(97f) Biomass Gasification Using a Fixed-Bed Downdraft Gasifier

The lime kiln, being an important unit in kraft pulping process, consumes a lot of energy in the form of natural gas or crude oil. The annual cost of the production of 1000 ADt/d of CaO from lime kilns is more than $6 million. As the cost of energy continues to increase, it is essential to find alternative fuel sources that are both suitable and economical to use in kraft pulp mills. Biomass, which refers to living and recently dead biological material, such as vegetation, or agricultural waste, is one of alternative fuel sources. It can be converted into a synthesis gas and possibly be used as a fuel in the lime kiln and other processes. This can potentially reduce the energy cost as well as net CO2 emission.

Biomass gasification has been widely studied over the years to convert wood waste and forestry residues into a combustible synthesis gas (syngas). In gasification, up to 80% of the energy in the biomass is converted into the gaseous form [1]. The produced syngas has the potential to be used in a wide range of systems to generate heat and electricity.

A gasification process is the partial combustion of biomass in the reactor with a gasifying agent such as steam, air, or pure oxygen at a temperature of about 800-1000oC [2]. The produced syngas is composed of H2, CO, CO2 and a small quantity of CH4. The heating value of the syngas depends on the gasification medium and the operating conditions, but roughly it could be about one-third that of the natural gas (37MJ/Nm3) [1]. It is known that the composition of the syngas produced from biomass gasification depends not only on the feedstock, but also on the gasifier design as well as gasification conditions [3].

The gasifier designs may be grouped into one of three categories: updraft [4], downdraft [5, 6] and fluidized bed gasifiers [7]. Due to the unique nature of the gasifier design, the characteristics of the produced syngas vary dependent on the gasifiers used, making the results hardly comparable. It is thus advantageous to have a single gasifier to systematically compare the characteristics of the produced syngas obtained from a broad range of the biomass of interest, making the results readily available to the industry. The feedstock under study includes wood waste, saw dust, slash, mill sludge, peat and agricultural residues, etc.

The approach of this research is to design and construct a bench-scale fixed-bed gasifier with biomass processing capacity of several kilograms per hour and to develop a standard characterization procedure. Then, the effect of the operation conditions, such as the air flow rate and the process temperature on the characteristics of the syngas will be studied.

EXPERIMENTAL

A downdraft gasifier has been designed and constructed for this study. It consists of a stainless steel outer cylinder, which is 56 cm high, 28 cm in diameter, and has a wall thickness of 8 mm. The combustion occurs within a smaller circular cylinder placed inside the outer cylinder. This reaction chamber is made of high temperature steel and is 15.2 cm high, 12.7 cm in diameter and 4 mm in wall thickness.

A grate is attached to the bottom of the reaction chamber to hold the feedstock. This grate is shaken regularly by two rods to let the ash fall on the base plate of the main chamber. Several feed-through holes have been devised on the base plate to let the oxidant (air) and thermocouples enter the chamber. Six type K thermocouples (Omega) are inserted into the reaction chamber from the base plate to continuously monitor and record the process temperature, 1 cm away from the reaction chamber inner wall and also on the chamber centerline.

Two 150 W cartridge heaters are inserted and secured on the reaction chamber walls. These heaters initiate the combustion of their surrounding biomass, once they are switched on for a short duration. Air is used as the gasification medium in this study. Four ¼ inch OD tubes uniformly supply air to four locations of the reaction chamber wall. A pressure regulator and a rotameter control the air flow rate from the compressed air line to the chamber.

The feedstock is fed to the chamber from the top. A funnel directs the feedstock to the reaction chamber. During the burning process, a lid is placed to close the chamber. Therefore the syngas has to move downward through the grate holes to enter the annulus between the reaction chamber and outer cylinder. The syngas then exits the chamber via two tubes welded on the chamber wall, and is directed to a Bunsen burner positioned in a fume hood where the syngas burns. A second Bunsen burner connected to a natural gas line is used to facilitate the burning of the synthesis gas. A valve is mounted on the exit syngas pipe line for sampling. For sampling this valve opens and the sample synthesis gas is taken using a gas tight syringe. A thermocouple mounted on the exit line measures the gas temperature at the exit.

