Is CFB the key to scaling up biomass?

The sustainability and green credentials of large-scale biomass-fired power generation have been questioned of late. According to Robert Giglio, advanced circulating fluidized bed boiler technology could well hold the answer.

Nations worldwide are taking steps to counteract global warming by reducing greenhouse gas emissions, with the European Union taking a lead role by setting its 2020 targets. In addition to ongoing efforts to increase efficiencies in power generation boilers, increased utilization of biomass fuels is central to achieving these targets.

Fossil fuel combustion releases carbon that was stored long ago in the earth, increasing net levels of CO2. Biomass, in contrast, is generally considered carbon-neutral because CO2 released from its combustion was previously removed from the atmosphere by photosynthesis, thus maintaining an equilibrium CO2 level.

An evolving biomass market

Until fairly recently the combustion of biomass fuels was predominantly limited to small industrial boilers located near the biomass source. This was mainly due to the limited availability of high-quality biomass and the lack of an efficient, large-scale boiler technology capable of reliably burning a wide spectrum of biomass fuels.

Today, the biomass marketplace has changed from local to global. Biomass fuels are dried and compressed into pellets to improve the economics and logistics of moving them around the globe. Pellets are currently being produced in Brazil, the US and Canada, for example, for transportation to Europe and Asia.

To date, biomass combustion systems primarily fire forest industry residues such as wood chips, sawdust and bark. However, as the demand for biomass fuels grows and the price for wood-based biomass rises, there is a growing interest in biomass from other sources, e.g. agricultural biomass, biomass residue and biomass wastes.

Some countries promote biomass use through environmental incentives that offset its additional transportation and logistics costs, while others only permit new plants that can use biomass. Such policies have amplified the interest for large-scale biomass-fired units with increased efficiency, extended availability and broader fuel flexibility.

Biomass properties vary considerably depending on their biological and regional origin, seasonality, farming and harvesting practices, and ultimately on their preparation and processing. This leads to broad variations in chemical composition and physical properties across different biomass types and even within the same type. These wide property differences are one of the key challenges of using biomass as a fuel for electricity production.

Large-scale biomass boilers

Circulating fluidised bed (CFB) boilers are ideal for efficient power generation, capable of firing a broad variety of solid biomass fuels in applications ranging from small combined heat and power plants to large utility power plants. My company has progressively advanced and scaled up CFB technology for biomass firing since the 1980s, starting from small multi-fuel boilers in pulp and paper mills to Advanced Bio CFB technology firing a range of biomass fuels.

This advanced technology further improves the well-known benefits of earlier CFB technologies to include unprecedented fuel flexibility, inherently low emissions, and high availability. Designs of efficient subcritical boilers firing 100 per cent biomass are available to 600 MWe. This technology also offers supercritical steam boiler designs for biomass and coal co-firing applications up to 800 MWe.

Advanced Bio CFB technology not only addresses the fuel issues related to biomass firing, but also conforms to plant requirements and optimises its investment factors.

Plant requirements include the type of boiler (i.e. utility or industrial), capacity, operational load range, steam data, emission limits and other legislated requirements. Investment factors include plant availability, fuel flexibility requirements and investment and operating costs. Consequently, economical boiler designs have been developed to fire easy-to-burn biomass, while more robust solutions are implemented as the biomass quality degrades and becomes more challenging to burn reliably.

Among the marketed biomass fuels, agricultural residue (agros), such as some straws, and olive and rapeseed residue are identified as the most problematic. These hold the highest potential to create operational difficulties such as agglomeration of fluidised beds and fouling and corrosion of convective heat surfaces.

These problems can be traced back to elevated concentrations of alkali, phosphorous and chlorine, which are typically much higher in most agros than in wood. This unfavourable composition is the main reason why the use of novel biomass fuels has been limited in energy production.

Detailed knowledge of biomass specifications and a full understanding of their variability of supply are paramount to designing boilers with the highest efficiency and availability, and to operating them in the most economical way. Design features of the Advanced Bio CFB technology (see Figure 1) address many of the abovementioned issues.

Figure 1: Advanced Bio CFB’s system layout and design features
Advanced Bio CFB's system layout

Early operating experiences

Examples of the ABC technology in practice include two power plants in Poland ” the Konin and the Polaniec power stations. Both plants fire 100 per cent biomass, including a considerable share of demanding agricultural residue.

The Konin power plant (see Figure 2) is the oldest generating plant owned and operated by ZE PAK SA.

Figure 2: Konin power plant combusts up to 20 per cent of agro fuels
Konin power plant

In July 2009, ZE PAK signed a contract for turnkey construction of a 55 MWe/154 MWth biomass-fired CFB boiler for the plant. In February 2010, the owner formally handed over the construction site to us for the preparation of site facilities and civil engineering work. Construction of the foundations for the boiler house and the auxiliary buildings and structures commenced one month later. The unit was commissioned in July 2012.

