Pyrolysis produces syngas (a mixture of CO and H2). This gas stream has to be converted to liquid fuel through a process called Fischer-Tropsch reaction. This integrated process is energy intensive and needs catalysts for the Fischer-tropsch reaction as well as gas cleaning process to remove sulfur.

Hence, I'd say this integrated processes (pyrolysis and Fischer-Tropsch) to produce liquid fuels is not energy efficient. It would be more efficient to use the syngas directly for energy production. Alternatively, you can separate the H2 from CO (after converting CO to CO2) and use H2 as an energy carrier.

Professor Yehia F. Khalil, Yale University, USA

Fellow of the University of Oxford, UK

Hi Tahir, I'd say the answer is a tentative "yes" if you want bio-oil as the primary product, and the comparison is made between pyrolysis and other thermochemical conversion processes. Unlike gasification, which typically operates at 700-900°C for biomass feedstock to produce syngas, most pyrolysis processes operate at 450-650°C to maximize the liquid (bio-oil) yield, typically 50-70 wt.% of feed on a dry basis.

Depending on the heating rate, there are slow, fast and flash pyrolysis processes. The severity of the reaction condition can be evaluated based on temperature, residence time and, to a lesser extent, pressure. For fast pyrolysis, the required residence time can vary from a second to perhaps a couple of minutes in order to complete moisture vaporization, heating and pyrolysis (thermal cracking) processes. Such process conditions can be readily achieved in a fluidized bed.

The key things that affact process economy are:

(1) Moisture content in the feedstock, which must be removed through drying process. This is because at typical pyrolysis temperatrues, water is merely an inert thermal mass. Drying is, however, a highly enregy intensive process. Heat integration is the key to minimize net thermal input for drying. Experience has shown that pyrolysis facilities can remain thermally neutral at up to 50-55% feed moisture with various levels of heat integration.

(2) Process heat requirement is a function of feed composition, moisture and operating temperature. How to supply heat to the process is another engineering challenge. Our experience has indicated that burning the non-condensable gases (a low-Btu gas) and a portion of the char should suffice to provide heat for both drying and pyrolysis. This can be done in either one or two vessels. The most effective way to transfer the heat to the pyrolyzer is probably through solids circulation in a two bed system, with one bed as pyrolyzer and the other as solids heater. All indirect heating options are unable to achieve the high heat flux (1 - 5 WM/m2) required for fast pyrolysis. Some direct heating options either have local hot spot issues (e.g. using thermal plasma, microwave heating) , or heating efficiency problems (e.g. hot gas as fluidizing agent).

(3) Reduce gas compression power consumption whenever possible. An unsuccessful example in this respect comes from an esablished national research facility. They proposed a large-scale bio-oil plant by fast pyrolysis 10 years ago. Unfortunately, due to poor chioce of a key design condition (rec gas to feed ratio), the pyrolysis system consumes > 30% of the power generated from an integrated power-generating unit, which has a huge impact on the techno-economic feasibility of the plant.

(4) Some processes use catalysts to reduce the oxygen content in the bio-oil. Further upgrading of bio-oil is required to meet fuel quality standards. This means one has to consider pyrolysis and upgrading as an integrated process chain in order to maximize the overall yield, energy efficiency and cost.

There are a number of other technical and engineering challenges asscoaited with fast pyrolysis to be resolved before the technology can be ready for the market. Hope this helps.

Thank you prof. Yehia F. Khalil and engr. Xuantian li fir your valueable comments