VIASPACE In the News
SOURCE: BIOENERGY INSIGHT MAGAZINE, JULY/AUGUST 2016, P. 46-47
KING OF THE GRASS WORLD
Move over wood, perennial grass is looking to have its moment in the spotlight
When one thinks of biomass energy, grass doesn’t automatically spring to mind. Images of burning wood or sugarcane bagasse normally permeate the brain. In fact, in most of the world, biomass energy means burning the latter. Wood and wood waste have many competing uses such as pulp and paper and construction materials. After Fukushima, Japan is also seeking wood pellets from around the world to meet its environmental goals.
Sugarcane bagasse is routinely used to produce heat and electricity for sugar mills and the excess is sold to local electricity grids. Sugarcane production bagasse production, however, is seasonal and not available for reliable 24/7 base electricity.
Nevertheless, perennial grasses are an attractive option as dedicated energy crops. Some can be harvested in the first year and subsequently harvested up to three times per annum. As in all agriculture, the cost of the biomass is proportional to the yield per acre. Higher yield means lower cost. Perennial grasses can be planted on marginal lands that are currently fallow and do not displace food crops.
Clean energy company Viaspace has developed a proprietary hybrid energy crop called Giant King Grass (GKG) which produces high yields in humid tropical climes. It can be grown in warm regions such as Africa, Central and South America, and South and Southeast Asia. GKG can be used there to produce electricity for local use, and it can be used as a feedstock to produce energy pellets, biochemicals and biofuels for export to Europe, Northern Asia and North America. Nevertheless, it does not withstand a hard freeze.
The crop is attractive in terms of water use and fertiliser use efficiency which is defined as the amount of water or fertiliser required to make one tonne of dry biomass. It has been tested extensively for use in direct combustion, anaerobic digestion and as a feedstock for cellulosic biofuels. In most properties it is very similar to corn straw, but has a much higher yield in terms of tonnes per hectare. GKG is a non- invasive species and is planted from cuttings like sugarcane.
The agriculture is also similar to the well-established techniques for growing sugarcane. GKG can be planted by hand or with machines, and it can be manually or mechanically harvested. It is currently growing in California and Hawaii in the US, and has been exported to St. Croix in the US Virgin Islands, as well as to South Africa, China, Myanmar, Pakistan, the Philippines, Nicaragua, Jamaica, and Guyana. GKG is inspected by the US Department of Agriculture prior to export and issued with a phytosanitary certificate.
A high quality direct combustion power plant can have 30% or higher efficiency from the lower heating value of the fuel to electricity produced. The direct combustion power plant can burn GKG or other fuels at moisture levels of 50% or lower. When GKG is harvested it has about 75% moisture and must be dried down to 50% or lower in order to be burned. A 12MW biomass power plant requires 18 tonnes per hour of GKG at 50% moisture (9 tonnes per hour of dry matter). This is equivalent to 36 tonnes per hour of freshly harvested GKG. The power plant operates 7,884 hours annually and therefore needs 284,000 tonnes of GKG to be harvested every year. With the high yield of GKG, in a tropical climate with irrigation and two harvests per year, the 12MW power plant can be produced on 840 hectares of land. This is 70 hectares per megawatt.
In a temperate area with a lower yielding grass and only one crop per year, the amount of land needed will be three to ve times larger. The boiler for direct combustion of agricultural straws and grasses must have a special design to accommodate the higher levels of chlorine in these fuels compared to wood. The chlorine causes both high and low temperature corrosion. Additionally, the ash from these fuels has a lower melting temperature that causes slagging. Simply putting grass or straw into a wood boiler will cause it to fail within a year. Special boiler designs are available and boilers designed for grass and straw can also burn wood or sugarcane bagasse.
A properly designed boiler can use a mixture of fuels such as GKG with rice husk. The sizes of direct combustion power plants for grasses and straws range from 10-35MW. In a tropical area, just in-time harvesting can be employed to reduce the amount of biomass that needs to be stored for the power plant. Logistics are simplified and costs are reduced if the power plant and plantation can be co-located.
The biomass power plant is environmentally-friendly. It requires a bag filter to capture particulates, but no other emission controls are needed to meet World Bank standards. Emissions are much lower than with a coal power plant, and further reducing nitrogen oxides and other emissions is straightforward using technology developed for existing power plants. GKG ash is a good fertiliser and can be put back on the fields.
GKG pellets have been tested by many companies and independent laboratories. GKG pellets have also been tested by SGS, Bio Energy Labs, and laboratories in Korea and China with similar results.
Wood pellets are routinely used to replace up to 20% of coal in European power plants to reduce carbon emissions. GKG pellets have the same potential. GKG has been converted at laboratory scale into a high- energy, carbon neutral, drop-in replacement for coal using a thermal catalytic process.
The resulting GKG biocoal has the same energy density as metallurgical coal, and is environmentally friendly as it burns cleanly without generating sulphur, mercury or other emissions found in coal. The biocoal is hydrophobic and can be stored outdoors like coal and pulverized in the same machines as coal.
The laboratory results indicate that GKG is an excellent feedstock for the production of biocoal using the hydrothermal process. The energy density of the biomass is increased significantly to the range typical of a good thermal coal (~24MJ/Kg). The ash content is similar to that of a good coal.
GKG has been extensively tested for biogas production using anaerobic digestion. Since lignin inhibits anaerobic digestion, the Giant King Grass is harvested every 60 to 90 days before it gets too woody, when it is to be used for anaerobic digestion.
The average biomethane production is 60.7L per kilogram of fresh GKG. With warm weather, sufficient water or irrigation, and good nutrition, 100 hectares of planted GKG can produce 1MW of biogas electricity.
Tibbar Energy is developing a 7MW biogas power plant using GKS on St. Croix, US Virgin Islands.
GKG can be digested alone or mixed with other organic matter such as manure, pig waste, food processing waste, slaughterhouse waste, vinasse from rum making or waste from cheese making. Researchers are using an open culture reactor similar to anaerobic digestion to produce biochemicals including medium-chain carboxylic acids, such as n-caproic acid (C6) and n-caprylic acid (C8).
In summary, GKG is an attractive, low cost feedstock for multiple biopower applications. GKG can also be used as a feedstock for cellulosic biofuels such as ethanol and butanol, drop-in biofuels, and for producing biochemicals and biomaterials. GKG has another important application as a high nutrition animal feed when it is harvested every two months at about 1.5- 2.0 m feet tall.
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This article was written by Carl Kukkonen, CTO, Viaspace. Visit www.viaspace.com