Unlike most biomass plants, the BioFlex160 is designed to be able to directly accommodate wet biomass. It is designed as a standardized plant that is more cost-effective than lignite, comparable to a coal-fired power station, and has superior flexibility and reduced material intensity, writes Aleksander Kovacevic


Biomass is a difficult fuel. It is variable in quality, moisture content, chemical composition, weight and impurities. It is not easy to handle, store or transport.

Most people perceive biomass as a low calorific value fuel: one cubic metre of biomass is typically equivalent to a cubic metre of compressed natural gas (CNG) (200 bars) in energy content, but it weighs three times more.

Alternatively, one cubic metre of biomass is similar in weight to a cubic metre of liquefied natural gas (LNG), but biomass has around one third of the energy content. However, both CNG and LNG require special exploration, development and production techniques and also specific handling and transportation infrastructure, while biomass is comparatively easier to produce and transport, as it is considered to be a dry bulk cargo.

The conventional view of biomass is that it is not transportable over long distances. However, we are now witnessing the commercial transport of biomass across oceans and continents. New transport technologies, and increases in transport efficiency and speed (turnover) of vessels, have made the bulk transport of biomass affordable to large-scale power plants (≥ 100 MWe) that have a much higher combustion efficiency than the typical small biomass plants (<50 MWe). The trade-off between transport costs and efficiency of the plant is now definitively resolved in favour of large scale biomass plants.

Small scale ‘biomass-to-power’ or ‘biomass-to-fuel’ plants constructed on the basis of public subsidies and land use patterns associated with such plants are increasingly regarded as stranded assets. The opportunity to use land for bulk production of biomass that is suitable for efficient large scale sustainable ‘biomass-to-power’ arrangements is emerging as an economically competitive alternative. Such competitiveness depends crucially on advances in plant efficiency and being fit for purpose.

Fluidized bed boiler technology has now advanced to such an extent that the technical challenges of burning biomass are mostly resolved. The most advanced vendors are now offering fluidized bed boilers at utility scale, and with boiler efficiencies that are comparable to that of fossil fuel boilers.

Co-firing biomass with fossil fuels in conventional steam cycle plants and conversion of these plants to burn biomass are also well established practices, although with some reduction in efficiency. However, steam parameters mostly remain comparatively lower than the latest fossil fuel plants. This limits the overall plant efficiency in a conventional steam cycle. Fluidized bed boilers do, however, have the advantage of being able to make use of various other opportunity fuels.

The ability to generate electricity from biomass creates an opportunity cost in addition to the use of land to produce (liquid or gaseous) biofuels, since the use of electricity in transport is emerging as more efficient than producing liquid fuels for combustion in mobile combustion engines.

Today’s consensus is that public subsidies have distorted the biomass market in favour of small power plants and expensive biofuels. Increasingly, the public policies that have resulted in this position are understood to lead to higher opportunity costs and consequent project risk. The growing strategic importance of land resources, land use and sustainable biomass production as a result of the Paris Agreement on Climate Change indicates that the efficiency of the value chain from land use to electricity generation will become a critical determinant of economic competitiveness.

There is a tendency between engineers and developers to view biomass as a lower-grade solid fuel that is suitable for use in a conventional steam cycle arrangement, subject to some boiler or burner modifications. This reflects the original approach to the use of gas in a conventional steam cycle for the generation of electricity, prior to the development of a more suitable – and more efficient – combined cycle, resulting in today’s highly efficient combined-cycle gas turbine arrangements. In summary, each different fuel type requires a different thermal cycle to achieve optimal results.

Steam and compression

Learning from this simple lesson, Castle European has designed a thermal cycle that is tailor-made for biomass-to-power: the Co-integrated Steam and Compression Cycle or CSCC®. This new cycle significantly exceeds the efficiency of a simple steam cycle when converting fuels with biomass properties to electricity. It responds to the volatility of fuel qualities and facilitates active optimization of processes that co-integrate into a comprehensive power plant.

Castle European is a UK company, based in Edinburgh, Scotland. Together with its industrial partners, Castle European has used this advanced cycle to design an innovative power plant using proven equipment. The result is the BioFlex160, a 160 MW modular power plant that may be placed in series and supplemented with a range of ancillary products including an advanced district heating module.

COP21 made climate change mitigation and remediation a political and market reality

Credit: United Nations

All components of the BioFlex160 are fully proven in existing industrial/utility scale arrangements operating at heavy duties to support >90 per cent availability. Furthermore, all components are manufactured by market leading vendors and accompanied by appropriate performance guarantees. Analysis of the simulated performance of the plant in various operating regimes reveals the following comparative performance:

Unlike the majority of biomass power plants, BioFlex160 is designed to be able to directly accommodate wet biomass, through energy efficient integration of an optional biomass dryer module at the power station site.

BioFlex160 is scalable up to around 350 MW, using the same standardized layout. The CSCC® thermal cycle supports supercritical steam parameters if desirable. However, it is difficult to identify the economic benefits and efficiency gains that would justify the increased investment cost for higher grade materials and more complex plant construction processes.

Castle European’s advanced district heating module combines with any BioFlex plant in a Decoupled Heat & Power (DHP) arrangement. An eventual increase in density would require certain technology advancements that are currently in the development pipeline.

Any CSCC® power plant is inherently fuel flexible, while its downstream arrangement augments the part-load power generation flexibility that the boiler is capable of delivering with minimal efficiency penalty. As a result, BioFlex is very suitable for load following in properly managed smart grids. In addition, the BioFlex160 module has black start capability, is resilient to most grid failures and can deliver an unusually wide variety of ancillary services compared to other power plant designs.

