Bio Coal or Green coal and Bio Methanisation are two ways of utilising agricultural waste to generate energy, both can be considered renewable since nature returns agricultural residues back through photosynthesis unlike fossil fuel use that consumes the reserves piled up over centuries in a short period of time.
The agrarian states of Punjab and Haryana have multiple crops in a year and the field has to be cleared of agricultural residue of every crop after every harvest for fresh sowing. Around mid – November, after the kharif crops, the field has to be cleared quickly and peasants burn the agricultural residue in large quantities. Large scale burning of agricultural residue has become a menace to the air quality in surrounding areas of Delhi where the wind velocity is low and smog remains entrapped for days together, playing havoc with the respiratory health of a large population. The high rise land locked urban settlement of National Capital Region has a thick boundary layer with very low wind velocity, thus the AQI of NCR around November becomes a subject of discussion every year.
As a corrective measure, agricultural residues have been converted to briquettes and pellets and co-fired in coal-fired plants to the extent of 5-10%. This involves the cost of conversion of agricultural residues to briquettes/ pellets besides the cost of transportation of briquettes/pellets from the point of manufacture to the point of use in coal fired plants. Alternatively, bio-methanisation of agricultural residue in the absence of air under the action of anaerobic bacteria is a well-known process. Methane as natural gas has got four-fold use, it is PNG in our kitchen, CNG on the road, fuel for Power-Plants based on combined cycle (with higher efficiency of 55% in contrast to a supercritical plant with efficiency of 43%) and raw-material for fertilizer. While transportation of briquettes could add significantly to its landed costs, natural gas can be utilized in the vicinity of its production in the kitchen or on the road. A techno-economic analysis is made in this article evaluating whether agricultural residues can be converted to pellets and briquettes or should they be bio – methanized.
Processing agricultural residues
Agricultural residues have a large volume and a low energy density, hence they require further processing. With the help of cutters and rackers, the residue is bailed by the bailers to increase the density, which further eases the handling and transportation of the residues across short distances. Hence, the bailed residues are converted to briquettes and pellets.
Pellets & briquettes
- Pellet is denser than briquettes as it has lesser moisture content.
- Pellet has got 20% higher calorific value.
- Pellet is more compact in size.
- Pellet has wider instrument acceptability.
- It gives higher efficiency as a fuel when fired.
Still Briquettes are more popular than the bio pellets as conversion of agricultural residues into briquettes requires very Low Capital Investment and Technology.
Both these residues can also remain in storage for a longer time without actually losing its own calorific value.
Co-firing biomass with coal is an attractive energy generating option from both economic and environmental points of view. Co-firing could be economic in the sense that biomass co-firing does not require major capital investments and uses the existing coal-fired power plant infrastructure with minor changes in the operating regime.
Agricultural residues when fired in the form of pellets or briquettes contain a lot of volatile matters. Hence, the mill outlet temp is to be maintained lower. This results in saving on investments due to reduced coal consumption in terms of reduction in not only mined cost of coal but also its transportation. Moreover, co-firing is a low-risk option for the production of renewable energy since the risks associated with major capital investments and raw material supplies are much smaller compared to other alternative uses of biomass (e.g., biomass to biofuel production). Additionally, direct co-firing is one of the most interesting and effective means of reducing GHG emissions from the coal fired power plants. Co-firing minimizes waste (e.g., wood waste, agricultural waste) and the environmental problem associated with its disposal. Finally, co-firing is a near term market for biomass. For the past decade, various forms of biomass fuels have been co-combusted in existing coal-fired boiler and gas-fired power plants.
Existing co-firing is driven by various co-firing technologies implemented in coal-fired power plants. Currently, there are three co-firing technologies widely used in coal plants:
- Direct co-firing,
- Indirect co-firing, and
- Parallel co-firing.
Direct co-firing: In direct co-firing, biomass is directly fed into the furnace with base fuel (e.g., coal). In this method, biomass can be milled with coal directly or separately prior to feeding them in the furnace.
The challenges associated with direct co-firing are: tendency of producing ash deposition (e.g., slagging and fouling); limited range of co-firing; and lack of flexibility to use different types of biomass.
Indirect co-firing: Indirect co-firing installs a separate gasifier to convert the solid biomass into a fuel gas commonly termed as sync gas.
It provides a number of benefits over direct co-firing that include: boiler slagging can be reduced since biomass does not directly feed into the boiler; gasification reduces gas residence time; and it has the flexibility to use different base fuels such as coal, oil, and natural gas.
