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Green | November 2011

Biomass gasification: Status and trends

A company operating a biomass gasification power plant should have a firm grip on the price of feedstock, which is one of the most critical determinants of its feasibility, says Narasimhan Santhanam as he takes a look at other aspects such as the market, trends, potential, and central and state government incentives.

While gasification as a technology has been prevalent elsewhere in the world as early as the 1940's, India pioneered the use of biomass gasification for power production in the recent decades. Following significant research and technology development or demonstration, the process has gained significant market acceptance after 2004. Even though it can produce power at small scales using locally available resources, the total amount of power production from it in India is relatively low as compared to combustion and co-generation which is still the predominant route for biomass to power. As per Energy Alternatives India (EAI) estimates, the total installed capacity of biomass gasification-based power production in India will be only 10 per cent of the total biomass-based power (cumulative of grid-connected and off-grid). Of the total, bagasse-based power generation has the lion's share, followed by combustion-based biomass power production. Currently, most power production systems in India using biomass gasification are off-grid and have been for captive consumption for an industry or for a community.

Market segments and applications
  • Industrial
  • Village and community
  • Biomass gasification for thermal applications
  • Biomass gasification as a replacement for diesel
  • Grid connected versus off-grid or captive applications
Biomass availability and potential

Of the total biomass potential in India which totals to over 34 GW, it is agro residues which constitute a lion's share of over 55 per cent followed by livestock waste, which constitutes about 25 per cent. The urban waste and industrial waste constitute 10 and 4 per cent respectively. In the current scenario, dedicated energy crop plantations constitute a negligible proportion of the biomass potential. However, it is expected that in future, dedicated energy crops will play a crucial role in the biomass to power sector. It has been estimated that the total potential per annum from the biomass surplus in India is about 17,400 MW. Of this, paddy straw and husk together are said to contribute to over 30 per cent of potential and cotton and wheat about 20 per cent each. Thus, these three (rice, wheat, cotton) together constitute about 70 per cent of the total available potential. Of the three, currently, paddy waste and cotton waste are the ones used predominantly. Punjab, Uttar Pradesh, Maharashtra, Haryana and Karnataka are states with high biomass potential. Together, they comprise close to 50 per cent of the total estimated potential for biomass in India.

Economics of power

While biomass gasification-based power production provides a number of benefits, especially to remote electricity needs, it is imperative that it is economically sustainable to operate. The primary cost components of a biomass gasification system comprise feedstock cost, capital costs (gasifier + gas engine + supporting equipment + land + installation) and operating expenses (including maintenance and repairs).

Capital costs

The total capex for a 1 MW grid-connected biomass gasification power plant would be approximately Rs 5.5 crore to as high as Rs 8 crore. The cost will, however, vary with the type of gasifier that is used. Given the fact that there are many different gasifiers under operation like the updraft, downdraft, fixed bed, fluidised bed etc., the cost will proportionately vary.

Operating Costs

Manpower cost: The typical requirement for a 1 MW biomass gasification power plant will be about 15 people comprising labourers, engineers, supervisors and manager. The approximate labour cost for a gasification plant will be about Rs 15 lakh and this will obviously escalate year-on-year.

Repairs & maintenance cost: Repairs and maintenance costs primarily comprise costs for cleaning the filters in the gasifier and periodical cleaning of the gas engines. The cost includes the cost of consumables, such as materials for repair (lube oil, gaskets, filter cloth, maintenance materials, chemicals for water treatment plant etc). The repair and maintenance cost will be around 5 per cent of the total capital cost for the gasifier and engine with an annual escalation of 5 per cent. If the total cost of the gasifier, engine and accessories are taken as Rs 5 crore per MW, then expenses towards repairs and maintenance will account to approximately Rs 25 lakh and this is bound to increase every year.

Cost of biomass: The cost of feedstock is one of the most critical determinants of the feasibility of biomass gasification power plants. It is critical that a company running such a power plant has a firm grip on the price of feedstock. The price of feedstock depends on factors such as the biomass used, availability of such biomass in the neighbourhood, scale of operation etc. The cost of feedstock could vary from a low of Rs 1,000 per tonne (for captive availability of local biomass waste) to Rs 200 per kg (for dedicated energy plantations undertaken by the power producer or for agro residues) to Rs 3,000 per kg (where the company is dependent on procuring biomass from the open market). In cases where fuel wood such as casuarina is used, the cost could be as high as Rs 4 per kg.

