Supplementing the use of coal with biomass — a woody or energy crop — has become desirable in the production of electricity for a number of reasons. Coal power plants that have tried to fire dried biomass have learned of the necessity of costly modifications to do so effectively. Yet at the same time, many coal-fired power plants have realized that the benefits of co-firing dried biomass are great enough to warrant the necessary modifications. Some coal-fired power plants, however, have not been able to make the necessary changes — or have not found them to be cost effective.

This article will look at the factors affecting thermal processing biomass to augment coal power plants during the production of electricity. Specific thermal processing technologies can help optimize such processes.

Torrefied Biomass vs. Dried Biomass

One way to increase the practicality of co-firing biomass in a coal plant is by using torrefied biomass rather than dried biomass. While this is not a new idea, the specifics are not always well understood.

Drying biomass merely removes the water present in the biomass. This is a necessary step — but not enough to fully prepare the biomass for use as co-firing biomass at coal-fired power plants. The inherent properties of dried biomass are such that plants using it must shut down more frequently than is typical for cleaning and repairs.

Torrefaction is the process whereby biomass — woody or otherwise — is heated in a low oxygen environment to a point where chemical changes start to occur. During the thermal processing, a portion of the dry solids are converted to gas (torrefaction gas, syngas, producer gas, etc.).

If dried biomass is used for co-firing in coal power plants, the lignins in the biomass materials — the organic polymers — act as glue. Lignins are especially problematic for pulverizers, which are equipment used to grind the coal and co-firing material into a fine dust. The pulverizers increase the surface area of the coal before it enters the firebox. Pulverizing lignins require more power, and the lignins can leave behind a gummy substance that must be cleaned.

When heated to the right temperature and time conditions with torrefaction, the biomass becomes the solid end-product that resembles coal visibly and physically. The process of torrefaction actually breaks the lignin bonds — inherent in biomass — that make biomass undesirable for co-firing with coal. This helps avoid problems such as gummed-up grinders and the interior of the boilers. (If not avoided, such conditions decrease plant efficiency and require more-frequent shutdowns for cleaning and repairs.) The end product is much more coal-like in physical characteristics, and the coal plant does not require modifications to the co-fire torrefied biomass.

A 3D rendering of a torrefaction system designed and built by Konza Renewable Fuels LLC. It scheduled to begin operation in 2018.

Densifying torrefied wood or biomass into a pellet form often is referred to as biocoal because it is so similar to coal and originates from biomass. Torrefied biomass also is hydrophobic. (Neither green nor dried biomass can be stored outside for any length of time because both absorb water and start composting.) Torrefied material can be stored outside in wet conditions indefinitely with little worry of decomposing or absorbing water. Also, because torrefaction removes moisture and converts part of the dry solids to gas, the final energy per unit weight of the final product is higher, decreasing the shipping cost per unit energy.

One drawback is that torrefied biomass has yet to be produced in large-enough quantities to prove widely beneficial to coal-fired power plants. By contrast, dried biomass is readily available.

Dried and torrefied wood samples are shown alongside pelletized biocoal.

Handling Feedstock Moisture-Level Inconsistencies

The nature of green biomass is the main hindrance to torrefying it on an industrial scale. (Green biomass is the term used to describe newly harvested biomass before processing.) Any internet research into the properties of biomass from trees, energy crops and other sources — especially moisture content — yields extremely varied numbers in all metrics.

Feedstock inconsistency can be a problem, but some equipment can be used to improve feedstock consistency. Industrial rotary-drum dryers have been used to handle the property variations of dissimilar crops for decades. Industrial rotary-drum dryers can be designed to process green biomass to a material of uniform moisture content. Tapping experience with other rotary-dried products allows rotary dryer manufacturers to better anticipate the challenges and power requirements that green biomass — trees in particular — present for grinding and sizing equipment.

Since 2009, Konza Renewable Fuels LLC, a manufacturer of torrefaction systems for biocoal, has performed many successful test runs on its torrefaction test facility. In a sample test, the test unit, which is composed of two rotary drums, converted green biomass with a gross heating value (GHV) of 8,660 BTU/lb at 0 percent mcwb (moisture content wet basis) to torrefied biomass with energy content of approximately 9,500 BTU/lb at 0 percent mcwb.1 Konza Renewable Fuels’ test unit is capable of producing torrefied wood in a continuous process.

