Next generation biomass fuel from forest to microchip
Biomass fuel offers a cleaner, renewable and carbon neutral way to produce energy, and wood has become a major source of this fuel for utilities and residential use.
Author: Thomas F. McGowan
Biomass fuel offers a cleaner, renewable and carbon neutral way to produce energy, and wood has become a major source of this fuel for utilities and residential use. As both feed to pellet mills, and competitor to pellets, microchips have emerged, due to a new generation of in-forest chippers. The microchippers produce a nominal 1/4″ long by 1/16″ thick chip (6 mm long×1.5 mm thick). When dried, microchip fuel has all of the positive properties desired for combustion: low moisture, good handling, near zero dust, little breakage and dusting when handled, and based on preliminary tests, no detectable CO generation in storage. In addition, cost including shipping is about 20% less than wood pellets. These savings are due to cutting about half the capital cost in production, and about half the electric power required. This study covers the subject of upgraded biomass fuels, from the raw resource through size reduction (e.g. chips and microchips), drying, densification and torrefaction. Practical information is offered including diagrams and photographs of actual equipment and installations to provide the reader with an understanding of the issues. Emphasis is on fuel preparation and transport for direct combustion of wood and biomass, and costs are provided for conventional and alternate fuels.
While pulp and paper mill boilers – and independent power producers in forested areas – use ‘hog fuel’, a rough mixture of bark and chips at about 50% moisture content, biomass can be upgrading by drying to 10% moisture for lower shipping cost and to enhance combustion. Some like to take it a step further via pelletisation to reduce transport volume. Others want to take it even further via torrefaction, to improve grindability and render it hydrophobic.
The North American biomass fuel export market is dominated by wood pellets, approaching $1 billion at delivered prices in Europe. The United States (US) and Canada now ship more than 6.4 million tons (5.8 million metric tons) of wood pellets to Europe, valued at some $800 million/year at $125/ton fob export price (€125/metric ton) [1,2]. Total worldwide pellet production is in the range of 30 million tons per year . Europe's 2014 demand was 81% of the global wood pellet supply .
There is an old saying in fuel preparation – ‘do only as much as the application requires, and no more’. So, why make the effort to dry wood? To reduce it in size? To make a wood pellet? To torrify it? Moreover, all this – just to burn it? The reasons are that upgraded fuels are a more standard commodity, have low and dependable moisture content, meet export standards for vector control, better combustion characteristics, have good flow properties, some are higher in bulk density and all have high energy density than raw wet wood. The European Union (EU) new target is 27% renewable energy by the year 2030, a major market driver for pellets. Worldwide pellet production is estimated at close to 30 million tons per year . However, pellets are costly, prone to dusting and crushing, turn to mush if they get wet, and also generate carbon monoxide (CO) while in storage.
Lower-cost, greener fuels are available with smaller carbon footprints. One that has potential is dry microchip fuel. This is created by a new generation of in-forest chippers which produce a nominal ¼″ long by 1/16″ (6 mm × 1.5 mm) thick chip. In addition, cost including shipping is about 20% less than wood pellets. These savings are due to cutting about half the capital cost in production, and about half the electric power required. Microchips could reduce the export fuel cost by $100 million (€90 million) per year.
New generation of chippers
Historically, in-field disk and drum chippers produced a typical 2″ × 2″ × 1/4″ (50 mm × 50 mm × 6 mm) chip for pulp wood or hog fuel use. This is the predominate size, but there was a range of over and undersize too. The new generation of microchippers is finding use for the pulp and paper market and wood fuel market. An example of diesel powered drum microchipper is shown in Fig 1. Examples of these machines can be found on manufacturer's websites. Microchipper manufacturers include Peterson [5,6], Vermeer , Morbark  in the US and LHM in Finland .
Fig 1: Diesel powered whole tree drum type microchipper (courtesy Peterson)
For fuel and pulp markets, the chips are blown into a 20–23 ton (18–21 metric ton) chip van, which then goes direct to the end user or to a rail head if rail transport is employed for longer hauls. In northern Europe, the maximum gross truck weight limit is 76 metric tons (84 short tons)
Comparing pellets and dry microchip fuel
Engineered Fuels [10,11] are those that undergo one or more beneficiation steps prior to being burned. For biomass, this would start with the base fuel of stem wood, tree trimmings, logging residue, windfalls, or mill residue. Beneficiation steps include size reduction, drying, pelleting, briquetting and torrefaction. Each step has its own costs and benefits.