The Konin CFB boiler is a state-of-the-art biomass-firing unit designed for burning clean woody biomass and up to 20 per cent of agricultural biomass including willow, straw, rapeseed residue, cherry stones and oat husk. The boiler design consists of the furnace and two high-efficiency solid separators, and the boiler design conditions are shown in Table 1.

kronin boiler design

To provide effective fuel management, all types of biomass are transported by trucks to the plant’s unloading station and conveyed to silos in the storage yard. There are three storage silos, each dedicated for forest and woody biomass, and five silos for agro biomass. Each agro fuel is fed from a separate silo, since the fuel mixture consists of straw pellets, oat husk, cherry stones, rapeseed cake and energy willow.

The system allows the dosing of different types of biomass using screw conveyors in order to feed a homogenous fuel mixture to boiler day silos. The mixture of different types of biomass is distributed evenly to the boiler by four feeding points. The fuel is fed to solid return chutes from external integrated heat exchangers and mixed with circulating material before entering the furnace.

The first steam was delivered to the turbine in April 2012, and the power block was synchronised two weeks later. The first operational experiences have been very good and data from performance tests show that all guaranteed parameters have been met.

The 1800 MWe coal-fired Polaniec power plant, owned by GDF Suez Energia Polska is the fifth largest power plant in Poland. In April 2010, GDF Suez Energia began construction of the world’s largest CFB boiler firing 100 per cent biomass at the Polaniec site.

Advanced Bio CFB combustion technology was selected to produce 205 MWe/447 MWth utilizing a broad range of biomass fuels while targeting high efficiency and availability in accordance with Polish regulations, which set the proportion of agro biomass at a minimum of 20 per cent and specify that the plant was in operation by end 2012.

The fuel considered for the new Polaniec biomass boiler is comprised of 80 per cent wood and 20 per cent agro biomass. The ‘wood’ fuel is clean forestry residues, while the ‘agro’ fuel includes a variety of agricultural biomass such as straw, sunflower pellets, dried fruit (marc) and palm kernel shell (PKS). The wood and agro fuel properties and the fuel mix properties are presented in Table 2.

palaniec boiler design

The alkali content of the fuel mixture with 20 weight-per cent of agro biomass is clearly higher than experienced earlier in large scale commercial CFB boilers with biomass fuels. To enable the use of this challenging fuel mixture with high efficiency and availability in the CFB boiler, a demonstration of the advanced agro CFB concept was carried out in a development program with supporting pilot testing. The results demonstrated CFB feasibility for the Polaniec project with the requested fuel range.

The Advanced Bio CFB design for the Poà…‚aniec project utilizes the main concept and features as shown in Figure 1. Risks related to high temperature chlorine corrosion, fouling and agglomeration potential were taken into account in the boiler design and operational concept.

The design of the CFB boiler has solids separators built from steam-cooled panels integrated with the combustion chamber. The steam-cooled separator design avoids heavy refractory linings. The design also features heat exchangers, located in separate enclosures at the bottom of the furnace adjacent to the main combustion chamber, as final superheating and reheating coils.

These heat exchangers are fluidised by clean combustion air which protects the high-temperature coils from the fouling and corrosive environment of the hot flue gas. The heat exchangers enable higher steam temperatures and higher plant efficiencies, along with good load-following capabilities and turndown ratios.

The Advanced Bio CFB design also incorporates a step grid which allows the effective transfer of heavy unfluidised particles into the bottom ash removal system without the need to shut down the boiler.

For emissions control, the CFB utilises a low and uniform temperature profile in the furnace and staged combustion. In addition to these combustion process-related measures, the boiler is equipped with an ammonia injection system and catalyst (SNCR+SCR) for controlling NOx emissions, and an electrostatic precipitator for controlling particulate emissions.

Initial operational experiences have been excellent. The boiler has operated well with various fuel mixes and with high efficiency. Boiler operation responds to requirements of electricity production, and full load range is in use on a weekly basis.

The boiler fulfills all of the Polish grid requirements with a 4 per cent minimum load change rate demonstrated solely with solid biomass fuels.

Since the beginning of hot commissioning, the boiler has been operated very close to the design values and on load levels from a minimum of 40 per cent MCR to maximum. Expected low emission levels have been confirmed during initial operation. Initial boiler availability has turned out to be good and still improving.

CFB & biomass: good bedfellows

Biomass has an important role in reducing the environmental effects of energy production both in pure biomass plants and in coal and biomass co-combustion. CFB boilers are capable of firing a broad variety of solid biomass fuels for small industrial plants as well as large utility power plants.

Advanced Bio CFB technology represents the state of the art for large-scale (up to 600 MWe) 100 per cent biomass fuel firing. Based on current reference plants, the technology has been proven to be highly efficient and fuel-flexible, with the capability of firing 20-30 per cent of difficult agro biomasses.

Thus, advanced CFB technology provides a solution for effective CO2 reduction in large-scale power generation with a broad range of solid biomass fuels, making it the ideal choice in meeting the market’s demand for fuel flexibility and low emissions.

Robert Giglio is vice-president of Foster Wheeler Power Group and is based in the US. For more information, visit www.fwc.com.

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