Flexibility by fast pyrolysis in renewable energy systems

With the increasing contribution of variable renewable energy sources in primary power production, there is a clear need to provide additional flexibility to ensure supply and demand remain properly balanced. The use of bioenergy for power production is a way to provide this flexibility using renewable resources, while also storage options are provided to accommodate the variable (seasonal) availability of solar and wind energy.

IEA Bioenergy Task 44 (Flexible Bioenergy and System Integration) used the following definition for flexible bioenergy: “Flexible bioenergy is defined as a bioenergy system that can provide multiple service and benefits to the energy system under varying operating conditions and/or loads”.

In this report, prepared by IEA Bioenergy Task 34 (liquefaction), the potential role of Fast Pyrolysis Bio-Oil (FPBO) in such flexible bioenergy system is discussed.

Full report available here: “Flexibility by fast pyrolysis in renewable energy systems”

Fast pyrolysis is a process in which organic materials are rapidly heated to 450 – 600 °C in absence of air. Under these conditions, organic vapours, permanent gases and charcoal are produced. The vapours are then quickly condensed to pyrolysis oil. Pyrolysis enables the transformation of difficult-to-handle biomass of different nature – biomass residues and organic waste materials – into a clean and uniform liquid. The world-wide production capacity of FPBO exceeds 100 kt/y.

The fast pyrolysis process is flexible with respect to the type of biomass and many different lignocellulosic biomasses like forest residues, sawdust, and agro-residues have been tested and can be utilized. Fast pyrolysis of (cheaper) residual biomass streams is currently at a lower development level, and the main challenge in the use of heterogeneous woody and non-woody biomass streams in fast pyrolysis lies in the production of a FPBO with a quantity & quality comparable to clean-wood derived FPBO by either improvement in the process and/or after treatment of the FPBO. FPBO is acidic and sensitive to ageing and therefore its storage and transport require some specific attention for trouble free operation. FPBO production from residual and non-woody biomass streams provides additional flexibility to the overall value chain. Local residue streams can be utilized and converted into FPBO as liquid bioenergy carrier, which can be utilized when needed.

Heat & flexibility:

FPBO can be used directly as renewable fuel to provide heat, and nowadays the majority of the FPBO produced commercially is used in heat generation applications. The production of the FPBO and its use can be decoupled in time and location which will give some short term flexibility. FPBO is also suitable to provide seasonal flexibility in heat demand applications, for example in district heating systems. The simplest form is the continuous, year-round, production of FPBO where the FPBO is stored in storage tanks and combusted on demand when needed.

Electricity & flexibility:

FPBO can be used for heat & power production by (co-)firing in conventional power plants, in internal combustion engines and gas turbines. Gas turbines and diesel gensets are designed for flexible operation in a power grid, with fast start-up and a rapid response to load variations. Therefore, they are very suitable for flexible operation and to balance varying renewable power from wind and solar. Diesel generators are widely used in off-grid application and for emergency back-up power. Such systems are mostly operated on fossil fuels, but FPBO could provide a renewable alternative.

Overall system efficiency improvements are possible when a hybrid system is considered in which various sources of renewable energy, e.g. solar power and FPBO engine, are combined (locally).

“Negative ancillary services”:

A negative ancillary service means the direct uptake of excess electricity or indirectly via hydrogen produced from electrolyzers (using excess electricity). Renewable electricity and/or hydrogen can be captured by (electro-)chemical upgrading of the FPBO to stabilized pyrolysis oil, advanced biofuels or even chemicals. However, it will be challenging for these processes to deal with intermittent power/hydrogen supply. At least a kind of short-term (hydrogen) storage will be needed to guarantee stable operation.