Parallel co-firing: It involves the installation of a completely separate external biomass-fired boiler in order to produce steam used to generate electricity in the coal-fired power plant. This steam generated from biomass-fired boiler is used to meet the demands of the coal-fired power plant and reduce the operational risk of a coal-fired power plant due to the availability of separate and dedicated biomass burners running parallel to the existing boiler unit.
Parallel co-firing provides a unique opportunity to increase the biomass percentage during biomass co-firing and avoid biomass-related contamination issues. However, this technology is proven more expensive than the direct co-firing approach since additional infrastructure is needed to support the system.
Typical boilers used in existing coal-fired power plants are Fluidized Bed Combustion (FBC) boiler, Pulverized Coal Combustion (PCC) boiler, Packed-Bed Combustion (PBCP) boiler, and Cyclone Boiler (CB).
Bio pellet as green coal
Agricultural residues, when burnt in open in the atmospheres, release a huge amount of ash, and unburnt carbon in the atmosphere. The unburnt carbon can cause a lot of diseases including respiratory issues.
It is also important that the amount of CO2, which is released after complete burning of bio pellets, is absorbed by the crops for the next cycle of photosynthesis.
We know that the thermal power plants have a higher efficiency in comparison to the traditional stand-alone biomass power plants, which makes this as a Cleaner Substitute.
Both briquettes and pellets are made by high compression without use of any chemical, which makes it more eco-friendly and pollution free. Because of this reason both briquettes and pellets are together known as Bio-Coal.
BIO pellets again are of 2 types: Torrefied and non-torrefied.
Torrefaction is nothing but the process of heating up the agricultural residue at 250-300 degrees centigrade in the absence of oxygen, which makes it brittle, less volatile, and black carbon powder. which is further processed as torrefied pellets.
In short, it’s nothing but roasting of wood or other biomass to remove moisture and low energy volatiles, increasing the energy density of the biomass.
STEP 1:- (Receiving And Storage)
The Woodchips are collected and stored to be used as biomass fuel.
STEP 2 (Drying)
The Wood chips are dried using a closed loop belt dryer.
STEP 3 (Torrefaction)
The wood chips are heated using the microwave technology, creating a charcoal like substance.
STEP 4 (Grinding and Pelletizing)
Torrefied wood is ground up and converted into pellets that produce up to 10% more energy than wood.
If we go through the properties, it is as good as coal. Therefore, it can be pulverized. Further, the agricultural residue, which doesn’t undergo torrefication gets converted to non-torrefied pellets. Torrefied and densified biomass has several advantages in different markets, which make it a competitive option compared to conventional biomass wood pellets.
Higher energy density: An energy density of 18–20 GJ/m³ – compared to the 26-33 gigajoules per ton heat content of natural anthracite coal – can be achieved when combined with densification (pelletizing or briquetting) compared to values of 10–11 GJ/m³ for raw biomass, driving a 40–50% reduction in transportation costs. Importantly, pelletizing or briquetting primarily increases energy density. Torrefaction alone typically decreases energy density, though it makes the material easier to break into pellets or briquettes.
More homogeneous composition: Torrefied biomass can be produced from a wide variety of raw biomass feedstocks that yield similar product properties. Most woody and herbaceous biomasses consist of three main polymeric structures: cellulose, hemicellulose and lignin. Together these are called lignocellulose. Torrefaction primarily drives moisture and oxygen-rich and hydrogen-rich functional groups from these structures, producing similar char-like structures in all three cases.
Hydrophobic behaviour: Torrefied biomass has hydrophobic properties, i.e., it repels water, and when combined with densification makes bulk storage in open air feasible. Torrefaction increases the storage period of the pellets and also increases its calorific value.
Elimination of biological activity: All biological activities are stopped, reducing the risk of fire and stopping biological decomposition like rotting.
Improved grindability: Torrefaction of biomass leads to improved grindability of biomass. This leads to more efficient co-firing in existing coal-fired power stations.
…To be continued
Sowjanya Rath is currently working as a Business Analyst in ESSPL, Bhubaneswar, Odisha. He is a Graduate (B. Tech.) in Electrical And Electronics Engineering from Silicon Institute of Technology. He has done internship at NTPC Corporate Engg., Noida.
Dr. Bibhu Prasad Rath is a graduate in Mechanical Engineering. He has worked in various functions in NTPC including Operation & Design. He is presently Additional General Manager in Project Engineering Division. He has also completed ICWA (Inter – 1995), and M.Tech (IIT Delhi – 2002) and PhD (in Business Administration – 2015) from Aligarh Muslim University.