Summary of Biomass Combustion Technologies

Direct combustion is the best established and most commonly used technology for converting biomass to heat. During combustion, biomass fuel is burnt in excess air to produce heat. The first stage of combustion involves the evolution of combustible vapours from the biomass, which burn as flames. The residual material, in the form of charcoal, is burnt in a forced air supply to give more heat. The hot combustion gases are sometimes used directly for product drying, but more usually they are passed through a heat exchanger to produce hot air, hot water or steam.

Combustion_Moving_Grate

The combustion efficiency depends primarily on good contact between the oxygen in the air and the biomass fuel. The main products of efficient biomass combustion are carbon dioxide and water vapor, however tars, smoke and alkaline ash particles are also emitted. Minimization of these emissions and accommodation of their possible effects are important concerns in the design of environmentally acceptable biomass combustion systems.

Biomass combustion systems, based on a range of furnace designs, can be very efficient at producing hot gases, hot air, hot water or steam, typically recovering 65-90% of the energy contained in the fuel. Lower efficiencies are generally associated with wetter fuels. To cope with a diversity of fuel characteristics and combustion requirements, a number of designs of combustion furnaces or combustors are routinely utilized around the world

Underfeed Stokers

Biomass is fed into the combustion zone from underneath a firing grate. These stoker designs are only suitable for small scale systems up to a nominal boiler capacity of 6 MWth and for biomass fuels with low ash content, such as wood chips and sawdust. High ash content fuels such as bark, straw and cereals need more efficient ash removal systems. Sintered or molten ash particles covering the upper surface of the fuel bed can cause problems in underfeed stokers due to unstable combustion conditions when the fuel and the air are breaking through the ash covered surface.

Grate Stokers

The most common type of biomass boiler is based on a grate to support a bed of fuel and to mix a controlled amount of combustion air, which often enters from beneath the grate. Biomass fuel is added at one end of the grate and is burned in a fuel bed which moves progressively down the grate, either via gravity or with mechanical assistance, to an ash removal system at the other end. In more sophisticated designs this allows the overall combustion process to be separated into its three main activities:

  • Initial fuel drying
  • Ignition and combustion of volatile constituents
  • Burning out of the char.

Grate stokers are well proven and reliable and can tolerate wide variations in fuel quality (i.e. variations in moisture content and particle size) as well as fuels with high ash content. They are also controllable and efficient.

Fluidized Bed Boilers

The basis for a fluidized bed combustion system is a bed of an inert mineral such as sand or limestone through which air is blown from below. The air is pumped through the bed in sufficient volume and at a high enough pressure to entrain the small particles of the bed material so that they behave much like a fluid.

The combustion chamber of a fluidized bed plant is shaped so that above a certain height the air velocity drops below that necessary to entrain the particles. This helps retain the bulk of the entrained bed material towards the bottom of the chamber. Once the bed becomes hot, combustible material introduced into it will burn, generating heat as in a more conventional furnace. The proportion of combustible material such as biomass within the bed is normally only around 5%. The primary driving force for development of fluidized bed combustion is reduced SO2 and NOx emissions from coal combustion.

Bubbling fluidized bed (BFB) combustors are of interest for plants with a nominal boiler capacity greater than 10 MWth. Circulating fluidized bed (CFB) combustors are more suitable for plants larger than 30 MWth. The minimum plant size below which CFB and BFB technologies are not economically competitive is considered to be around 5-10 MWe.

Major Considerations in Biopower Projects

In recent years, biopower (or biomass power) projects are getting increasing traction worldwide, however there are major issues to be tackled before setting up a biopower project. There are three important steps involved in the conversion of biomass wastes into useful energy. In the first step, the biomass must be prepared for the energy conversion process. While this step is highly dependent on the waste stream and approach, drying, grinding, separating, and similar operations are common.

In addition, the host facility will need material handling systems, storage, metering, and prep-yard systems and biomass handling equipment. In the second step, the biomass waste stream must be converted into a useful fuel or steam. Finally, the fuel or steam is fed into a prime mover to generate useful electricity and heat.

One of the most important factors in the efficient utilization of biomass resource is its availability in close proximity to a biomass power project. An in-depth evaluation of the available quantity of a given agricultural resource should be conducted to determine initial feasibility of a project, as well as subsequent fuel availability issues. The primary reasons for failure of biomass power projects are changes in biomass fuel supply or demand and changes in fuel quality.

Fuel considerations that should be analyzed before embarking on a biomass power project include:

  • Typical moisture content (including the effects of storage options)
  • Typical yield
  • Seasonality of the resource
  • Proximity to the power generation site
  • Alternative uses of the resource that could affect future availability or price
  • Range of fuel quality
  • Weather-related issues
  • Percentage of farmers contracted to sell residues

Accuracy is of great importance in making fuel availability assumptions because miscalculations can greatly impact the successful operation of biomass power projects. If biomass resource is identifies as a bottle-neck in the planning stage, a power generation technology that can handle varying degrees of moisture content and particle size can be selected.

Technologies that can handle several fuels in a broad category, such as agricultural residues, provide security in operation without adversely affecting combustion efficiency, operations and maintenance costs, emissions levels, and reliability.

