Organic Rankine Cycle technology can be used to generate heat and power from renewable sources. Over the last 10 years ORC technology has been successfully demonstrated for application in small, decentralized biomass CHP plants, as Ilaria Peretti writes.
Over the last 10 years, ORC technology has proven its value for small, decentralized biomass CHP plants up to around 5 MWe.
Typical systems are based on the following steps:
- Biomass fuel is burned in a combustor made according to the same, well-established techniques used for hot water boilers. These combustors and accessories – elements such as filters, controls, automatic ash disposal and biomass feed mechanisms – are safe, reliable, clean and efficient.
- Hot thermal oil is used as heat transfer medium, providing several advantages, including low pressure in the boiler, large inertia and insensitivity to load changes, simple and safe control and operation. The adopted temperature (about 315°C) for the hot side also ensures a very long oil life. Using a thermal oil boiler avoids the need for licensed operators, as required for steam systems in many European countries.
- An Organic Rankine Cycle turbogenerator converts the available heat to electricity. Through the use of a properly formulated working fluid and an optimized machine design, both high efficiency and high reliability can be achieved. The condensation heat of the turbogenerator produces hot water at typically 80°C-120°C, a temperature suitable for district heating and other low-temperature uses such as wood drying and cooling through absorption chillers.
The ORC unit is based on a closed Rankine cycle, using a suitable organic fluid. In Turboden’s standard biomass cogeneration units, silicon oil is used. Figure 1 shows a cogeneration plant in a biomass application.
ORC technology functions similarly to a traditional steam turbine, but instead of water, the ORC system vapourizes a high molecular mass organic fluid, offering cycles with superior electric performance (up to 10 MW) and several mechanical advantages:
- slower turbine rotation
- lower pressure
- no erosion of piping and blades.
The ORC turbogenerator is pre-assembled onto one or more skids and can be easily transported. The thermodynamic cycle and relevant components are illustrated in Figure 2. Figure 3 illustrates the differences between turbines that work with water and turbines that use high molecular mass working fluid. Advantages of ORC turbogenerators are:
- high cycle efficiency
- very high turbine efficiency (up to 90%)
- low turbine mechanical stress due to low peripheral speed
- low turbine RPM, allowing the direct drive of the electric generator without gear reduction
- no erosion of blades, due to the absence of moisture in the vapour nozzles.
- simple start-stop procedures
- automatic and continuous operation
- no operator attendance needed
- quiet operation
- high availability. Partial load operation down to 10% of nominal power
- high efficiency event at partial load
- low O&M requirements: about 3-5 hours/week
- long life.
APPLICATION OF ORC UNITS TO WOOD PELLET PRODUCTION
In conventional, heat-only plants for pellet production, belt or rotary dryers are used to dry sawdust to the necessary moisture content for pellet process. – see Figure 4. Here, heat-only pellet production plants are compared with a CHP solution based on a biomass combustion system, an ORC unit and a belt dryer fed by hot water coming from the ORC condenser.
In a pellet production plant based on a biomass combustion system and a direct rotary dryer, hot gas coming from the combustion chamber is diluted with an ambient air stream in a suitable mixing chamber to obtain gas at a temperature compatible with the highest inlet temperature acceptable in the dryer (usually around 300°C). Higher gas temperatures at dryer inlet would lead to lower pellet quality, and also increase the risk of sawdust firing.
A feed system supplies the drum dryer with the wet biomass, which comes into direct contact with the hot drying gas, thus evaporating the excess water content up to the process requirements.
A typical pellet production plant based on a biomass combustion system and a rotary dryer usually includes:
- biomass burner (hot gas generator)
- mixing chamber including hot gas distribution device
- wet biomass feed device
- drum dryer
- dried product discharge system
- drying gas cleaning unit
- fire detection and sprinkler system
- system control device.
As an alternative to rotary dryers, indirect belt dryers are often adopted in pellet production plants – see Figure 5. This technology requires a hot water boiler, generally biomass fuelled. The hot water produced through the belt dryer is utilized to generate a hot air stream that flows into a special web belt, thus evaporating the water content of the sawdust.
Figure 4. Schematic diagram of a biomass heat only plant for pellet production based on direct rotary dryer
Therefore, within the belt dryer, there is no direct contact between hot combustion gas and wet biomass, since the hot air stream used as drying medium has not been mixed with hot combustion gas. In the dried product, the dust, particle and ash content that usually comes from combustion gas, is avoided.
Figure 5. Schematic diagram of a biomass heat only plant for pellet production based on a belt dryer
Furthermore, due to the lower drying air temperature (usually between 70°C and 110°C), the risk of sawdust firing is also much reduced.
Figure 6. Schematic diagram of a CHP biomass plant for pellet production based on belt dryer coupled to an ORC unit
A typical pellet production plant based on a belt dryer usually includes:
- hot water biomass boiler
- wet biomass feed device
- hot air generation (hot water/drying air heat exchanger).
- drying web belt
- dried product discharge system
- drying air cleaning unit (if required by local regulations)
- fire detection and sprinkler system
- system control device.
