Energy crops pellets are technically feasible . A lot of progress by companies developing technology and recent research produce densification in biomass from herbaceous crops or by mixing herbaceous and woody biomass.

This week, a publication from BIOMASS MAGAZINE, reported that Penn State Researchers are working to develop a solid understanding of energy crops and their densification behavior, which could remove much of the guesswork in creating a quality pellet. 

Creating a pellet or briquette from energy crops may seem more like a secret family recipe than an exact process. Begin with a selected feedstock, dry accordingly, add binders if necessary and compress generously. Sometimes, even if the recipe is dead-on, the end product may turn out differently than expected.

Densifying energy crops presents unique challenges to producers seeking to use grassy feedstocks, such as miscanthus and switchgrass, for pellet production. Moisture, binder and energy requirements are just a few of the issues inherent in creating a pellet or briquette made from energy crops, says Emily Heaton, assistant professor of biomass crop production at Iowa State University.  “Sometimes there is not enough moisture or enough binder, depending on the pelleting process and getting it to stick together, and then the energy that is required to get it to stick together,” Heaton says. “You can always get it to stick together, but what does it take to do that?”.

Napier grass pellets, had several difficulties to be desnfied but they are already feasible and demonstrated at commercial level in Brazil. Mixing raw materials and improving methods can allow stable and durable products..

 Napier grass biomass pellets

Gaining an intimate understanding of energy crops and how they behave in the densification process could help producers remove some of the guesswork in creating a quality pellet or briquette.  ellet producers may experience difficulties in creating a uniform product, even if it seems they are handling the material with the same methods every day.

Napier grass can produce great yields in tropical countries and several measures are required to improve drying methods and logistics.

Bulk density of straw and grass raw material is low, whether it is chopped or in bales, varying between 50 to 150 kg/m3 , so also the energy density of raw material becomes low. It is possible to recompress the raw material before the transport or even before the pelleting process. In the feed industry it has been used two pelleting mills in series for compressing (Payne 1994). If straw  material can be recompressed the transport cost will be lower, and also traditional wood pellet press  and other process devices work better and more efficiently with the densified rawmaterial.  Densifying biomass allows the material to be handled and stored more easily. The delivery option  for the densified biomass will be determined by the distance of the transportation radius.

The key of success is probably to use particulate compaction background to avoid problems including pelletizer clogging and crumble-prone pellets.  In addition to clogging issues, the feedstock’s physical properties can cause problems within the pelletizing process. Compared to woody biomass, energy crops like switchgrass, contain less lignin and tend to have more issues, such as clogging pelletizing machines, and sometimes require additional materials to create a pellet

A drawback of the agricultural fuels is the ash content and the behaviour of them during combustion. Nitrogen, sulphur and chlorine contents of several alternative raw materials and the chemical ash contents requires specific measures and solutions for the ash smelting properties of the alternative pellets.

Agrecol Corp., a Madison, Wis., grower of native plants and seeds, began producing biomass pellets four years ago. According to Mark Doudlah, president of Agrecol, the company began making pellets to deal with the high quantities of MOG (material other than grain) byproduct produced during seed-cleaning. “It didn’t belong in a landfill,” he says, “and composting is quite messy. Land-spreading for us wasn’t a good option, either. We decided, let’s densify and burn it.” Doudlah says the company modified equipment from a retired feed plant to pelletize MOG, and later biomass from the fields. Agrecol uses the pellets to heat its facility and sells the rest.

To make pellets, the biomass must first be cleaned to remove contaminants. The clean biomass is then ground in a hammer mill or chipped to a uniform size, which must be less than the thickness of the pellet that will be produced. Grinding down biomass helps to reduce the horsepower the pellet mill must produce. If the biomass is high in moisture, it must be dried to approximately 10 percent moisture content.  While the lignin content in wood is generally enough to bind pellets, other forms of biomass require special conditioning to strengthen them. Sometimes binders such as starch, sugars, paraffin oils, or lignin must be added to make the biomass malleable.

Some remarkable studies

• Effect of miscanthus addition and different grinding processes on the quality of wood pellets

In this study the mechanical properties of wood miscanthus mix pellets for domestic use were investigated. The influences of a proportion of miscanthus of up to .2kgkg-¹ to the quality of wood pellets were tested. Furthermore binder addition (.2kgkg-¹ of potato starch) and two different grinding processes (impact comminution under dry conditions and shearing comminution under wet conditions) were tested to improve the pellet quality. According to existing quality standards physical properties for the produced pellets have been determined to evaluate the quality. The results showed that for producing standardized pellets with miscanthus proportion the addition of a binder is necessary in the case of impact comminution. When using a shearing comminution for the raw material preparation no more binder addition is necessary to fulfil the quality criteria even with a miscanthus proportion of .2kgkg-¹. The shearing comminution leads to a highly fibrous material that can be more compacted in the press channels than normal shavings and furthermore these fibres have a high potential to create interlocking bonds. Another effect might be a successful mechano-chemical activation of binder substances like proteins in the raw materials through the wet preparation step. The influence of miscanthus addition and different grinding processes on the quality of wood pellets is investigated. Miscanthus addition weakens the pellet structure. Physico-mechanical quality can be improved by using shearing comminution instead of impact comminution.