This paper covers a wide range of steps, but concentrates on dry microchip fuel and its comparison with pellets. Microchip fuel is a new biomass product aimed primarily at the industrial export market, and secondarily at the national and international market for residential and commercial wood heating. The term dry microchip (DMC) Fuel™ has been trademarked by TMTS Associates, Inc., an engineering consulting firm focusing on biomass projects. TMTS has also tested DMC Fuel™ properties, developed flowsheets and quantified cost reductions to help promote its move into the marketplace.
Fig 2 shows the traditional hog fuel produced by whole tree chippers and also by coarse-grind hammermills (also known as hammer hogs). As shown in the photograph of weathered hog fuel, it is relatively course, with top size of about 2″ (5 cm).
Fig 2: Hog fuel
Fig 3 shows both microchip fuel (at left) and wood pellets (at right). Fuel wood pellets are about 3/8″ (10 mm) diameter × 5/8″ (16 mm) long. Dry microchip fuel's smaller size makes drying much easier and faster (as compared with the old standard 2″ × 2″ × 1/4″ whole tree chips or 2″ hog fuel), getting the job done with shorter dryer residence time and smaller dryer shells. Combustion (including drying, devolatilisation/pyrolysis, carbonisation, and carbon burnout steps) is much more rapid for dry microchip fuel, due to the thinner 1/16″ against 3/8″ (1.5 mm × 16 mm) (one sixth the thickness) minimum dimension, and for some combustors, a final grind before the fuel entering the furnace may not be required for dry microchip fuel.
|PELLET||DRY MICROCHIP FUEL|
|well-established commodity||lower capital costs|
|high bulk density||lower power, labour, and maintenance cost|
|low-moisture content||low-moisture content|
|–||lower-cost per ton|
|–||greener fuel: less power per ton used in manufacture|
|–||no pellet mill or pellet cooler|
|smaller dryer shell|
|dusting and crushing in transport and handling||new product|
|loss of integrity on wetting; protection from weather required, silo storage||lower bulk density|
|need for hammer milling/grinding to <1/16″ (1.5 mm) particle size before pelletising||higher ocean transport cost and fuel use|
|high energy and maintenance pelletising step, need pellet coolers||change in storage design to open roofed building instead of silos|
|high cost and power consumption in manufacture; high cost per ton|
|CO generated in storage should have been . CO levels have been found that are many times the National institute for Occupational Safety and Health (NIOSH) immediately dangerous to life and health level of 1200 ppm, with CO concentrations of 1460–14,600 ppm in cargo holds for ocean shipped pellets|
Table 1: Pellet and dry microchip comparison
The properties of microchip fuel vary little with type of wood, with the exception of slightly higher calorific value and volatile fraction for softwoods (conifers) wood as compared with hardwoods (broadleaf), as would also be the case with pellets based on the wood employed to make them.
Microchip fuel ash content will vary with feed stock, and, if sand and fines are excessive, screening to remove sand and fines can be done as part of the production process after drying. Any resultant fines can be used as dryer fuel. It is worth noting that ‘fines’ is a term with many meanings. It is defined as <3.15 mm (International Organization for Standardization (ISO) 17225-1:2014 table for wood chips and hog fuel or ISO 17225-4:2014 for graded wood chips), whereas others (ISO 14688-1:2002 for soils) show fine sand as 0.063–2.0 mm and the Casagrande/Unified Soil Classification System has the same at 0.075–0.425 mm. Regardless of the processing system, once dried when cohesiveness of sand and wood is reduced, sand can be removed from larger wood particles via shaker screens, trommels, and other separators.
Pellet bulk density is in the range of 45 lb/cu ft (720 kg/m 3), whereas microchip fuel is about 15 lb/cu ft (240 kg/m 3). Cost for truck shipping differs little in practise between the two fuels, as both can make a full truckload weight, and a standard 48′ (15 m) chip van can transport a full truckload (20 tons plus/18 metric tons) of microchip fuel.
