The furnace is alight and the feed is running: why the future is bright for biomass CHP plants
Biomass combined heat and power plants (BMCHP) produce electrical energy from biogenic solids – and additional heat that can be used efficiently. For the combustion process to run smoothly, the mass must first be processed mechanically to obtain a homogenous unit that is free from extraneous material. Vecoplan supplies systems for storing, dosing and conveying the different material for this purpose. The machine builder tailors and matches these systems to the particular application – for example, at Burmeister & Wain Scandinavian Contractor, Vecoplan has designed fuel charging and processing systems at five sites for the Danish manufacturer and BMCHP operator.
In BMCHP, untreated wood chip, mature timber, agricultural waste and cut branches are incinerated in boilers at over 1,000°C to heat water. The resulting steam drives a turbine that is coupled to a generator to produce electricity. Some steam is left over and is used to heat water for district heating by means of a heat exchanger. The energy of the biomass is therefore used twice: once for producing electricity, and once for recovering heat. In Germany in 2016, around 13% of all electrical energy was obtained from renewable sources such as this, and also from wind, water and solar power. Just less than 80% was obtained from fossil fuels such as oil, gas and coal, and approximately 7% from nuclear power.
“Renewable energies are becoming ever more important for coping with the increasing demand for electricity worldwide. Oil is coming to an end and is also more expensive, the combustion of coal gives rise to greenhouse gases, and nuclear waste is difficult to dispose of,” says Ralf Rosenkranz from Vecoplan, citing three examples. However, availability plays a major role when it comes to regenerative energy sources. Wind and water power cannot be used everywhere, and solar energy is highly dependent on the sun’s radiation. The importance of biogenic fuels for generating electricity reliably and continuously therefore increases further, as mature timber in particular is available in sufficient quantities in the short-, medium- and long-term. The main markets for BMCHP are currently still to be found in Europe – but Africa, the USA and Asia are on the way up.
EFFICIENT PROCESSING BEFORE COMBUSTION
Demands on power stations that have to provide an increasing amount of electrical energy are therefore escalating. They must work reliably, efficiently and safely, often around the clock and seven days a week. For smooth and economic combustion, the biogenic solids must first be homogenised, that is to say mechanically processed before combustion. “For example, the charging material may have a very high water content. Also, there are often foreign materials such as nails that contain iron, or leftover wood with over-long pieces in the mass. These can damage the machines and disrupt the combustion process,” explains Rosenkranz. Vecoplan develops the necessary systems and components to prevent this.
The machine builder’s tasks also include services such as planning, consultation, overall project management, and comprehensive service as well as installation, commissioning and maintenance work. This enables the company to adapt its technology to the particular application. “Biomass power plants are usually designed for long-term operation,” reports Rosenkranz. “The fuel that is to be used is exactly defined for every plant.” Although the type of material is known, its source and therefore its quality may vary, particularly when transport service providers do not always deliver the biomass from the same suppliers.
FREE FROM EXTRANEOUS MATERIAL
When the pre-shredded biomass arrives at the BMCHP, Vecoplan provides various components, such as pull rod dischargers, bunkers and its patented Toploader storage system, for receiving the material depending on the way it is discharged. These are filled, for example, from the open side by semi-trailers, tilting containers or wheel loaders on a flat floor surface. Discharging to the subsequent conveyor equipment takes place on the opposite side via an inclined ramp.
In most cases, the fuel is transported in sizes P63 or P100. The maximum length of the particles is therefore less than 200 or 250mm. Foreign bodies, such as metals and minerals, must be removed to obtain a homogenous mass. For this purpose, Vecoplan installs magnets in the lines to remove ferrous metals such as screws and nails from the mass. Depending on the material, non-ferrous separators are also incorporated into the line. “For example, a high aluminium content can significantly impair the combustion process,” explains Rosenkranz. Disc or star screening machines are used to separate over-length wooden slats from the biomass. The excess lengths removed go directly to slow-running single or double shaft shredders from the VAZ or VNZ ranges. These work particularly energy efficiently.
The processed biomass is usually transported by open trough belt or closed drag chain conveyors. “In the case of drag chain conveyors, an endless chain with dogs runs in a closed trough. Depending on the type of dog and trough, they can move the material horizontally, on an incline or vertically. For storage, Vecoplan supplies different systems, such as pull rod dischargers, loading and unloading conveyors, top loaders, concrete silos or open-air storage, to suit the volumes transported. “We size all components of the process line according to the charging cycles and the fuel requirement,” says Rosenkranz.
INDIVIDUALLY MATCHED TECHNOLOGY
Burmeister & Wain Scandinavian Contractor also has faith in these systems. The Danish firm tasked Vecoplan with designing an individual charging and processing system for new and mature timber at five locations in the UK.
“We carried out the engineering, supplied the machines and the electrical controls, took responsibility for the steelwork and installation and subsequently trained the staff,” explains Rosenkranz.
As well as high availability, technical safety presented a particular challenge. One of the sites is in Snetterton in the English county of Norfolk. Vecoplan incorporated pendulum rod dischargers to be able to tip the already shredded material safely. The suspended rods enable bulk and free-flowing material with a maximum grain size of 40mm to flow uniformly and controllably onto the removal station. Lumpy material is diverted by the oscillating bars, and dust development is reduced by the low drop height.
At three other locations, the material is loaded via push rod dischargers. From here, the biomass goes either directly to a vibratory conveyor, which serves as a discharge conveyor, or directly to a conveyor belt. Dosing conveyors are used at the fifth plant. As intermediate bunkers, these are capable of depositing different materials in exact quantities onto the conveyor system. To remove foreign materials, the lines in all plants are equipped with magnets and non-ferrous separators. The mass passes to star or disc screens via drag chain conveyors. In Snetterton, for example, samples are also taken automatically to ensure that the quality is right.
Here – unlike in the other BMCHPs – the material is buffered in boxes. These are equipped with loading and discharge conveyors. A controllable distribution and discharge conveyor suspended on steel wire ropes fills and empties the containers evenly. The two adjacent storage boxes can be handled by means of a chassis. Worm conveyors and drag chain conveyors transport the biomass onward to the boiler. “We have also implemented dryer charging in one of the plants,” says Rosenkranz. The material is received via a push rod discharger and the dryer is fed via worm conveyors and drag chain conveyors. A drag chain conveyor is also responsible for discharging the dryer.
A SINGLE SOURCE AND A SINGLE POINT OF CONTACT
When it came to equipping the new biomass combined heat and power plant, Burmeister & Wain placed its entire trust in Vecoplan. Design, planning and implementation were carried out almost simultaneously from a single source. There was therefore only one point of contact responsible for all projects. Meetings could be combined, installation and commissioning staff were able to service several sites, and spare parts and consumables are suitable for all Vecoplan systems. “We have developed an efficient and tailor-made solution for each site,” explains Rosenkranz.
“We were also able to comply with the strict quality standards that are typical of the country as well as meeting the high demands on project management and documentation.”