Biomass Plants

Key Features

The combustion process of biomass involves a complex series of physical and chemical processes. These processes are influenced both by the intrinsic properties of the biomass fuel and by the specific design and operational parameters of the combustion system used. The combustion process can be broadly classified into the following key stages:

Drying: Biomass fuels, particularly those derived from wood, often contain significant amounts of moisture. The initial stage of combustion involves the evaporation of this moisture. The energy consumed in this drying process is taken from the combustion itself, which can lower the temperatures in the combustion chamber and consequently affect the overall efficiency of the process.

Pyrolysis: As the temperature increases, the biomass undergoes thermal decomposition in the absence of sufficient oxygen for complete combustion. This process, known as pyrolysis, breaks down complex organic molecules into simpler volatile compounds. These volatile substances, mainly in gaseous form, are combustible and represent a significant portion of the energy potential of the biomass.

Gasification: This phase occurs in parallel with pyrolysis, particularly in combustion systems with a limited supply of oxygen in the initial stages. During gasification, the biomass reacts with a limited amount of oxygen or steam, leading to the formation of combustible gases such as carbon monoxide, hydrogen, and methane. The precise composition of this gas mixture depends on the specific gasification conditions.

Combustion: This final stage is characterized by the rapid reaction of the volatile gases and char produced in the previous stages with oxygen. This exothermic reaction releases heat and light and, ideally, results in the formation of carbon dioxide and water as primary products. The efficiency of combustion, and thus the emission profile, depends heavily on factors such as temperature, residence time, and the degree of mixing between fuel and air.

It is important to note that the distinct stages described above do not occur in strict isolation. There is a significant degree of overlap and interaction between these processes. For example, in some systems, drying, pyrolysis, and gasification may occur simultaneously in different zones of the combustion chamber.

Pretreatment of Biomass

These techniques aim to address the challenges associated with the variability of fuel quality and to optimize aspects such as handling, storage, and combustion efficiency. Here is a summary:

Chipping of Woody Biomass: This involves reducing the size of woody biomass materials such as logs and branches. The use of chippers, which are machines specifically designed to cut wood into smaller, uniform pieces called chips, is highlighted. Different types of chippers, such as drum chippers and disc chippers, are employed based on factors such as the desired chip size and the volume of material being processed.

Pretreatment of Waste Wood: There is often a need for specialized pretreatment of waste wood due to the potential presence of contaminants such as metals, glass, and plastic. These contaminants can be harmful to combustion systems and lead to increased emissions.

Baling and Packing: This technique applies mainly to herbaceous biomass such as straw and hay. Baling, typically done after harvesting, compresses these materials into manageable units (bales). This densification not only simplifies handling and storage but also reduces transportation costs.

Pellets and Briquettes: These represent more refined forms of biomass fuel. Pelletization involves grinding the biomass into a fine powder and then compressing it under high pressure to form small, uniform pellets. Briquetting follows a similar principle but produces larger, brick-shaped units. Both processes significantly increase the energy density of the biomass and improve handling characteristics.

Drying: The intrinsic moisture content of biomass, particularly fresh wood, poses a considerable challenge in combustion. Drying aims to reduce this moisture content, thereby increasing the effective calorific value of the fuel. Various drying techniques are available, including solar drying, where the biomass is spread out to dry naturally under sunlight, and mechanical drying methods, which use heated air or exhaust gases to accelerate the drying process.

Storage and Transport of Biomass

There are key aspects of biomass storage and transport:

Long-Term Storage: There is a need for long-term storage of biomass fuels to bridge the gap between production and use. This is particularly important for fuels such as wood chips, which are often produced seasonally. The design of long-term storage facilities is essential to minimize costs, given the relatively low energy density of biomass.

Short-Term Storage: Biomass combustion plants require a short-term storage system with an automatic discharge mechanism. This system ensures a continuous supply of fuel to the combustion unit, enabling consistent operation.

Fuel Handling: The movement of biomass between long-term storage areas and the short-term storage that feeds the combustion plant is typically managed by cranes or wheeled loaders.

Transport Systems: Once the biomass is in the short-term storage area, various types of conveyors are used to transport it to the combustion unit. These include:

• Sliding Bar Conveyors: Suitable for larger biomass units such as briquettes.

• Chain Conveyors: Similar to sliding bar conveyors, they are also suitable for handling briquettes.

• Screw Conveyors: These versatile conveyors are used for transporting pellets and other smaller biomass particles.