Overseas shipment is by major carriers (e.g. NYK Bulk, MOL and K line) with chips shipped in 3.6 million cu ft (100,000 m 3) capacity ships, and pellets in dry bulk 1.8 million cu ft (50,000 m 3) capacity ships. Typical receiving ports are Rotterdam, Antwerp, Liverpool, and Stockholm. Two major US East coast export ports are Savannah and Brunswick, GA.
An additional advantage of microchip fuel is the potential outsourcing of the drying step to existing tolling facilities to enable low risk, fast start-up of production and bypass the need to expend capital and time on equipment procurement, permitting and construction.
Drying is traditionally done in a direct fired rotary dryer followed by a cyclone and baghouse (Fig 4). High excess air or recycled flue gas is required to reduce hot gas temperature into the dryer and reduce the tendency to ignite embers that can burn pinholes in bag filters and start baghouse fires. When drying pine chips, an regenerative thermal oxidiser or other oxidiser maybe required to control volatile organic compounds (VOCs) and blue haze. Dryer fuels maybe natural gas, fuel oil or dry wood. Dry wood is usually burned in vertical cyclonic burners such as that shown in Fig 5.
Fig 4: Direct fired dryer, furnace, cyclone, and baghouse
Fig 5: Vertical cyclonic wood burner to supply hot gas to dryer
Another approach is to use an indirect fired hot oil or steam tube dryer, which reduces dryer exhaust flowrate and a small amount of tramp air (Fig. 6 ). This more concentrated exhaust can be taken into a burner and burned for fuel recovery to heat the hot oil or steam. If this is done, there is no need for a VOC oxidiser.
Fig 6: Indirect fired hot oil heated dryer
Power and fuel use for pellets and dry microchips
The production of dry microchips eliminates the pellet grinding step to ∼1/16th of an inch top size (1.5 mm) before passing it through the pellet mill. Fig 7 a is a cross-section of a hammermill, where swinging hammers grind and break material up until it can pass through chosen size holes in an abrasion resistant grate. The pellet mills traditionally (as shown in Fig 7 b) use internal rollers that force the ground and dried wood through thick rotating dies. Both the grinding and the pelletisation processes use considerable power (about 125 hp or 93 kW per ton per hour pellet production) for their motors. The coal-equivalent fuel used by an electric utility to produce power for such hammermills and pelletisers is equal to about 7% of the heat available in the pellet product. Both inlet moisture content and temperature control of the process is critical to make strong pellets. In terms of carbon dioxide (CO2) emissions, the 125 hp per ton per hour to grind and pelletise would generate 216 lb (98 kg) of CO 2 per ton of pellets using coal-based electric power.
Fig 7: Hammermill and pellet mill
a Hammermill cross-sectional end view
b Rotary pellet mill cross-sectional end view
Reproduced with permission, biomass and alternate fuels systems: an engineering and economic guide, American Institute of Chemical Engineers (AIChE)/John Wiley & Sons, 2009
Diesel is used to fuel the in-forest chippers. While it varies with species, a rough estimate is 2.25 wet tons of microchips produced per gallon (3.8 l) of diesel fuel burned , in the chipper engine. The diesel use represents 0.7% of the product wood's calorific value.
Comparative fuel cost
A range of fuel costs are presented in Table 2, based on prevailing US prices. The coal, gas and oil costs are those paid by utilities for use in power generation. Hog fuel is burned directly at pulp and paper plants for steam and power. European prices for natural gas and fuel oil are higher than in the US and vary by country. Note that political decisions (e.g. the EU 2030 goals for renewable fuel) ultimately trump normal market pricing. Another factor in pricing is local and national fuel taxes In Finland, Sweden, and Denmark there is a CO 2 tax for fossil fuels in production In Sweden and Denmark there is also Sulfur Dioxide (SO 2) tax. Also keep in mind that burning coal entails extensive air pollution control systems, with significant cost for sulphur, particulate and mercury removal. One must take care in adding up delivered fuel cost as well as other operating and maintenance costs at point of use to make appropriate economic comparisons. On the basis of US pricing, pellet costs are more than all fuels in Table 2, except fuel oil.