Pellets from miscanthus

• Effect of process parameters on pelletisation of herbaceous crops

Straw, sawdust and other herbaceous crops benefit from pelletisation, as the process increases the materials bulk density therefore reducing transportation and storage costs; providing better material feeding with less dust formation. This study investigated a pressure pelletisation method of switchgrass for five types of material preparation: raw cut switchgrass, raw shredded switchgrass, torrefied switchgrass, switchgrass combined with heavy pyrolysis oil and wheat straw. The effects of pelletisation pressure and temperature on the quality of pellets were evaluated in terms of density, mechanical strength and durability. Temperature had a greater effect than pressure on pellet quality, where at elevated temperatures, the lignin present in the biomass softened and acted as a binding agent. The cut switchgrass produced more desirable pellets over shredded switchgrass due to an additional binding effect of intertwined fibres. The torrefaction of grass was not an attractive pre-process as the pellets were very brittle and possessed little mechanical strength and reduced bulk density. At elevated temperatures, with a grass to tar ratio of 2:1, the pellets were twice as strong as pellets made by cut switchgrass. The increased durability was a result of lignin present in the biomass and the heavy oil dispersing inside the pelletiser, which filled in the gaps between the switchgrass fibres upon heating. Finally, the pellets were burnt in a fixed bed combustor and the ignition rate and average burning rates were evaluated. The results have provided an indication to how the pellets would perform compared to other fuels. Future work should focus on the standardisation of herbaceous crop pelletisation and in particular, upper and lower bound limits for the moisture content should be investigated, as moisture was found to have a significant impact on pellet quality.

• Critical review on the pelletizing technology

An often mentioned hurdle for biomass utilization are the logistics inherent to an agricultural product; harvesting, oisture, storage, transportation, quality uniformity etc. Typically, biomass is delivered to the bio refinery in bulk by railroad cars or by trucks in the form of chopped forage or baled hay. One way for handling of biomass crops more efficiently is densifying them into bales, pellets, cubes or briquettes to reduce the bulk volume of the material. Although each method has pro’s and con’s, pelleting seems to have the greatest number of advantages. Although pelleting ads costs, pelleted material is floodable and allows the fuel to handled and stored easily and transported more economically. In addition, pelleted biomass is very homogenous fuel. Pelletizing decreases the moisture content and allows the pellets to be burned more efficiently.

This study shows several implications on the use of herbaceous biomass to produce pellets in Europe.

• Biomass torrefaction technology: Techno-economic status and future prospects

Torrefaction is a promising bioenergy pre-treatment technology, with potential to make a major contribution to the commodification of biomass. However, there is limited scientific knowledge on the techno-economic performance of torrefaction. This study therefore improves available knowledge on torrefaction by providing detailed insights into state of the art prospects of the commercial utilisation of torrefaction technology over time. Focussing on and based on the current status of the compact moving bed reactor, the authors identify process performance characteristics such as thermal efficiency and mass yield and discuss their determining factors through analysis of mass and energy balances. This study has shown that woody biomass can be torrefied with a thermal and mass efficiency of 94% and 48% respectively (on a dry ash free basis). For straw, the corresponding theoretical energetic efficiency is 96% and mass efficiency is 65%. In the long term, the technical performance of torrefaction processes is expected to improve and energy efficiencies are expected to be at least 97% as optimal torgas use and efficient heat transfer are realised. Short term production costs for woody biomass TOPs (torrefied pellets) are estimated to be between 3.3 and 4.8 US$/GJ_LHV, falling to 2.1–5.1 US$/GJ_LHV in the long term. At such cost levels, torrefied pellets would become competitive with traditional pellets. For full commercialisation, torrefaction reactors still require to be optimised. Of importance to torrefaction system performance is the achievement of consistent and homogeneous, fully hydrophobic and stable product, capable of utilising different feedstocks, at desired end-use energy densities.

• Effects of compressive force, particle size and moisture content on mechanical properties of biomass pellets from grasses

A 2006 study analyzed mechanical properties of wheat straw, barley straw, corn stover and switchgrass that were determined at different compressive forces, particle sizes and moisture contents. Ground biomass samples were compressed with five levels of compressive forces (1000, 2000, 3000, 4000 and 4400 N) and three levels of particle sizes (3.2, 1.6 and 0.8 mm) at two levels of moisture contents (12% and 15% (wet basis)) to establish compression and relaxation data. Compressed sample dimensions and mass were measured to calculate pellet density. Corn stover produced the highest pellet density at low pressure during compression. Compressive force, particle size and moisture content significantly affected the pellet density of barley straw, corn stover and switchgrass. However, different particle sizes of wheat straw did not produce any significant difference on pellet density. The relaxation data were analyzed to determine the asymptotic modulus of biomass pellets. Barley straw had the highest asymptotic modulus among all biomass indicating that pellets made from barley straw were more rigid than those of other pellets. Asymptotic modulus increased linearly with an increase in compressive pressure. A simple linear model was developed to relate asymptotic modulus and maximum compressive pressure.

Pellets from Switchgrass

• Natural binders and solid bridge type binding mechanisms in briquettes and pellets made from corn stover and switchgrass

This article, is focused on micro-structural analyses (i.e., light microscopy, scanning electron microscopy, and UV auto-fluorescence imaging) of corn stover and switchgrass briquettes and pellets. The authords showed that the natural binders in these biomass materials created “solid bridge” type bonding between particles in the briquettes and pellets. The potential natural binding components in these biomass materials are water soluble carbohydrates (2.2–7.9% d.b.), lignin (8.8–9.2% d.b.), protein (3.6–3.9% d.b.), starch (0.4–1.0% d.b.), and fat (0.7–0.9% d.b.). The natural binders in the biomass can be expressed or activated (softened) under high pressures in the presence of moisture (e.g., water soluble carbohydrates) and in some cases increased temperature (e.g., lignin, protein, starch, and fat). When pressure is removed and the binder cools, it hardens or “sets up” forming bridges or bonds between particles, which has the effect of binding them together and making the resulting product more durable. Furthermore, activating (softening) the natural binding components through moisture and temperature in the range of glass transition is essential to produce highly durable briquettes and pellets.