• Belt Conveyors: These conveyors are used to move pellets and can be used in combination with screw conveyors for efficient fuel delivery.

• Pneumatic Conveyors: The use of pneumatic conveyors, which use air pressure to transport biomass particles, is particularly effective for handling fine materials such as sawdust.

Inclined Moving Grate Combustion

Furnaces with inclined moving grates use a grate composed of fixed and moving rows of grate bars, positioned on a slope. The moving sections of the grate alternate between forward and backward horizontal movements, facilitating the transport of fuel along the grate. This movement offers several advantages:

• Mixing: Effectively mixes burned and unburned fuel particles.

• Surface Renewal: This action constantly renews the surface of the fuel bed.

• Uniform Fuel Distribution: Contributes to a more uniform distribution of fuel across the grate surface, which is crucial for even primary air distribution.

A well-designed and controlled grate ensures homogeneous fuel distribution and a consistent ember bed. This homogeneity is essential for ensuring even primary air distribution through the grate, which in turn helps prevent issues such as:

• Slagging

• Excessive fly ash

• High excess oxygen demand, leading to greater heat loss

The grate bars are typically made of heat-resistant cast iron and incorporate small channels along the sidewalls for primary air supply. The narrow design of these bars aims to optimize primary air distribution throughout the fuel bed.

Operational and Control Aspects:

• Movement Frequency: The source emphasizes the importance of precisely adjusting the movement frequency of the grate bars. Excessively high frequencies can lead to incomplete carbon combustion, resulting in higher unburned carbon content in the ash and insufficient grate surface coverage. Conversely, insufficient frequency can hinder proper fuel transport and combustion. To address this issue, infrared sensors are often installed above the grate sections. These sensors monitor the height of the fuel bed, allowing dynamic control of grate movement to maintain optimal combustion conditions.

• Air-Cooled Systems: Furnaces with air-cooled grates, where the primary air also serves to cool the grate system, are particularly suitable for fuels with higher moisture content, including bark, sawdust, and wood chips.

• Water-Cooled Systems: Furnaces employing water-cooled grates are recommended for drier biomass fuels or those with low ash sintering temperatures.

Flue Gas Condensation: Maximizing Heat Recovery

• Significant Energy Savings: Flue gas condensation emerges as a highly effective method, capable of recovering up to 20% of the energy input from the biomass fuel (relative to the Net Calorific Value). This results in substantial improvements in the overall plant efficiency.

• Dust Precipitation: This process offers the additional benefit of achieving dust precipitation efficiencies ranging from 40% to 75%, contributing to cleaner emissions.

Operating Principle: The core of a flue gas condensation unit consists of three key components:

• Economizer: Recovers sensible heat from the flue gases.

• Condenser: Extracts both sensible and latent heat from the flue gases through condensation.

• Air Preheater: Uses the recovered heat to preheat the combustion air and the air used to dilute the saturated flue gases before they enter the stack.

Factors Influencing Heat Recovery:

• Biomass Moisture Content: Higher moisture content in the fuel increases the potential for heat recovery.

• Excess Oxygen Levels: Reducing excess oxygen in the flue gases increases the dew point, thereby increasing the amount of latent heat recoverable at a given temperature.

• Return Water Temperature: Lower return water temperatures from the heating plant increase the amount of latent heat recoverable during flue gas cooling.

• Air Humidification for Improved Recovery: An innovative approach involves integrating an air humidifier into the flue gas condensation system. This humidifier moistens the combustion air by injecting condensate water, increasing the moisture content of the flue gases and, consequently, the potential for heat recovery.

Reducing Excess Oxygen: A Simple Efficiency Boost

• Direct Impact on Efficiency: The source emphasizes that minimizing the excess oxygen content in the flue gases directly results in higher plant efficiency.

• Technological Solutions:

  • Oxygen and CO Sensors: The use of oxygen and carbon monoxide sensors in tandem at the boiler outlet facilitates precise control of secondary air supply, optimizing combustion and minimizing excess oxygen.

  • Improved Gas/Air Mixing: Enhancements in the quality of mixing between flue gases and air within the furnace also contribute to reducing the excess air ratio required for complete combustion.