Feedstock price fluctuations have been one of the main reasons why biomass gasification based power production has been unviable in many regions in the country. The feedstock price is the single-most influential factor that determines the viability of the power plant. The following examples will show why feedstock is such a crucial factor which influences the business.
  • At the end of 2010, the price of rice husk price was around Rs 1,800 per tonne while in the beginning of 2009, it was selling at about
  • Rs 600 per tonne, implying a 300 per cent increase within 2 years.
  • In some regions of Bihar for instance, farm and crop waste was sold at Rs 1 per kg in 2007, it increased to Rs 2 per kg by the beginning of 2010 and by the end of 2010, it had increased to Rs 3 per kg.
  • A similar trend is seen in the prices of many other agro wastes when procured from the open market. A general thumb rule that is being mentioned by many biomass-based power producers is that, on an average, the price of biomass feedstock had doubled between the end of 2009 and end of 2010.
Levelised cost of electricity (LCOE)

The levelised cost of electricity (LCoE) represents the total cost of power production (Rs/kWh), after factoring in all the cost components. The LCoE for biomass gasification-based power depends on how the critical parameter - cost of biomass is managed. Since most of the other expenses will be controlled by the developer it is the feedstock cost that influences the LCoE over the long run. The levelised cost can vary from Rs 3.36 to Rs 4.36, depending on the cost of feedstock.

Revenue

Revenue for a biomass gasification plant will be from the following three main sources:

1. Feed-in tariff from the utility (in the case of grid-connected power plants)
2. Sale of charcoal which is a co-product
3. Returns from CDM (carbon credits)

From feed-in-tariffs
  • Considering an average power production of about 6 million to 6.5 million units per year, a feed-in tariff of Rs 4.5 per unit will guarantee revenue of Rs 2.7 crore to Rs 2.9 crore. If one chooses the REC + APPC route instead of the FiT route for revenue, returns from the sale of power and renewable energy certificates (RECs) will total to more or less the same value of Rs 2.7 to Rs 2.9 crore.  
From sale of charcoal
  • Given the amount of charcoal generated per kg of biomass used is 0.05 kg, the annual charcoal production will be 450 tonne per year which will guarantee a revenue of Rs 45 to Rs 65 lakh depending on the selling price of charcoal.  
From carbon trading (CDM)
  • For a 1 MW gasification plant, 4,500 certified emission reductions (CERs) could be accrued per year, which would translate to approximately Rs 27 lakh per year.  
Pay-back period

In the best case scenario, where biomass costs are low (especially if the company has excellent control over biomass prices from dedicated plantations) and with accelerated depreciation and capital subsidy benefits, the power producer could achieve pay-back within 4 years. Under more likely circumstances where the biomass cost is on the higher side but the government benefits are completely realised, the payback period is about 5 years. A note of caution however is that, the payback period is critically dependent on the feedstock cost. If the biomass cost is much higher than Rs 4 per kg, the payback periods could be quite long and hurt the project's viability.

Gasification technology and components

Biomass gasification transforms biologically derived organic, carbonaceous materials into gas mixtures namely carbon monoxide, hydrogen, carbon dioxide and methane, charcoal and other volatile organics etc. This is achieved by allowing the biomass feed-stocks decomposition at elevated temperatures (700° to 900°C) with a controlled amount of oxygen and/or steam. The predominant product is carbon-monoxide and hydrogen gas mixture called 'producer gas' and it serves as a gaseous fuel. The producer gas obtained by the process of gasification can have an end-use for thermal applications or for mechanical or electrical power generation.

Thermal energy to the order of 5 MJ is released, by flaring 1 m3 of producer gas in the burner. Flame temperatures of up to 1,550 K can be obtained by an optimal pre-mixing of air with producer gas. For applications that require thermal energy, gasifiers can be a good option as a gas generator and retrofitted with existing devices. Some devices to which a gasifier can be retrofitted are dryers, kilns, furnaces, boilers etc.

For power generation, the producer gas can either be used in mono or dual-fuel mode in reciprocating engines. In case of mono-fuel mode of operation, the gas is fuelled to a spark-ignition (SI) engine whereas in the dual-fuel mode it is operated along with a small quantity of liquid fuel (high-speed diesel, furnace oil or bio-diesel) in a compression ignition (CI) engine. The choice of mode of operation is entirely dictated by the economics of operation.

The gasification system could be primarily classified as fixed bed or fluidised bed gasification system.

