Pyrolysis and Combustion of Pulverized Wheat Straw in a Pressurized


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Energy & Fuels 1996, 10, 649-651

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Articles Pyrolysis and Combustion of Pulverized Wheat Straw in a Pressurized Entrained Flow Reactor† Jan Fjellerup,*,‡ Erik Gjernes, and Lars K. Hansen Department of Combustion Research, Risø National Laboratory, DK-4000 Roskilde, Denmark Received October 2, 1995. Revised Manuscript Received January 22, 1996X

Within the past decade, there has been an interest for pressurized combustion and gasification of solid fuels in power plants due to the potential for high efficiency. The utilization of new types of solid fuels for pressurized combustion and gasification depends on char yield and char reactivity at relevant conditions. The pressurized entrained flow reactor designed at Risø is introduced. Pyrolysis and combustion at 10 and 20 bar pressure have been studied using pulverized wheat straw. Samples of partly reacted particles are collected, and the conversion is calculated using the ash tracer technique. The pyrolysis experiments show a yield larger than the yield from the proximate analysis. The pyrolysis is completed in about 1s, and the yield is the same for 10 and 20 bar pressure. The combustion experiments show a high reactivity with oxygen, and the effects of pressure on combustion are discussed using the 10 and 20 bar experiments.

Introduction The interest in using biomass as a fuel arose during the 1970s due to the increase in conventional fossil energy prices. In many countries, biomass represents a domestic energy source which can assure a secure supply of raw material to the energy system. Furthermore, the use of biomass as a fuel can decrease environmental problems such as the CO2 increase in the atmosphere caused by the use of fossil fuels. However, at present power plants using biomass have a low power efficiency. In Denmark, e.g., the small scale combined heat and power (CHP) plants using straw have a power efficiency of less than 25%.1 Thus there is an incentitive to increase the power efficiency of biomass fuelled power plants. One of the solutions may be combustion or gasification in combined cycle power plants, where the biomass is converted under pressure. Different research reactors have been used to investigate fuel reactivity at pressurized conditions. Pressurized pyrolysis of coal in entrained-flow type reactors has been studied at, e.g., Advanced Fuel Research2 and Pennsylvania State University,3,4 while pressurized combustion of coal char has been studied in an entrained† This paper is part of the Symposium on Biomass Fuels section that was published in the March/April issue of Energy Fuels. ‡ Now at NKT Research Center, Sognevej 11, DK-2605 Broendby, Denmark. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Energistyrelsen. Teknol. varmeproduktionsanlæg 1995 (in Danish). (2) Serio, M. A.; Solomon, P. R.; Heninger, S. G.. Coal Pyrolysis in a high pressure entrained flow reactor. Prepr. PapsAm. Chem. Soc. Div. Fuel Chem. 1986, 31 (3), 210-221. (3) Fatemi, M. Pyrolysis of a subbituminous coal in a high-pressure entrained-flow reactor. Ph.D. Thesis, The Graduate School, Department of Material Science and Engineering, Pennsylvania State University, 1989. (4) Lee, C. W.; Jenkins, R. G.; Schobert, H. H. Structure and Reactivity of Char from Elevated Pressure Pyrolysis of Illinois No. 6 Bituminous Coal. Energy Fuels 1992, 6, 40.

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flow reactor at Brigham Young University.5 Pressurized pyrolysis of biomass has been performed at the Royal Institute of Technology, Sweden.6,7 Risø’s pressurized fuel laboratory was established in 1992 with the aim of studying pressurized pyrolysis, combustion, and gasification of solid fuels. At present the laboratory has a pressurized entrained flow reactor (PEFR) and a pressurized thermogravimetric analyzer (PTGA) for the study of reactivity of solid fuels at high pressure. The PTGA can give reactivity data for slow heating rate processes, while the PEFR can give reactivity data for fast heating rate processes, such as gasification and combustion in an entrained flow reactor. In the present paper the PEFR facility is presented and pyrolysis and combustion of pulverized wheat straw at elevated pressure are discussed. Experimental Section Pyrolysis and combustion of pulverized wheat straw were performed in a PEFR.8 The reactor is operated such that the reactant gas flow is large compared to the particle flow. This implies that during combustion or gasification of solid fuels, the bulk concentration and temperature are well controlled. The system layout of the facility is seen in Figure 1. The main components are the gas mixing system, pressure vessels, furnace, movable probe, gas outlet, and conditioning system. The system consists of a vertical cylindric furnace fed with a reaction gas of a specified composition. The furnace consists (5) Monson, C. R.; Germane, G. J.; Blackham, A. U.; Smoot, L. D. Char Oxidation At Elevated Pressures. Combustion Flame 1995, 100, 669-683. (6) Sjo¨stro¨m, K.; Guanxing, C. Properties of Char Produced by Rapid Pressurized Pyrolysis of Peat. Ind. Eng. Chem. Res. 1990, 29, 892895. (7) Zanzi, R. Rapid pyrolysis of biomass at high temperature. Lic. Eng. Thesis from the Department of Chemical Engineering and Technology, Royal Institute of Technology, Stockholm, 1994. (8) Hansen, L. K.; Fjellerup, J.; Stoholm, P.; Kirkegaard, M. The Pressurized Entrained Flow Reactor at Risø. Design Report, 1995; RisøR-822(EN).

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Energy & Fuels, Vol. 10, No. 3, 1996

Fjellerup et al. Table 1. PEFR Facility Performance pressure (bar) primary gas flow (Nm3/h) reaction gas flow (Nm3/h) reactant gas flow (Nm3/h) N2 O2 CO2 H2 CO CH4 H2O (kg/h) reaction gas preheating (°C) reactor temperature (°C) fuel (coal or biomass) up to 10 mm (g/h)

0.1-80 0.3-6.0 1.5-30 0.5-30 0.3-6.0 0.7-15 0.7-15 0.3-6.0 0.3-6.0 0.5-10 600-1500 600-1500 10-4000

Table 2. Size Distribution Based on Mass (Sieve Method) (wt %) size (µm)

wt %

size (µm)

wt %

>1000 500-1000 250-500 125-250

0 0.2 6.2 25.0

63-125 32-63