The Sustainable Australia Report 2013 advises that global trends such as population growth and growth of the middle class in Asia will place increasing demands on energy, water and food systems.
In Australia one such impact, caused by population growth within its major cities, is the increased cost of waste landfill as sites reach capacity limits.
Biomass waste-to-energy systems help address this issue by reducing both landfill and reliance on fossil fuels.
What is biomass?Fossil fuels are formed from biomass and while both can produce energy, biomass is sourced from the atmosphere as part of the planet’s balanced carbon cycle, and can be therefore considered “carbon neutral”.
Biomass is organic matter including forest and mill residues, agricultural crops and wastes, wood and wood wastes, animal wastes, livestock operation residues, fast growing trees and plants, industrial and some municipal wastes such as construction and demolition wastes.
Biomass is the only renewable energy source that can provide on-demand energy, heat and electricity, as well as carbon-based fuels.
Selecting the right biomass energy systemThere are several processing technologies to convert biomass into energy, each with specific application depending on the biomass resource.
These are typically thermal (direct combustion, pyrolysis, gasification), or biological (anaerobic, fermentation) processes.
Since thermal processes are the most flexible, as they can process all forms of biomass, and the majority of proven commercial applications for heat and power utilise direct combustion and gasification technologies, this article will focus on these two technologies
Most projects only consider the biomass fuels that are readily available at the time they are initiated. However, after five years, many plants operate with a very different fuel mix to that for which the plant was originally designed.
It is therefore important to select a technology that allows for changes in fuel composition, for example, different energy value, moisture content, density and size. Also, the chosen technology should have the ability to accept fuels in liquid or gaseous form.
Environmental / Permit issues
Public opinion can have a significant influence on the ease or difficulty that a project experiences during the planning and permitting stages.
To avoid potential difficulties, the technology should be demonstrably the best available to minimise emissions and environmental impacts.
The technology supplier
It is important to question both the maturity of the technology and the experience of the supplier. Is the technology innovative yet commercially proven? Is it out-dated?
Or is it a research and development project in disguise? Is the supplier a recent start-up or an established company with many references to its name?
What about the financial strength of the supplier? And finally, will the project investors and lenders support the project based on the selected technology supplier?
Thermal biomass systemsA thermal biomass energy system is comprised primarily of a fuel handling system and a biomass boiler system.
Fuel receiving and handling system
The fuel receiving and handling system can be very simple or highly automated, depending on the fuel and the plant operating philosophy.
It will normally include weighing, unloading, screening, storage, reclaiming and transfer systems and should be designed to ensure reliable, consistent fuel supply to the biomass boiler.
Biomass boiler system
There are a number of biomass boiler technologies that convert biomass to steam or energy, however, for this article, we will consider only mature commercialised technologies. These are suspension fired boilers, grate boilers and fluidised bed boilers.
[Pictured below is a typical boiler island.]
1. Suspension fired boilers
Suspension fired boilers consist of a horizontally mounted tube, or combustion chamber, with an open end. A fan blows combustion air into the boiler, and fuel is introduced from the side with high velocity air.
Combustion is promoted when fuel and air mix as they swirl through the combustion chamber. Hot gases exit at the open end.
The suspension fired boiler normally has the lowest capital cost, however it has limitations. Fuel needs to be sized to ≈5mm or less and must have less than 15% moisture (wet basis).
The burner typically operates above 1250oC, and this causes thermal NOx to be formed. In addition, a percentage of the nitrogen contained in the fuel is converted to NOx during combustion.
The NOx generated can be partly alleviated by selective non-catalytic reduction (SNCR) but the effectiveness of this treatment is limited because of the short reaction time available due to the boiler design.
2. Grate or stoker boilers
Grate boilers are available in many forms including slotted, pinhole, shaker and traveling. The grate supports a bed of fuel that allows air to pass up through the bed.
The bed typically contains up to 45 minutes of fuel inventory; a feature that limits the boiler’s ability to adjust quickly to load changes or variations in the fuel composition heating value.
The fixed grate spacing restricts the ability to switch fuels, (for example wood chips to sawdust). Moisture must be kept quite constant, and since the burner operates above 1200oC, thermal NOx is an issue, as per the suspension type boilers.
3. Fluidised bed boilers
Fluidised bed boiler technology can be used in thermal oxidation and gasification operations, and has the advantage of inherently lower emissions and greater fuel flexibility.
The fluidised bed comprises a layer of sand-like material that is suspended by an upward flowing stream of air. When the air velocity is sufficient to lift the sand particles and keep them in suspension, the bed resembles a violently boiling pot of water, hence the term “fluidised bed”.
The turbulence in the bubbling bed acts to efficiently spread the fuel as it is fed to the boiler and the sand particles act to abrade the fuel as it is oxidised to expose fresh fuel to the combustion process.
These systems are flexible because they can accept a wide variety of fuels (singly or blended) through the same plant even with varying moistures (5%-55%) and high ash contents (55%).
Fluidised bed systems typically have stable operating characteristics because the turbulent nature of the operation results in even distribution of temperature and oxygen.
Fluidised bed thermal oxidation and gasification processes will be described here.