Biomass Drying: A Specific Consideration

• Potential Benefits: Although not always economically sustainable, drying biomass fuel before combustion can offer some advantages:

  • Reduced storage volume

  • Prevention of self-ignition

  • Minimized dry matter loss during storage

• Economic Viability: The economic feasibility of biomass drying depends on several factors, including investment costs, operating expenses (electricity, labor), and the availability of low-cost or free preheated air sources, such as solar air collectors or preheated air from flue gas condensation units.

Optimizing Plant Utilization Rate

• Crucial for Cost-Effectiveness: Given the high investment costs associated with biomass combustion plants, maximizing their utilization rate is essential for economic profitability. To achieve this, it is necessary to ensure:

  • High annual utilization: The biomass system should operate at least 85% of its annual capacity.

  • Integration of heat recovery: The importance of integrating heat recovery systems, such as flue gas condensers or economizers, is emphasized.

Technical and Economic Standards for Biomass District Heating Plants in Austria

To ensure economically viable investments in biomass energy, Austria has established specific technical and economic standards for biomass district heating and combined heat and power (CHP) plants. Compliance with these standards is a prerequisite for obtaining investment subsidies for new biomass district heating or CHP projects in Austria.

Although the source does not provide a complete list of these standards, it highlights their importance and points to external resources ([30, 31]) for a detailed understanding.

Key Aspects of the Standards Highlighted in the Source:

• Plant Utilization: The standards emphasize achieving optimal plant utilization. This is crucial to offset the high investment costs typically associated with biomass systems.

• Heat Distribution Networks: Efficiency within the heat distribution network is another focus of the standards. This underscores the importance of minimizing heat losses during distribution to consumers.

Sizing Biomass Boilers for District Heating

When determining the size of a biomass boiler for a district heating or controlled heat CHP plant, the main factor is the energy demand, which includes both heat and electricity requirements. However, this assessment goes beyond simply examining current needs. It is essential to consider projected future energy demands to ensure that the selected boiler capacity remains adequate in the long term. This means anticipating potential growth in the area served by the district heating system.

Simultaneity Factor: Another key consideration is the simultaneity factor, which reflects the reality that not all consumers will have their maximum heat demand at the same time. This factor is influenced by the number and type of consumers connected to the network. For example, large district heating systems with diverse consumers tend to have a lower simultaneity factor (around 0.5) compared to microgrids (which approach 1) where consumption patterns are more uniform.

Annual Heat Production Line and Load Management: Given the fluctuating nature of heat demand throughout the year, a crucial step is establishing the annual heat production line. This line provides a visual representation of heat demand at different times. Typically, district heating networks experience peak demand during winter and lower demand in summer. To optimize cost efficiency, boiler planning distinguishes between base load (the relatively constant energy demand) and peak load (the highest demand for shorter periods). Biomass boilers are often designated for covering the base load due to their operational characteristics and fuel cost structure. Peak loads, on the other hand, are often met using fossil fuels or liquid biofuels for economic reasons. Alternatively, the integration of heat accumulators into the system can help manage peak load situations.

The information provided underscores the importance of achieving a high number of full-load operating hours for the biomass boiler. By carefully sizing the boiler and distinguishing between base and peak load demands, plant operators can maximize the economic benefits of biomass combustion.

Understanding a Biomass CHP Plant with ORC Cycle

A biomass combined heat and power (CHP) plant using an Organic Rankine Cycle (ORC) generates both heat and electricity from biomass fuel. Here is a breakdown of the process:

1. Biomass Combustion and Heat Transfer: The process begins with the combustion of biomass in a specialized furnace. Unlike traditional steam systems, the heat generated in this furnace is used to heat a thermal oil. This thermal oil acts as an intermediate heat transfer fluid.

2. ORC System and Power Generation: The heated thermal oil then circulates through a heat exchanger, transferring heat to a separate closed loop containing an organic fluid with a lower boiling point than water. This heat input vaporizes the organic fluid, driving a turbine connected to a generator to produce electricity.

3. Heat Recovery for Cogeneration: After expansion through the turbine, the organic fluid is cooled in a condenser. The heat released during condensation is captured and used for various heating purposes, such as district heating or industrial processes. This simultaneous production of heat and power is what defines this configuration as "cogeneration."

4. Closed-Loop Operation and Efficiency: The ORC system operates in a closed loop, meaning the organic fluid is continuously circulated through the system, vaporizing and condensing. The use of an organic fluid with a lower boiling point allows efficient operation at lower temperatures compared to conventional steam cycles, making it particularly suitable for biomass applications.

