Technical and economic potential for combined heat, power and bio-oil production in power plants-CHPO

It is known that the fast pyrolysis of biomass is one way to produce liquid from biomass. This is the most intensively investigated pyrolysis process at present. The key issues are rapid heating and rapid quenching in order to obtain liquid as much as possible. External heat generally is supported by sand in fluidized-bed reactors, or a hot wall such as ablative (vortex and rotating blade) reactor. The main heat transfer modes are conduction and convection. Pyrolysis oil is a dark-brown, free flowing liquid fuel that derived from plant materials such as biomass and waste. Its density is about 1.2kg/litre and heating value is 16-19 GJ/ton. A general problem encountered when using biomass pyrolysis to generated bio oil is that the ‘oil’ contains a high level of oxygen, including water and water –soluble components. This is not miscible with petroleum-based liquid. Further on, bio-oil generated by fast pyrolysis are very acidic and corrosive and often chemically unstable over time. A higher viscosity of bio-oil also makes it difficult pumping and handling. In order to increase the quality of bio-oil, catalytic fast pyrolysis is employed.

At KTH, a previous research of catalytic fast pyrolysis with steam has been conducted. Using the KTH developed catalysts and the process; it is possible to produce the organic liquid component of the bio-oil with low O/C ratio and having a higher heating value on dry basis of 35.24 MJ/kg.

The CHP plant considered in this study is Hörneborgsverket and it produces electricity, district heat/cooling, and process steam to nearby industries. The CHP plant uses a bubbling fluidized bed boiler from Metso Power and a multiple stage steam condensing extraction turbine with district heat water condenser system. The boiler has a nominal capacity of 130 MW with 540°C and 140 bar live steam parameters. The steam turbine has a nominal electricity output of 40 MW.

The methodology used for the simulation is that first standalone units are modelled using Aspen Plus simulation tool for both the pyrolysis and CHP plants. For the pyrolysis plant: the Bio-oil capacities considered are 5MW,10 MW,20 MW,30 and 40 MW on lower heating value (LHV) basis. For the CHP plant, the part load (PL) operations considered are: 50 %, 60 %, 70 % and 100 %. The standalone units are integrated and there are 20 scenarios simulated. Critical parameters during the integration are also investigated by examining the simulation results. The optimum possible scenario is selected based on the analysis of the critical parameters. A simplified economic analysis is also carried out in order to determine the minimum production cost of the bio-oil produced.

The study has shown that the underutilized boiler capacity of the CHP plant can be made use of during low production seasons as well as during winter at an optimum bio-oil production capacity of 20 MW. The integration process has its limitations and possibilities as stated in this report and this needs to be further investigated by using the results from this study as an initial step.

Two main limitations were identified:

1. The boiler's flue gas after the air preheater, which should not exceed 80 Nm3/s. This limits how large capacity pyrolysis plant can have.

2. The boiler fuel handling systems which cannot handle flows under 7 kg/s. This limits how low loading degree which can be driven with a given bio-oil production.

The 20 MW bio-oil capacity integrated with the part load operations 60-100 % will enable the plant to run during the winter and part of the summer season if the case of 20 MW-50% PL is seen as a hindrance for integration. On the other hand, if the case of 20 MW-50% PL is seen as a minor problem, the plant will be able to maintain the existing operation hours per year. For higher capacities ≥30MW the critical parameters stated in the results section, have put a limit to the integration concept, however if the influence factors is minimized by further investing in small retrofit activities like in the flue gas condensing section and the need for additional water is also compensated by the added benefits from the integration, then it might be possible to consider higher bio-oil production integrated plant. The total investment cost based on the reference scenario results is 276 Million SEK. The bio-oil production cost and selling price are subject to different factors and the estimated production cost (4.7 SEK/kg or 752.1 SEK/MWh) is only indicative. For full load operation the limiting factor for integration is observed to be the flue gas flow after the air preheater. The bio-oil production cost is sensitive to the feed stock cost and the catalyst replacement rate.