PRELIMINARY EXPERIMENTS

Operating the gasifier in steady state conditions, i.e. having a stable burning of the biomass, while the process temperature is nearly constant, is one of the most important issues in gasification. Preliminary experiments revealed that when the combustion was initiated without preheating the chamber and the biomass, a larger amount of tar was produced which was condensed in the form of biofuel at the bottom of the outer chamber. This is because of the low temperature of the process, which hinders the cracking and reduction of large molecules of tars to light-weight hydrocarbons such as CH4. To resolve this problem, the chamber was preheated by the purge nitrogen gas. However, due to sever heat loss in the pipelines, no significant rise in the biomass and reaction chamber temperature was observed.

As a second way to preheat the chamber, the 150 Watt cartridge heater was contiguously turned on and off at 15 s intervals to warm up the chamber. This was done, once the chamber was purged by nitrogen gas and before the gasification air was introduced to the chamber.

The temperature profiles of the reaction chamber centerline and wall, and the syngas during the warm-up period showed that the temperature profiles level off after about 2200 s. This indicates that just using the 150 Watts cartridge heater, the maximum temperature achievable is about 300ºC.

During the gasification process including the warm-up period for an air injection rate of about 10 L/min, the temperature within the reaction chamber decreased first upon introduction of air, and then started to increase once the combustion was initiated. The maximum temperature of about 800ºC was recorded. To investigate whether the combustion could be sustained without keeping the heater on, the heater was turned off, however, the rapid temperature drop indicated that the combustion was terminated once the heater was turned off. Therefore, the heater was turned on again and the temperature increased again due to overheating of the heater. The heater suffered a failure and the temperature started to decrease.

It was observed that a continuous flow of white smoke was produced and exited the gas line. Also, a dilute oily liquid was condensed on the bottom of the outer chamber. Preheating the chamber resulted in the formation of a lower amount of tar, which was also less concentrated compared to the case where the chamber was not preheated.

FUTURE PLAN

The gasifier has been constructed and successfully tested with wood chips. The first goal is to achieve steady-state operation of the apparatus and also to determine the burning rate and therefore the valid range of the air flow rate that could be employed. Then, for a valid range of air flow rate and temperature below 1,000ºC, the syngas will be characterized to determine its composition and also the tar content.

REFERENCES

[1] Austermann, S., Whiting, K. J., ?Commercial Assessment: Advanced Conversion Technology (Gasification) for Biomass Projects?, Biomass Energy Centre, UK (2007).

[2] U.S. Department of Energy, "Biomass Program: Thermochemical Platform." Energy Efficiency and Renewable Energy, (2007).

[3] Paisley, M. A., Biomass Energy. ?Kirk Othmer Encyclopedia of Chemical Technology?. John Wiley & Sons, Inc. June 2007.

[4] Yang, W., Ponzio, A., Lucas, C., and Blasiak, W. ?Performance Analysis of a Fixed-Bed Biomass Gasifier Using High-Temperature Air?, Fuel Processing Technology Vol. 87, pp. 235-245 (2006).

[5] Lv, P., Yuan, Z., Ma, L., Wu, C., Chen, Y., Zhu, J. ?Hydrogen-rich Gas Production from Biomass Air and Oxygen/steam Gasification in a Downdraft Gasifier.? Renewable Energy Vol. 32, pp. 2173-2185, (2007).

[6] Garcia-Bacaicoa, P., Bilbao, R., Arauzo, J., and Salvador, M. L. ?Scale-Up of Downdraft Moving Bed Gasifiers (25-300kg/H)- Design, Experimental Aspects and Results?, Bioresource Technology Vol. 48, pp. 229-235 (1994).

[7] Li, X. T., Grace, J. R., Lim, C. J. ,Watkinson, A. P., Chen, H. P. and Kim, J. R. ?Biomass Gasification in a Circulating Fluidized Bed?, Biomass and Bioenergy Vol. 26, pp. 171-193, (2004).