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Optimizing Biomass Boiler Efficiency

How to generate heat with biomass boilers (pellets, wood chips, wood). Types, design and sizing criteria, incentives

The biomass boiler is a heat generator that harnesses the energy produced by the combustion of biomass instead of that produced by gas, diesel, or LPG.

Currently, due to their characteristics, biomass boilers represent a sustainable alternative to heat pumps.

The essential conditions required for the installation of a biomass boiler are:

  • an adequate space to be allocated for a technical room;
  • the possibility to install a chimney;
  • availability of space for fuel storage.

To efficiently perform the energy calculation of a building served by a biomass boiler and comply with regulations, you can use a thermal engineering software that allows you to define climatic data and thermal bridges to then select the most suitable biomass system for your needs; the software will automatically perform all the necessary analyses.

Let’s analyze in detail how a biomass boiler works, its advantages, and the tax benefits provided.

What is a biomass boiler?

A biomass boiler is a heat generator that harnesses the energy of biomass, i.e., organic materials of plant origin, to produce heat through combustion. Heating with biomass allows the use of a renewable source par excellence, protecting the natural carbon dioxide cycle, saving on combustion materials, and protecting the environment.

Biomass boiler: how it works

The operating principle of a biomass boiler is practically identical to that of a common gas boiler. The only difference is the type of fuel used for the production of thermal energy used for heating the water circulating within the system. The operation is based on the combustion of biomass inside a specific combustion chamber.

The biomass is fed through an automatic system that burns it, generating heat that is then transferred to a heating circuit. This circuit then distributes the heat throughout the building. In certain contexts, the energy produced by the boiler can also be used to generate electricity through a turbine.

Biomass boiler: types

Biomasses are differentiated into:

  • liquid: derived from the pressing and refining of oily seeds or other particular plant parts;
  • gasified: (biogas, syngas) obtained from solid biomasses with specific processes. Regarding biogas, for example, it is obtained through a biological process at low temperatures by which, in the absence of oxygen, the organic substance is transformed into a gaseous mixture of methane and carbon dioxide;
  • solid: generally used as fuel in thermal plants.

Among all solid biomasses, woody biomasses play a significant role, thanks to their wide availability and ease of storage and combustion.

Woody biomasses include materials from the agro-forestry sector and wood industries such as:

  • pellets: wood processing waste compressed into cylinders. This material is used in stoves for direct room heating or in boilers to heat water for radiators and/or underfloor radiant panels;
  • wood chips: natural wood from forests shredded with or without bark;
  • sawdust: derived from industrial waste;
  • wood logs: made of pressed wood residues;
  • firewood logs: trunk portions from forests.

A biomass boiler stands out for being able to be fueled with one of the aforementioned types of fuel, or through a combination of them. In the latter case, it increases versatility and usefulness for end-users.

The most common type consists of reverse flame boilers where the flame does not develop upwards but downwards. In this way, with the fuel load present in the upper part of the boiler, the firebox can be loaded to the maximum without all the wood burning at once.

When choosing a boiler, the first aspect to consider is the type of fuel you want to use. Let’s see some examples.

Pellet boiler

Pellets are a biomass fuel obtained from untreated wood residues, cylindrical in shape and usually with a length of up to 3 cm. These residues are reduced to sawdust, dried to reduce the moisture content, and subjected to a subsequent pressing process, where pressure and heat allow the typical cylindrical shapes to be formed.

Wood chip boiler

Wood chips represent a biomass at a very low cost, easy to obtain, and simple to produce. They are obtained through wood shredding, a process that uses a machine called a wood chipper, capable of reducing the waste resulting from cutting, pruning, and wood processing for other purposes into chips.

These chips, up to a maximum length of a few centimeters, are left to rest for several years to lose absorbed moisture, dry out, and become ready for combustion. Like pellets and wood in general, the higher the drying level of the wood chips, the higher their calorific value.

Wood boiler

A wood-fired boiler is certainly the most valid solution. However, it is essential to ensure that the wood is adequately dried and stored to minimize the moisture content and is subsequently cut into usable pieces.

Choosing the biofuel

The correct choice of biofuel depends on various factors, including:

  • energy density;
  • moisture content;
  • particle size;
  • ash quantity.

It is essential to consider that the biofuel must meet both the technological and environmental requirements of the heat generator, and the optimal choice of fuel may vary from situation to situation, as there is no universal solution.

In general, low-quality biofuels have high moisture content, variable particle size, and a low ash melting point. They are usually used in large plants capable of offsetting the high costs associated with fuel management and exhaust gas cleaning.

On the contrary, high-quality biofuels, more manageable, are often preferable for small plants.

Energy density is a critical factor to consider, as it affects the amount of space required for biofuel storage and management. For example, pellets have an energy density of about 3100 kWh/m3, while wood chips can vary considerably depending on quality and moisture, with values ranging from 630 to 860 kWh/m3.

Biofuels with high energy density, such as pellets, are more suitable for storage in small spaces or less frequent deliveries, while wood chips are preferable when ample storage space is available, and high thermal loads are required.

However, the specific fuel specifications will be provided by the combustion technology supplier, and it is always advisable to refer to European or national technical regulations to ensure compliance with the minimum requirements of biofuels for both physical parameters (dimensions, moisture, density) and chemical parameters (calorific value, energy density, ash content, or other chemical elements).

Designing biomass heating systems

We can outline the activities required for the design of a biomass heating system in the following phases:

  • ideation;
  • definition;
  • implementation;
  • delivery;
  • management.