|WASTE OR FUEL||GROSS CALORIFIC VALUE, BTU/LBA||APPROXIMATE COST, $/MM BTUB||COMMENT (SOURCE, DATE)|
|natural gas industry/utility||23,896||$3.23/2.67||US natural gas supply (Energy Information Agency (EIA) 08/10/16, first 5 mo. 2016)|
|no. 6 fuel oil||18,266||$10.00||at ∼$1.5/gal sale to end user including transport and tax; price varies with sulphur content (EIA, 08/10/16, 2105 average; based on $0.996/gal wholesale refiner price)|
|coal||9000–15,000||$2.08||$45.66/ton; cost based on 11,000 Btu/lb (EIA, 08/10/16, 2014 US utility average)|
|wood mill residue||∼4250 at 50% mc||$2.20||$18.74/ton delivered (Georgia Forestry Commission, 7/3/14). Local availability.|
|wood, whole tree chips||∼4250 at 50% mc||$2.55||$21.75 per ton delivered (Georgia Forestry Commission, 7/3/14)|
|wood pellets||∼7650 at 10% mc||$8.17||average price for export 2015 ∼$125 USD/ton for industrial grade|
|dry microchip fuel||∼7650 at 10% mc||$6.86||estimated $20/ton lower-cost than pellets ∼$105 USD/ton|
a To convert to MJ/kg, divide Btu/lb by 430; net calorific value is about 90% of gross calorific value for gas and oil.
b To convert from $/MM Btu to $/GJ, divide by 1.06. As of 8 November 2016, 1.10$ US dollars (USD) equals 1 €.
Table 2: Comparative fuel costs
Capital and operating cost
There are significant power savings in production when shifting to dry microchips from pellets due to reduced grinding power and elimination of pellet presses (see Fig. 8 ). This also reduces the carbon footprint for dry microchips, and the ratio of energy used in producing a fuel versus energy in the fuel product.
Fig 8: Flowsheet for pellets and differences for microchip fuel
How much more fuel does it take to transport the lower density fuel in larger volume/lower bulk density ships? A study done by the University of Iowa  for export grain shipments shows net ton-miles per gallon of fuel for various sized ships, with fuel use decreasing up as deadweight tonnage (dwt) goes up. Wood pellets in a dry bulk carrier at 1.8 million cu ft (50,000 m 3) capacity would load 40,000 short tons (36,400 metric dwt) at 45 lb/cu (720 kg/m 3) foot density. Dry microchips in a chip carrier with 3.6 million cu ft capacity ships would load 27,000 short tons (24,500 metric dwt) at 15 lb/cu foot (240 kg/m 3) density. Pellets would require 8.1 gallons (31 l) of fuel oil for the ship per ton transported versus 9.7 gallons (37 l) or 20% more fuel for ocean transport for dry microchip fuel for the 4500 mile (7200 km) trip. This transport fuel use is equivalent to about 9% of the energy contained in a ton of either wood fuel.
More detail on carbon footprint estimates can be found in .
Elimination of equipment from the flowsheet (the hammermill, multiple pellet mills, pellet cooler, interconnecting conveyors etc.) cuts purchased equipment cost by 57%. Total savings for power, maintenance, labour, reduced amortised cost of capital is in the range of $30–$35/ton of dry product. Estimates of the premium for transatlantic shipping of lower bulk density product via chip carriers suggest an added cost of $15/ton, resulting in a net savings on dry microchip fuel delivered to the EU of about $20/ton when compared with wood pellets.
Product regulation and specifications
In addition to grading by industry standards for industrial (higher ash) and residential (lower ash) markets, multiple parameters are tested on wood pellets:
- fines content, bulk density, diameter, and length;
- calorific value, chloride, and moisture content;
- pellet durability index; and
- ash content.
