hemi-cellulose and lignin components. This affects the complexity, yields, and costs of biofuels produced.
Torrefaction is a proven technology that is on the cusp of being scaled up to commercially viable levels. And torrefaction is significantly less capital intensive as biorefining and the costs of producing TW on a commercial scale is relatively low.
TW can be produced at a price that allows it to compete with coal and it can be used in lieu of coal or in combination with coal in boiler and power applications.
In the countries within the EU, where significant carbon taxes are in place, a pent-up demand for TW exists and on net-back basis, prices TW commands in the EU are significantly higher than prices now being paid for coal in the US by industrial users.
The opportunities for marketing TW in the US are fast improving, as the demand for a clean green fuels is increasing, due in part on more stringent environmental regulation and in part on increasing prospects that fossil fuel users will ultimately have to pay carbon taxes.
Torrefaction plant construction is being encouraged by currently available government backed loans and loan guarantees, tax incentives and grants directed specifically at renewable energy projects.
In short, TW is the bio-energy option that is ready to go.
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