The following part of this study looks at a CHP solution based on a biomass ORC unit and belt dryer. A typical pellet production plant based on a biomass combustion system and an ORC unit requires minor changes to conventional heat-only plant for pellet production with belt dryer.
This means that, in addition to the installation of CHP biomass pellet plant, retrofitting of an existing pellet plant based on a hot water boiler coupled to belt dryer can easily be implemented, simply by replacing the hot water boiler with a thermal oil boiler feeding the ORC unit – see Figure 6. Hot water will be actually available downstream the ORC condenser.
Financial results are calculated in terms of the discounted payback time of the additional investment required by the cogeneration solution. The sensitivity of the results to variations in plant size and electricity value is investigated.
Figure 3. Differences between turbines that work with water and turbines that use high molecular mass working fluid
The following boundary conditions are assumed:
- constant fuel cost (biomass): €20/MWh (US$26/MWh)
- equivalent electricity value variable between €0.10 and €0.24/kWhe ($0.13 and $0.31/kWhe)
- ORC size: from Turboden 4-CHP Split to Turboden 22-CHP Split (about 2.2 t/h – 10 t/h pellet production)
- constant hot water feed temperature to belt dryer: 90°C.
Biomass costs at this level will strongly impact the economic feasibility of the biomass based heat-only solutions assumed as reference case.
The results show that, in this difficult scenario, plants with an installed power above 1500 kWe exhibit a good feasibility regardless of the technical solution for the dryer considered as reference case for the heat-only plant.
For equivalent electricity values around €0.20/kWhe ($0.26/kWhe), plants in the power range from 600 kWe also remain competitive. A higher fuel price has a strong impact on smaller plants in the range below 600 kWe, which can be considered competitive only if indirect air drying with a belt dryer is assumed as reference technology for the heat-only plant.
The economic analysis shows cogeneration units based on thermal oil boilers and ORC units, coupled with indirect belt dryers as heat suppliers for pellet plants, are viable economic option under a broad range of conditions.
Plants starting from 2.2 t/h pellet production can be competitive starting from an electricity value of €0.18/kWh ($0.23/kWh). Due to the additional income from electric energy generation, this solution also reduces the risk from higher biomass costs, being able to generate positive cash flows at much higher fuel costs than the heat-only solution. These operating conditions exist in many European countries where new pellet production capacity is under construction, such as Germany, Austria, Italy, Belgium and the UK.
For a pellet plant size above 8 t/hr, a cogeneration plant may be a good solution in countries with incentives for renewable energy production, especially if fuel costs are negligible. In this case the feasibility is good starting from electricity values in the range of €0.10kWh ($0.13kWh), which can be considered a long-term average buying rate for industrial customers in many countries. In particular, this gives excellent medium-term application opportunities for new plants in Eastern Europe, Russia and North America.
The operating conditions described apply to a large share of the new production capacities planned worldwide, both concerning economic conditions (electricity value and biomass cost) and plant size. The available operational data confirm that the actual process efficiencies are even higher than the figures assumed in this study.
Land Energy to use biomass power in wood pellet production
Wood pellet firm Land Energy is developing a series of new production plants, as it seeks to move from being a trading company towards becoming a manufacturer of biomass fuel. The company has ordered a biomass CHP system to use wood fuel to generate the power and heat needed to run the manufacturing process. A CHP system has been ordered from Turboden to use Organic Rankine Cycle technology to generate about 2 MW of electricity and about 9.5 MW of heat energy.
The company aims to convert a potato processing site in Wombleton, North Yorkshire into a wood pellet plant, and has lodged plans with Ryedale district council. A similar project at Presteigne in mid-Wales would take £10 million ($15 million) in investment to convert an animal feed plant into a wood pellet facility. Land Energy’s primary markets for its wood pellets are likely to be domestic biomass heating and small commercial heating systems in schools and offices.
Currently, the company has wood pellets delivered to its Yorkshire site before distributing the project to end users.
Land Energy has signed an agreement with German CHP specialists Gammel Engineering as plant designer for all its future production plants. The company has already had experience developing more than 150 biomass CHP projects, mainly in Germany and central Europe.Innovative technology
The technology involves biomass fuel being burned to heat oil within a boiler, which is then used within an Organic Rankine Cycle unit to vapourize an organic working fluid that drives a turbine to generate power. The ORC also heats water alongside the power generation, which can be used for space heating purposes and hot water supply. Waste heat from the system is recovered to continue heating the boiler, with the organic working fluid cycling back around the system to keep driving the turbine.
Studies of the system have suggested that turbine efficiencies of around 88% can be achieved, while there are also said to be advantages from using a system that does not rely on high-pressure steam to drive the turbine. There are currently 113 Turboden plants already in operation, including 15 in the pellet sector across Europe, where they are used to supply the heat needed for drying sawdust to produce wood pellet fuel.
Ilaria Peretti is country manager for the Biomass Department of Turboden, in Brescia, Italy.Email: Ilaria.firstname.lastname@example.org
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