Consistent and reliable supply of biomass is crucial for any biomass project

Identification of potential sources of biomass fuel can be one of the more challenging aspects of a new biomass energy project. There are two important issues for potential biomass users:

  • Consistent and reliable biomass resource supply to the facility
  • Presence of harvesting, processing and supply infrastructure to provide biomass in a consistent and timely manner

Biomass as an energy source is a system of interdependent components. Economic and technical viability of this system relies on a guaranteed feedstock supply, effective and efficient conversion technologies, guaranteed markets for the energy products, and cost-effective distribution systems.

The biomass system is based on the following steps:

  • Biomass harvesting (or biomass collection of non-agricultural waste)
  • Preparation of biomass as feedstock
  • Conversion of biomass feedstock into intermediate products.
  • Transformation of intermediates into final energy and other bio-based products
  • Distribution and utilization of biofuels, biomass power and bio-based products.

Overview of Biomass Handling Equipment

The physical handling of biomass fuels during collection or at a processing plant can be challenging to conveying equipment designers, particularly for solid biomass. Biomass fuels tend to vary with density, moisture content and particle size (some even being stringy in nature) and can also be corrosive. Therefore biomass fuel handling equipment is often a difficult part of a plant to adequately design, maintain and operate.

Biomass_Conveyor

The design and equipment choice for the fuel handling system, including preparation and refinement systems is carried out in accordance with the plant configuration. This is of special importance when the biomass is not homogeneous and contains impurities, typically for forest and agro residues. Some of the common problems encountered have been the unpopular design and undersized fuel handling, preparation and feeding systems.

The fuel handling core systems and equipment are dependent on both the raw fuel type and condition as well as on the conversion/combustion technology employed. The core equipment in a biomass power plant include the following:

  1. Fuel reception
  2. Fuel weighing systems
  3. Receiving bunkers
  4. Bunker discharge systems (stoker, screw, grab bucket)
  5. Fuel preparation
  6. Fuel drying systems
  7. Crushers
  8. Chippers
  9. Screening systems
  10. Shredding systems
  11. Grinding systems (for pulverised fuel burners)
  12. Safety systems (explosion relieve, emergency discharge, fire detections etc)
  13. Fuel transport and feeding
  14. Push floors
  15. Belt feeders
  16. Conveyers and Elevators
  17. Tube feeders
  18. Fuel hoppers and silos (refined fuel)
  19. Hopper, bunker and silo discharge
  20. Feeding stokers
  21. Feeding screws
  22. Rotary valves

To enable any available biomass resource to be matched with the end use energy carrier required (heat, electricity or transport fuels) the correct selection of conversion technologies is required. Since the forms in which biomass can be used for energy are diverse, optimal resources, technologies and entire systems will be shaped by local conditions, both physical and socio-economic in nature.

As the majority of people in developing countries will continue using biomass as their primary energy source well into the next century, it is of critical importance that biomass-based energy truly can be modernized to yield multiple socioeconomic and environmental benefits.

Is Green Car Fuel A Reality?

drop-in-biofuelsVehicles remain a huge global pollutant, pumping out 28.85Tg of CO2 in Maharashtra alone, according to a study by the Indian Institute for Science in Bangalore. However, vehicles cannot be discarded, as they form the lifeblood of the country’s towns and cities. Between electric vehicles and hybrids, work is being done to help rectify the situation by making use of green car fuel and technological advancements.

Emissions continue to be a huge issue, and there are two main options for helping to rectify that. The first is electric, which is seeing widespread adoption; and the second, biomass fuel, for more traditional vehicles. Between the two, excellent progress is being made, but there’s much more to be done.

How electric is helping

Electric cars are favoured heavily by the national authorities. A recent Times of India report outlined how the government is aiming for an all-electric vehicle fleet by 2030 and is pushing this through with up to US$16m of electric vehicle grants this year. Green vehicles are obviously a great choice, improving in-city noise and air pollution whilst providing better vehicular safety to boot; a study by the USA’s MIT suggested that electric vehicles are all-around safer than combustion.

However, where EVs fall down to some extent is through the energy they use. As they are charged from the electricity grid, this means that the electricity is largely derived from fossil fuels – official statistics show that India is 44% powered by coal. Ultimately, however, this does mean that emissions are reduced. Fuel is only burned at one source, and oil refining isn’t done at all, which is another source of pollutants. However, as time goes on and the government’s energy policy changes, EVs will continue to be a great option.

The role of biofuels

Biofuels are seeing a huge growth in use – BP has reported that globally, ethanol production grew 3% in 2017. Biofuel is commonly a more favoured option by the big energy companies given the infrastructure often available already to them. While biofuel has been slow on the uptake in India, despite the massive potential available for production, there are now signs this is turning around with the construction of two US$790m biofuel facilities.

Biofuels are increasingly being used to power vehicles around the world

The big benefit of biofuel is that it will have a positive impact on combustion and electric vehicles. The Indian government has stated they intend to use biofuel alongside coal production, with as much as 10% of energy being created using biofuel. Therefore, despite not being emission-free, biofuel will provide a genuine green energy option to both types of eco-friendly vehicle.

Green car fuel is not entirely clean. The energy has to come from somewhere, and in India, this is usually from coal, gas, and oil. However, the increase in biofuel means that this energy will inevitably get cleaner, making green car fuel absolutely a reality.