5. Advantages of ORC in Biomass Cogeneration:

   • Lower Operating Temperatures: ORC systems can generate power efficiently from low-temperature heat sources, making them ideal for biomass combustion systems, which often have lower flue gas temperatures compared to fossil fuel systems.

   • Flexibility and Partial Load Operation: ORC units can adjust their power output more effectively than traditional steam turbines, allowing better adaptation to varying heat and electricity demands.

   • Reduced Maintenance: The use of thermal oil as an intermediate fluid reduces the risk of corrosion and scaling in the ORC system, resulting in lower maintenance requirements.

The source material does not provide specific details on the structure of a biomass CHP plant with an ORC cycle; however, by combining the information provided on ORC technology and general biomass energy production, this explanation offers a comprehensive understanding of the process. You may want to verify this information independently.

Characteristics of Biomass Ash and Their Influence on Combustion System Behavior

Biomass ash, a byproduct of biomass combustion, exhibits unique characteristics and behaviors in combustion systems.

1. Ash Formation and Fractions:

   • Origin and Transformation: The elements that form ash exist within the biomass in three forms: integrated into the organic matter, contained as mineral granules within the fuel, and present as external contaminants. During combustion, these undergo various transformations, releasing some elements into the gaseous phase and leaving behind residual ash particles.

   • Different Ash Fractions: Combustion processes typically produce three distinct ash fractions:

     • Bottom Ash: The largest fraction, composed mainly of residual ash particles and mineral impurities, collects on the grate of the combustion system.

     • Cyclone Fly Ash: The finer particles, predominantly inorganic, are carried away by the flue gases and captured in cyclones located after the combustion unit. This fraction consists mainly of coarse fly ash.

     • Filter Fly Ash: The finest ash particles, formed primarily from volatilized ash compounds, are captured by high-efficiency filters downstream of the flue gas path.

2. Ash Composition and Its Significance:

   • Variable Composition: Biomass ash exhibits significant compositional variations depending on the biomass source, with wood-based fuels generally having lower ash content (often less than 7%) compared to herbaceous fuels such as straw (up to 12%).

   • Major Elements and Their Influence: Silicon (Si), calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), and phosphorus (P) constitute the major elements in biomass ash. Their relative proportions influence the behavior of ash at high temperatures, affecting the design and operation of combustion systems.

   • Trace Elements and Environmental Concerns: Biomass ash contains varying levels of trace elements, some of which can be harmful to the environment and human health. Monitoring and managing these trace elements is crucial for sustainable biomass use.

3. High-Temperature Behavior in Different Combustion Systems:

   • Grate Combustors: In these systems, common for domestic and smaller industrial applications, the fuel bed temperatures typically reach 1000-1200°C. While some ash sintering (partial melting) is expected, excessive melting can lead to operational challenges such as clogged airflow and difficulties in ash removal.

   • Fluidized Bed Combustors: These systems, prevalent in medium-scale applications, operate at slightly lower temperatures (below 900°C) compared to grate systems. The constant movement of the bed material promotes heat transfer and reduces the risk of large molten ash formations.

   • Pulverized Fuel Combustors: Used in large-scale applications, particularly for co-firing biomass with coal, these systems reach very high temperatures (around 1600°C). The short residence time at these high temperatures requires careful consideration of the behavior of inorganic material and its impact on ash formation and deposition.

4. Ash Deposition and Its Implications:

   • Slag Formation: Primarily driven by the melting of ash particles, slag deposition, common in high-temperature zones such as furnaces, can lead to reduced heat transfer efficiency and operational disruptions. The chemical composition of the ash, particularly the presence of alkali metals acting as fluxes, influences slag formation.

   • Convective Pass Fouling: Occurs when volatile inorganic species in the flue gases condense on cooler surfaces in the convective section of the boiler. Potassium compounds and phosphates are the main contributors to this type of fouling in biomass combustion.

   • Mitigation Strategies: Effective ash deposition management involves a multi-faceted approach, including careful combustion system design, appropriate fuel selection, controlled combustion parameters, and the implementation of online cleaning mechanisms.

5. Impact on Flue Gas Treatment Equipment:

   • Challenges in Particulate Removal: The presence of fine and sub-micron aerosol particles in biomass flue gases poses a challenge for conventional particulate removal equipment such as electrostatic precipitators.

   • Potential for Equipment Degradation: Biomass ash can affect the performance and lifespan of flue gas treatment systems through mechanisms such as surface blockage, chemical poisoning.