Ideation phase

In the design process of a biomass system, the initial phase includes a preliminary evaluation of the project’s sustainability for a specific site. A common mistake in this phase, especially when the experience of the involved parties is limited, is to base decisions on solutions that have worked elsewhere. However, this approach is not always recommended because each project is unique and requires a tailored approach.

Each project has its own peculiarities, so it is essential to design and adapt each element to the specific needs of the users.

To avoid excessive costs in the initial phase of the process, you can proceed in stages: a pre-feasibility phase, in which all inappropriate sites must be filtered out and excluded from the project, and subsequently a more detailed feasibility study that aims to quantify some of the key variables that allow for a better evaluation of the proposal.

Definition phase

During the project definition phase, it is necessary to further delve into the technical and financial details.

In this phase, decisions are made regarding the size of the generator and the type of fuel used. These factors, which influence all other aspects of the system, must be identified based on a clear understanding of the heat requirements and the specific user requirements of the site.

Identifying potential problems that could compromise the realization or operation of the project in advance can save time and money in later stages.

Common problems include difficulties in fuel delivery at certain times of the year or the presence of communities sensitive to noise or vehicular traffic, which could influence design decisions.

Consulting stakeholders during this phase can be extremely useful to gather useful information and address foreseeable obstacles. Stakeholders may include site managers, plant maintenance or construction staff, biofuel suppliers, and end-users. Even local residents can be considered stakeholders if they are affected by plant activities.

Implementation phase

During the project implementation phase, roles are assigned to different actors, and various activities are carried out, involving the client.

Maintaining good communication among all participants in this project development phase is important. The interconnection between the different elements of the project implies that any changes from the specifications established in the initial phase could affect other system components.

Regular meetings with stakeholders are essential to adequately consider any changes to the project without compromising the future functionality of the plant and without creating problems for the parties involved. This phase concludes with the commissioning of the boiler.

Delivery phase

Once the system is built, the plant is handed over to the operator who manages it or directly to the owner.
Modern biomass plants are usually efficient, but their performance can significantly decrease if operators do not have adequate skills or are not properly trained.

The main aspects on which personnel should be trained include proper management, routine maintenance, ash cleaning, possible failures that may occur during normal operation. The delivery phase naturally includes the necessary testing to verify the performance compliance with what was defined in the design phase.

Management phase

After the plant is delivered, there is a period of familiarization with the equipment and its use, especially considering variations in thermal loads and fuel quality.

Monitoring the plant’s performance is also a valuable tool for early detection of any problems within the system.

Boiler sizing

Properly designing the boiler is essential to ensure optimal system operation. While fossil fuel boilers, such as gas boilers, can dynamically adapt to heat demand, biomass boilers are less reactive and require accurate sizing to avoid inefficiencies and operational issues.

Usually, these boilers are oversized to reduce the risk of not fully meeting user needs.

Biomass boilers are less reactive, and if oversized, they can generate continuous cycles of ignition and shutdown, with negative consequences such as poor combustion quality, low efficiency, increased emissions, excessive component wear, and a higher likelihood of malfunctions.

To solve this problem, a distinction must be made between the average thermal load and the peak load already during the system design phase.

According to this approach, boilers are then sized to meet only part of the peak load, with performance varying depending on the model and technology. For example, a wood chip boiler should operate, in regime, at least at 30% of its useful thermal power, while a pellet boiler can decrease to 25%.

An optimization of the system, also from an economic point of view, may involve the use of a smaller-sized boiler capable of meeting 50-60% of the peak load. Additionally, to manage additional thermal loads, other smaller boilers can be integrated, fueled with fossil fuels or biomass and equipped with thermal storage. The latter is highly recommended as it increases the system’s efficiency; a boiler sized for 50% of the peak load can meet up to 85% of the demand when combined with adequate thermal storage.

When heat demand is more constant over time or does not show significant variations, the system can be sized to autonomously cover peak demands.

In cases where the heating system is part of significant renovation works, it is advisable to review the thermal loads downwards because often measures to improve building performance, such as improving thermal insulation and reducing heat losses, are introduced.

Pros and cons of a biomass boiler

Choosing a biomass boiler involves some pros and cons to consider. Among the main advantages are:

  • low environmental impact, due to the very low level of CO2 emissions;
  • effective savings on consumption;
  • low cost of pellets or biomass in general.

Among the disadvantages, instead, there are:

  • bulky size;
  • need for an external technical room;
  • regular maintenance and cleaning;
  • requirement for a separate flue;
  • inconvenience related to fuel refilling;
  • higher cost compared to gas boilers.

Among these disadvantages, the most significant is undoubtedly the need for an external technical room. Unlike a room heater, the boiler requires installation outside the building.

How to maintain a biomass boiler?

Routine maintenance of biomass technological systems, including the boiler, must be carried out at least once a year. This maintenance concerns both the heat generator and the flue system. Performing proper maintenance is important to ensure maximum efficiency and minimize harmful emissions into the atmosphere. Furthermore, biomass heating systems must undergo an energy efficiency control report by a qualified maintainer.

How much does it cost to install a biomass boiler?

The cost of a biomass boiler can vary significantly based on various factors, including the brand, model, power, system complexity, and required installation. In general, biomass boilers may have a higher initial price compared to traditional gas boilers, but it is essential to consider the potential long-term savings on energy costs and the possible access to tax incentives or energy efficiency financing.