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 power 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 energy 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.