Fixed bed gasification system

The fixed bed gasification system consists of a reactor or gasifier with a gas cooling and cleaning system. The fixed bed gasifier has a bed of solid fuel particles through which the gasifying media and gas move. On the basis of these factors, it is classified into principal types namely,
  • Updraft fixed bed gasifiers
  • Downdraft fixed bed gasifiers
Fluidised-bed gasification system

A fluidised-bed gasifier is a vessel with dimensions made in such a way that the superficial velocity of the gas maintains the bed in a fluidised condition at the bottom of the vessel, with a change in cross-sectional area above the bed that lowers the superficial gas velocity below fluidisation velocity to maintain bed inventory and act as a disengaging zone. The fluidised bed looks like a boiling liquid and has the physical properties of a fluid. In fluidised-bed gasification of biomass the gas is air, oxygen or steam, and the bed is usually sand, limestone, dolomite or alumina. The gas acts as a fluidising medium. It is principally classified into:

Bubbling fluidised bed gasifiers
Circulating fluidised bed gasifiers.  

Components of gasification technology

Fuel storage area: The fuel storage area is where the biomass feedstock is treated and stored. The stored amount of fuel is dependent on the respective configuration of the plant (performance range, fuel logistics and plant operating state). Besides the amounts of biomass feedstock, it is also necessary to pay attention to the storage of auxiliary fuels (propane gas, diesel fuels, etc.) for various co-combustion purposes. The storage area is also essential for combustible operational supplements (lubricating oils, washing emulsions for the operation of the gas cleaning plant, etc) and residue from plant operation.

Biomass sizing system: Gasification plants usually incorporate a size reduction unit to cut the biomass into smaller pieces prior to feeding into the gasifiers.

Fuel conveyance system: The fuel conveyance system is essential for transportation of feedstock from the storage area to the fuel feeding system. Depending on the size of the fuel and storage conditions, various discharge systems such as walking floor, travelling grate etc., could be used for the transport of the fuel from the storage area

Fuel feeding system: Feeding of the fuel (feedstock) into the gasifier is normally carried out by means of a clocked conveyance system that is activated by the output regulation of the entire system. Fuel feeding is usually carried out via a gas-tight transfer canal that prevents gas leakage.

The gasifier: It is an enclosed reaction vessel where biomass material transformation occurs under partially oxygenated, extreme temperature conditions to generate producer gas which will be subsequently used for production of heat and electricity after processing.

Gas cooling systems: Gas cooling is done to lower producer gas temperature. The gas cooling systems usually comprise heat exchangers and the type and efficiency of the heat exchanger will vary according to the application requirements of producer gas.

Gas cleaning systems: Gas cleaning is essential to provide uniform producer gas quality. The gas cleaning systems comprise of cyclones and filters.

Water treatment plant: Water is required for cooling and scrubbing of gas prior to the supply of gas to the engine. Cooling water of about 80 M3/hour is required on a continuous basis. To optimise on the utilisation of limited resources, the system will usually recycle the wash water. The power plant will have a water treatment plant for continuous filtration and purification of water.

Cooling tower: A cooling tower is provided to cool recycled cooling water after water treatment to maintain its temperature within the prescribed limits.

Char extraction unit: The char extraction unit consists of a screw blender for intermittent extraction of char or ash. The charcoal, which has commercial value, is milled to the required size, bagged and sold.

Central and state govt incentives

Feed in tariffs: For most renewable energy power projects, support from the government in the form of incentives and high feed-in tariffs is critical for their success. It must however be noted that biomass gasification power projects, while they too will become more economically feasible with government incentives, do not require as much as assistance from the government as do some others such as solar power plants (both PV and CSP). In fact, where the power producer is able to control biomass prices below a reasonable level, biomass gasification-based power plants have levelised costs of electricity production competitive with grid power.

Most Indian states have specified feed-in tariffs for biomass power. In addition, biomass gasification power plants can also avail of other financial incentives from the government.

Biomass gasification

Biomass gasification is a thermo-chemical process wherein solid carbonaceous materials, containing inorganic and organic constituents, react with air or oxygen to provide sufficient exothermic energy to produce a primary gaseous product containing mostly CO, H2, CO2 and light hydrocarbons (with volatile and condensable organic and inorganic compounds). Most of the inorganic constituents in the feedstock are discharged as bottom ash. The raw gas is then cooled, filtered, and cleaned or scrubbed to provide producer gas that can be used in energy conversion devices such as internal combustion engines or gas turbines.

The author is founder and Director, Energy Alternatives India (EAI). Views are personal.
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