Fluidised bed thermal oxidation systemsIn the process of thermal oxidation (otherwise known as combustion), efficient oxidation and low emissions are governed by the three ‘Ts’ - Temperature, Time and Turbulence. Drying, volatilization, ignition and thermal oxidation all take place in the fluidised bed.
The vapour space above the bed is sized to allow adequate retention time for the gases, ensuring complete mixing and full conversion of fuel to energy.
Over-fire fans provide effective turbulence and assist mixing in the vapour space. The high combustion efficiency limits the emission of volatile organic compounds (VOCs) and other harmful gases.
[Pictured above is a fluidised bed thermal oxidation unit.]
The temperature in the vapour space is typically between 850-950oC; thermal NOx is not an issue as this is below the temperature where thermal NOx is formed. The temperature in the bed is also lower at ≈650oC.
Where nitrogen is contained in the fuel, some of this may convert to NOx during thermal oxidation. This can be addressed with a selective non-catalytic reduction (SNCR) system.
SNCR systems work more efficiently in a fluidised bed boiler because of the turbulent nature and effective mixing that occurs in the vapor space. Typically 75% of the NOx can be reduced in this system, compared with up to 50% NOx reduction in other biomass boiler systems.
Where the fuel composition indicates that SO2 is likely to be generated, this can be reduced inside the vessel with the introduction of limestone into the bed. At the correct temperature, the limestone is calcined and reacts with the sulphur compounds to reduce SO2 formation.
Fluidised bed systems can adjust quickly to load changes or variations in the fuel composition heating value. This is because they operate with only a few minutes fuel inventory.
Some suppliers have features that enhance the effectiveness of the technology such as:
• Using refractory lined vessels instead of water wall boilers. This has the benefit of maintaining a consistent vapor space temperature for a significant time, ensuring very high combustion efficiency with very low CO, NOx and VOC emissions.
• Designing the bed system to allow fuel sizing up to 100 mm, and accepting a variety of fuel compositions. This reduces the investment required for fuel handling and preparation. Opportunistic fluidised bed biomass boiler operators are able to use many different fuels as a result.
• Incorporating specially designed boiler tubes in the fluid bed zone. These have a very high heat transfer coefficient because of their location and allow a significant reduction in the boiler convective surface area, flue gas volume and therefore footprint.
Fluidised bed gasification systemsGasification systems partially oxidise the biomass at temperatures between 600-1600oC.
Operating in a sub-stoichiometric manner (less oxygen than is required for complete oxidation), a synthetic gas (syngas) is produced that contains primarily CO, CO2, H2, H2O, CH4, along with other hydrocarbons, inert gases and contaminants.
[Pictured above is a thermal oxidation unit.]
Air, oxygen, steam, and carbon dioxide can all be used for partial oxidation, and the heat value of the syngas will vary depending on the fuel and the gas mixture. The syngas can be used for heating or to produce biofuel.
Gasification is now the politically preferred method of energy recovery from biomass and wastes. It results in the formation of CO rather than CO2, and the gas volume is significantly less than for thermal oxidation, depending on the gas mixture.
Fluidised bed technology is very well suited to gasification applications. They operate at temperatures around 650-750oC.
The fluidised bed gasifiers share the same mechanical components as fluidised bed thermal oxidisers, however they operate with lower specific gas flow through the underfire air fans and no overfire air is used. SNCR and limestone systems are not used with fluidised bed gasification.
Fluidised bed combined thermal oxidation & gasification systemsIt is possible to utilise gasification and combustion processes together to generate heat.
The gasification and combustion can be performed in separate adjacent vessels, however some fluidised bed boiler system suppliers have direct coupled the gasification and combustion stages for plant footprint and capital cost benefits.
In these cases, the gasification occurs in the fluidised bed, and the gas, along with char, tar and ash, enters the secondary reaction chamber above the bed where multiple jets of air generate turbulence to complete the oxidation process.
The design of the oxidation chamber allows for the required residence time, and the jets of air provide turbulence and temperature control. In this way gaseous pollutants can be prevented from forming and emissions are kept at very low levels.
Other benefits include:
• utilises biomass and waste fuels with varying moisture contents of 5-60%.
• destroys other plant VOCs (for example from press vents, dryers etc).
• operates with fuels that have low ash softening temperature characteristics.
• reduces fan power requirements due to lower specific volume flow, compared with thermal oxidation systems.
Track recordTo date Outotec has installed over a hundred biomass fluidised bed thermal oxidation and gasification systems across a broad range of applications, from municipal waste to power generation and ethanol plants.
Outotec can also provide O&M and servicing for these installations, as well as upgrades to existing equipment such as boiler retrofits.
SummaryBiomass and waste-to-energy boiler systems have an important role to play in the world today. The selection of boiler technology and technology supplier is critical for a successful project.
There are some innovative, yet commercially proven technologies that will satisfy the investors, the regulator and the public, particularly with consideration of environmental issues.
Most importantly, the selected technology must be appropriate for the proposed fuel and have the ability to accept completely different fuels in the future.
About the author
Mark Weatherseed joined Outotec in 2004 and is currently the Technology Sales Manager for Hydrometallurgy. He is also responsible for Outotec’s Energy from Waste Technology in South East Asia Pacific. Mark holds a B Eng (Hons) in Mechanical Engineering from the University of Liverpool, UK.