It is able to co-fire various opportunity fuels with biomass, and this capability can be further enhanced with certain boiler modifications and options. It is suitable for installation at any location where sufficient transport infrastructure is available to deliver the required volumes of biomass.

In a DHP arrangement, BioFlex160 may deliver up to 240 MWth of useful heat for industrial drying, industrial heating or district heating. This arrangement, in conjunction with an appropriate heat distribution management tool, provides extraordinary flexibility capable of supporting intermittent renewables: a single BioFlex160 DHP unit supports the intermittency expected from more than 600 MW of installed wind power capacity.

As a result, this plant could be considered as an essential ingredient to any ‘smart city’ concept. Taking into account that urban and metropolitan areas are still growing and absorbing more population and economic activity, smart urban energy emerges as a critical component of the resilient energy system of the future.

The International Renewable Energy Agency says the biomass market could grow to 165 GW by 2025

Credit: Mott MacDonald

Castle European’s team is experienced in smart heat distribution management and has proprietary tools that deliver tailor-made solutions to any urban area with or without existing conventional district heating infrastructure. As a result, any DHP module attached to a BioFlex (or other CSCC®) power plant is a viable, proven, large scale and economically competitive renewable heating solution.

The CSCC® power plant is inherently carbon capture ready. Its flue gas stream has a high CO2 density and other characteristics that make it more suitable for carbon capture than a coal fired plant. BioFlex160 is designed as ‘carbon capture-ready’ in its standard layout. As a result, any CSCC® power plant may become ‘carbon negative’ at a relatively affordable cost. This provides an opportunity to deliver a BECCS (Bio Energy CCS) carbon offset service to the market that appears to be needed in most climate change models.

Heading to market

BioFlex160 is designed as a standardized power plant that is more cost-effective than lignite and comparable to a coal fired power plant, with superior flexibility and reduced material intensity.

There are scientific and technical disputes around climate change and the role of power generation. However, with the 2015 Paris Agreement, active climate change mitigation and remediation is now a political and market reality. Without prejudging the scientific and technical basis of the debate, developers need to acknowledge the difficulty in obtaining financing for coal power projects. Intermittent renewables and demand volatility are already realities in many markets. Dispatchable power plants of the future will therefore need to have a higher degree of generation flexibility and greater fuel flexibility in order to optimize economic outcomes.

The consulting firm GlobalData estimates current biomass-to-power installed capacity to be around 106 GW. The International Renewable Energy Agency (IRENA) forecasts that the market could grow to 165 GW by 2025, and may continue to increase towards 2030. On an annualized basis, this is equivalent to a growth rate of 10 per cent per annum.

AVOID2, a UK government-funded climate change research programme involving a multi-disciplinary consortium including the Grantham Institute, has presented a requirement to install 52 GW-100 GW of new biomass power capacity with BECCS per year for 10 years in order to limit global temperature increases to 2oC.

The installation rate of new coal power capacity during the last 10 years was between 66 GW and 98 GW per year. Installing new biomass power in line with AVOID2’s estimates would result in the replacement of 1627 GW of currently available coal power capacity within 10 to 15 years, taking into account the forecast reduction of the market due to energy efficiency, declining utilization rates and the increase in market share of intermittent renewables.

AVOID2 calculates a requirement for 1000 GW of installed power with BECCS capacity. To be economic, such plants should have a high utilization rate and produce about 8000 TWh of electricity per annum. Biomass availability may range between 60 EJ and 80 EJ per year. Producing this volume of electricity from that volume of biomass requires an efficiency of at least 40 per cent in combination with a highly effective BECCS.

If even 50 per cent were correct, this massive increase in biomass installation capacity and introduction of BECCS will require at least the following:

• Deployment of technologies and components that are available now;

• Rededication of the majority of the world’s current and existing steam power manufacturing capacity for the production of biomass plants;

• Significant standardization of large scale biomass-to-power units in order to streamline delivery;

• Commoditization of CO2 and CO2 offset markets in order to boost biomass-to-power profitability and competitiveness as well as to motivate its rapid deployment.

Research and development into the optimization of the carbon capture process from CSCC® power plants is ongoing. However, a combination of existing technologies already presents a viable, economic solution based on known equipment.

Furthermore, CSCC® has a modest material intensity. This provides an opportunity for the recycling of materials used in retiring coal-fired power plants at minimal energy penalty, contributing to a decrease in industrial energy demand and contributing to the decarbonization of the steelmaking process. The decrease in continuous industrial electricity demand, the decrease in utilization rates, the increase in demand for scrap steel and the internalization of external costs (including perceived climate change and health impacts) are likely to shape the market place for biomass CSCC® power plants.

Security and resilience

The CSCC® power plant is intended to facilitate system level change with smart electricity and heat distribution, and to provide security and resilience of supply to urban areas. Its deployment can reduce transmission grid loads and facilitate increased intermittent renewable energy.

Transport of biomass to the urban vicinity or metropolitan areas provides an economy of scale and logistics (storage, drying, use of residuals, etc) for construction wood that will increasingly become a key material for urban renewal as well as an immediate cost-effective carbon sequestration option. A CSCC® power plant is intended to use locally available fuel to the maximum extent possible, enhance land use and local employment on a permanent basis and inject new vitality to the local economy.

In this context, CSCC® is highly suited to any energy system that requires dispatchable power capable of interfacing with intermittent renewables in a highly flexible and efficient manner, values security of supply, is carbon neutral (potentially carbon negative, enabling carbon offset of gas or transport) with the added benefit of being able to supply renewable heat at scale, all without subsidy in the open market.

Aleksander Kovacevic is consultant technology development director at Castle European and an author with the Oxford Institute for Energy Studies