Comparatively, microchip fuel requires only moisture content, particle size range, ash content, and calorific value tests. Table 3 shows general specifications for microchip fuel.
|size||nominal top size 1/4″ × 1/16″ (5 mm × 1.5 mm)|
|±10>#/td###||8–12% typical, wet basis|
|gross calorific value||±7650 Btu/lb (18 kJ/kg)||varies with hardwood (less), softwood (more)|
|bulk density||±15 lb/ft 3 (240 kg/m 3)||approximate|
|ash, dry basis||0.5–2>#/td###||varies with source; can be reduced by screening|
|angle of repose||−35°||poured angle of repose|
Table 3: Generic dry microchip fuel specifications
A major issue for export is moisture content. Wet wood is generally banned from shipment due to the potential for transporting parasites such as pine wood nematode (PWN). PWN treatment as per the ‘56/30’ rule requires a core temperature of at least 56°C for 30 min for wood chips. Hence, the drying step is mandated for overseas shipment.
While wet wood chips tend to pack leading to arching and bridging in storage and handling systems, dry wood is freer flowing, and is stronger and less prone to jams and hang-ups.
In-house testing was done on bulk density and poured angle of repose (see Figs 9 and 10). The bulk density and angle of repose are reported in the above table.
Fig 9: Angle of repose testing tools
Fig 10: Poured conical pile of dry microchips
Combustion tests with a pilot scale grate burner were done by Karlsruhe Institute of Technology (KIT) wood pellets, larger, and slightly higher moisture content wood chips and DMC Fuel ®. Fig 11 shows the comparative combustion properties, with DMC Fuel® showing good potential for its use on grate type burners .
Fig 11: KIT combustion test results
Table 4 is a list fuel related international standards. Others exist such as American Society of Testing and Materials (ASTM) and wood industry tests related to marketing of wood pellets, e.g. Pellet Fuel Institute Standards Program, which provides product data including bulk density, size, durability, fines content, moisture and chloride content, and ENPlus and CANPlus ‘Quality Seal’ programmes.
|ISO 17225-1:2014 fuel specification and classes – general requirements (Table 4 for wood chips and hog fuel)|
|ISO 17225-2:2014 fuel specification and classes – graded wood pellets|
|ISO 17225-4:2014 fuel specification and classes – graded wood chips|
|ISO/TS 17225-8:2014 fuel specification and classes – graded thermally treated densified biomass fuels|
|EN 14778:2012 (ISO 18135) sampling|
|EN 14780:2012 (ISO 14780) sample preparation|
|EN 14918:2009 (ISO 18125) method for the determination of calorific value|
|ISO 17827-1 particle size analysis|
|ISO 17830:2016 particle size distribution of disintegrated pellets|
|ISO 18846:2016 determination of fines content in qualities of pellets|
|ISO 17828:2016 bulk density|
|ISO 18122:2015 ash content|
|ISO 18134-2:2016 moisture content simplified method|
|ISO 17831-1:2016 mechanical durability – pellets|
|ISO 16948:2014 determination of total content of C, H, N – instrumental methods|
|ISO 16994:2015 determination of total contents of S and Cl|
|ISO 16967:2015 determination of major elements (Al, Si, K, Na, Ca, Mg, Fe, P, and Ti)|
|ISO 16968:2015 determination of minor elements (As, Ba, Be, Cd, Co, Cr, Cu, Hg, Mo, Mn, Ni, Pb, Se, Te, V, and Zn)|
|CEN/TS 15370-1:2006 (ISO 21404) determination of ash melting behaviour|
Table 4: List of primary international standards for solid biofuels
While wood pellets are the current logical choice for biomass for industrial/utility use in the Europe, dry microchip fuel has potential for lowering costs and reducing the carbon footprint associated with biomass fuel, and making it yet a greener and less costly fuel source. The reduced capital cost and electrical use result from elimination of the grinding and pelletisation steps. When dried, microchip fuel has all of the positive properties desired for combustion: low moisture, good handling, near zero dust, little breakage when handling, and based on preliminary small-scale TMTS tests no detectable CO generation in storage. In addition, fuel can be stored in simple, lower-cost, roofed buildings instead of traditional pellet silos.
We would like to thank the personnel from the Karlsruhe Institute of Technology that provided figure 11: Daniela Baris, H.-J. Gehrmann, H. Mätzing, and D. Stapf, H. Seifert.
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