Multi-fuel boilers

A multi-fuel plant is capable of burning a wide range of biomass and sorted waste fuels and is designed to meet both current and future waste fuel requirements.

Our multi-fuel technology is the result of decades of experience with biomass and waste-to-energy. With more than 80 years of waste-to-energy innovation and more than 30 years of biomass experience, we have developed a unique integration process for these fuels. Our experience has given a singular ability to combine know-how and technology from both business sectors and this has resulted in the introduction of our new multi-fuel technology.

Landskrona Stad, Landskrona, Sweden

Multi-biofuel steam boiler – the new generation

Based on our Strängnäs plant and previous operational experience, we were able to optimise the design of our multi-fuel and offer Landskrona a “second generation,” state-of-the-art multi-fuel plant. The Landskrona boiler plant is, in fact, based on optimised experiences with our plants in Örebro, Strängnäs, and Fiskeby in addition to our decades of waste-to-energy experience.

Detailed engineering with Computational Fluid Dynamics (CFD)

In order to achieve the best possible design of the system, we used Computational Fluid Dynamics (CFD) as a tool for detailed engineering. CFD simulation is an effective method of evaluating different design alternatives that are otherwise too expensive, time consuming, or impossible to test. CFD is a method used for solving the fluid flow equations with numerical methods used for optimising the flue gas modelling.The water-cooled grate is integrated in our tree-pass boiler with vertical superheaters and a vertical economiser. The high reliability of boilers with this design and layout has been proven during the last 10 years at multi-fuel fired plants such as Örebro.
Our secondary air system, VoluMix™ has been designed via CFD.

CFD illustration of the Strängnäs plant

CFD illustration of the multi-fuel fired plant in Strängnäs, Sweden

The Landskrona boiler plant design

The fuel is fed into the combustion chamber through a chute with a pusher. The chute and pusher are waste-to-energy technology, which gives a sturdy and robust feeding system. The grate is a water-cooled vibrating grate.
The primary air is preheated to approximately 170°C in order to stabilise the combustion and enable the system to handle variations in size and heating value.
The furnace design is similar to the centre flow furnaces used in waste-to-energy plants (e.g. Reno-Nord). This configuration provides a long grate for burning out the ashes and ensuring a good mix¬ing of the flue gases leaving the furnace by the secondary air system at the entrance of the post combustion chamber.

Inconel® for corrosion protection

Since multi-fuel often contains waste material, the plants have an increased risk of corrosion. In order to avoid problems with slagging at the furnace bottom side walls, these are kept free of refractory. Instead, Inconel® cladding is used on the bottom part of the furnace to prevent corrosion.
The post-combustion chamber is covered with castable refractory in order to maintain two seconds of residence time above 850°C in the entire cross section of the post-combustion chamber. Fur¬thermore, the residence time zone extends the ammonia injection temperature window, enabling good NOx reduction.
The boiler is designed with a second radiation pass with two baffle walls in order to ensure a low uniform temperature of the flue gas before the convection part.  The second pass can be cleaned with our fully automatic Vølund Online Boiler Washing System during operation.
The roof and the upper part of the rear wall in the second pass are lined with Inconel® as well.
The convection heating surfaces are placed in the third pass.  The flue gas temperature is approximately 700°C at the entrance to the third pass, which is a safe temperature n order to avoid significant fouling of the heating surfaces.  The convection pass is cleaned with steam soot-blowers.
The economiser is a free-standing vertical type with soot-blowers.

Advantages to this technology

Some of the many advantages to our water tube boiled include:

  • High water circulation
  • Unheated downcovers
  • Good adjustment of circulation with load variation
  • Relatively unaffected by pressure variations
  • High-quality steam
  • Self-supporting structures
  • Simple wall structure
  • Fully welded boiler
  • Standard tubes and materials used for construction
  • Suitable for installation of all kinds of soot-cleaning equipment
  • The furnace is designed in accordance with the requirements made by the CFD modelling
  • Fully drainable

Multi-biofuel steam boiler Strängnäs Energi AB in Sweden: 2 types of fuels, 2 different sets of steam parameters

In 2008 we provided a multi-fuel boiler for Strängnäs Energi AB in Sweden. The Strängnäs plant is designed for two different sets of steam parameters in order to improve the electrical efficiency when burning clean biomass. When burning waste, the steam parameter is adjusted to avoid corrosion in the boiler.
In order to benefit from green certificates for the initial 10 years’ operation of the plant, virgin wood (GROT) and waste wood chips are the main fuels. The plant is designed to burn refuse-derived fuel (RDF) based on wood, paper and plastic, and a fraction of peat.
The Strängnäs plant is equipped with a water-cooled vibration grate due to its ability to handle low melting point metals.

The boiler design

The boiler has two empty radiation passes which cool the flue gas sufficiently before the convection part to avoid clogging the convection pass. The superheaters and economiser are in a vertical arrangement. The steam parameters have been adjusted to avoid corrosion in the boiler when burning waste fuels.
The steam is expanded through a steam turbine with a 13 bara extraction of 7MW process steam. The energy produced by the boiler is to be used for high-efficiency electricity production, process steam for neighbouring industries and district heat heating for the town of Strängnäs.

Read more about Strängnäs here