Augmenting Torrefaction with Rotary-Drum Dryer Technology

With a uniform moisture content, downstream processing of the material appropriately is much easier. This is true whether the biomass is ultimately used for strand board, pellets, shavings or other end products. A rotary-drum dryer used in series with a torrefaction reactor can address problems of inconsistent infeed. The dryer creates a uniformly dried product from varyingly sized and moisture-content biomass before it is torrefied in the torrefaction reactor.

In the process, the uniform and dry material exits the first rotary-drum and enters a second rotary-drum thermal-processing system. The torrefaction reactor heats the infeed material to temperatures between 482 and 536°F (250 and 280°C) in a low oxygen environment. Internal flighting in the rotary-drum dryer and torrefaction reactor lift and separate the infeed product. This helps ensure that the product contacts the gases long enough to be heated all the way through. In the dryer stage, this dries the entire particle to a uniform moisture content throughout, regardless of size. Likewise, in the torrefaction reactor, this action heats the entire particle to the required temperature throughout, regardless of size.

This torrefied wood, produced in Konza Renewable Fuels’ test unit, shows uniform torrefaction throughout the wood particle.

When a particle remains in the gas stream for the entire duration of its residence time in the dryer drum, one of two outcomes typically occurs:

• If the particle remains in the dryer only long enough for the surface to dry, it tends to remain moist on the interior.
• If the particle remains in the gas stream long enough for the interior to dry as well, the shell likely overdries. It also can potentially experience damage.

Allowing the particle to rest during the drying process between bursts of falling through the gas stream and being pneumatically conveyed toward the exit solves both of these problems. This is accomplished with three dryer design characteristics:

• The dryer flighting design.
• The designed fall distance per rotation.
• The slow rotation of the drum, which allows each particle to rest out of the gas stream.

The resting periods allow heat to penetrate into the core of the particle to drive moisture from the center outward. Overheating the particle or heating it too intensely can overdry the shell and potentially damage it while the interior heats.

The flighting pattern used in the dryer and torrefaction reaction also creates an environment such that smaller particles spend less time in the drum and larger particles more time.

Biomass Sources

Any number of crops or byproducts can be torrefied and pelletized into biocoal. Farmers also can raise biomass crops such as switchgrass, miscanthus, bamboo and eucalyptus specifically for torrefaction.

Some types of biomass will grow where no food crops will grow. This means previously unusable land can become productive. Recent advances in aquaponics technology also can be applied toward growing energy crops using fish waste as fertilizer and less water than conventional crops.

In conclusion, being able to co-fire biomass in the form of biocoal without modifications to a coal-fired power plant is nearly realized due to recent advances in torrefaction technology. While the technology needed to make biocoal has existed for several decades, the challenge has been producing it on a large scale at a reasonable ROI. Industrial rotary-drum dryer technology may be just the key to unlocking large-scale biocoal production. 

Rotary Dryer Design for Torrefaction System

This schematic shows the process flow for a two-drum rotary torrefaction system. The hot gases flowing from the dryer combustion chamber to the dryer drum are the combination of combustion gas, torrefaction gas and reheated dryer recycle gas. Green biomass enters the dryer drum from the system infeed hopper.

The product tumbles through the dryer drum conveyed by the hot dryer gases. The dryer outlet hopper separates most of the product from the dryer outlet gas. The dryer outlet gas flows through the cyclone bank, where the remaining particles separate from the gas stream. The dryer outfeed screw transfers the product separated by the outlet hopper and the cyclone bank to the torrefier drum.

The dryer outlet gas exiting the top of the cyclones is reheated in the heat exchanger bank. It then splits between the heat recovery system, dryer inlet and torrefier drum inlet. The heat recovery system heats the reheated dryer vent gas entering it to a temperature high enough to destroy the volatile organic compounds (VOCs) in the torrefaction gas. The hot, clean gases from the heat recovery system provide heat to the heat exchanger bank and then exit the system through the stack.

Dry biomass and reheated dryer outlet gas enter the torrefier drum. The product tumbles while being heated to a temperature that drives off about 30 percent of the dry solids and 10 percent of the energy. It becomes a coal-like substance that is roughly 9,500 BTU/lb. The torrefier outlet hopper drops the product into the torrefier outfeed screw.

Torrefaction gas recycles to the beginning of the process to contribute energy and gaseous volume to the dryer drum.