BIOGASIFICATIONKarthik Gopalakrishnan and Dr. Terry Walker

Biosystems Engineering Department, Clemson University

October 2011 Clemson, SC

About Gasification

Gasification is a process that converts organic or carbon based materials into carbon monoxide, hydrogen, carbon dioxide and methane. This is achieved by reacting the material at high temperatures in the absence of oxygen so that the material does not combust but gasifies to produce what is called syngas or synthetic gas. Gasification involves two basic processes.

The first process involves the pyrolysis of raw biomass at temperatures around 6000C. The second process involves the char conversion, which is the solid residue from pyrolysis, into gaseous products. Biomass fuels have more volatile components (70-86% on a dry basis) than  coal; hence pyrolysis plays a proportionally larger role on biomass gasification than in coal gasification. The solid byproduct of pyrolysis is char. Char is mainly carbon and some percentage of ash. (Easterly et.al 1996) In the second gasification step, char conversion, the remaining carbon after pyrolysis undergoes classic gasification reaction and/or combustion. It is this latter combustion reaction that provides the heat energy required to drive the pyrolysis and char gasification reactions. Due to the high reactivity of biomass almost all of the biomass feed including the char are converted to syngas products in a single pass through the gasifier system. The simple design includes a separate gasification chamber and char combustor. The char can be further oxidized to produce combustible gasses such as carbon monoxide (CO), Hydrogen (H2) and traces of methane (CH4) (Goswami et al, 1986). Advantages• Syngas is considered to be more efficient than direct combustion of original fuel since it can be combusted more efficiently at higher temperatures or even in fuel cells•Syngas can be burned directly in gas engines, used to produce methanol and hydrogen, or converted via the Fischer Tropsch process into synthetic fuel• All biodegradable wastes can be gasified and used in a gasifier• Carbon neutral, low-greenhouse gas emitting fuel source• Syngas is clean of sulfur, NOX and other particulate emissions into the atmosphere •Carbon emissions from a biomass source are captured by future crops, which take the place of the harvested crop, making the process carbon neutral

Table 1 shows the heating value of biomass from hardwood and other agricultural waste. The nitrogen and sulfur contents are also shown to be very minimal. Table 1- Comparison of heating values of Biomass and coal (Easterly et.al 1996 )

Equipment Design

Gasification as discussed, includes two steps. The first step is the conversion of dry biomass into char and gasses. The second step involves the combustion of the char or solid residue into carbon dioxide. The figure below describes a model of an indirect gasifier that can be used for any kind of biodegradable waste material as well as biomass.

Figure1-Biomass gasification for syngas production (P Lv et.al 2007)

 

The above reactor model is made of stainless steel and is externally heated by two electric furnaces. Along the total height of the reactor, there are 5 temperature and pressure taps for temperature and pressure detection. Below the reactor, the air distributor installed is for better air distribution. The biomass can be fed into the reactor through an auger driven by a variable speed motor. Air is used as the fluidizing agent and is obtained from the air compressor.  Before the air enters the reactor, it may be preheated to 65 °C for better performance. The steam of 154 °C is produced in the steam generator. Before the steam flows into the reactor above the biomass feeding point, it is metered by a steam flow-meter. The produced gas exits the reactor, then passes through a cyclone, which is heated to 200 °C to prevent the tar contained in the gas from condensing in it. Tar can be periodically collected and checked for carbon content using a GC fitted with an FID detector .

Reaction Chemistry  The combustible substance of a solid fuel is generally composed of carbon, hydrogen and oxygen. Carbon dioxide is produced as result of the carbon reaction with oxygen. The combustion reaction is exothermic and generally yields a theoretical oxidation temperature of around 14500C. The reactions are as follows.

C + O2 CO2 (1) ( + 393 MJ/kg)2H2 + O2 2 H2O (2) (- 242 MJ/kg) In the next stage of partial combustion, water, carbon dioxide and other un-combusted materials are partially cracked in a pyrolysis environment where reduction takes place. Reactions (3) and (4) are the main reduction reactions and being endothermic have the capability of reducing gas temperature. Consequently the temperatures in the reduction zone are normally 800-10000C.  C + CO2 2 CO (3) (-164.9 MJ/kg)C+ H2O CO + H2 (4) (-122.6 MJ/kg)CO + H2O CO2 + H2 (5) (+42 MJ/kg)C + 2H2 CH4 (6) ( +75 MJ/kg)CO2 + H2 CO +H2O (7) (-42.3 MJ/kg)(Lv et al. 2007) On an average 1 kg of biomass produces about 2.5 m3 of producer gas at S.T.P. In this process it consumes about 1.5 m3 of air for combustion. For complete combustion of wood about 4.5 m3of air is required.

Future ResearchThere have been several feedstock's tried and tested for gasification, but still further scope exists to study the binding properties of algae with switch grass which could bring about a breakthrough in this area. Algae consists of lipids which when pelletized with switch grass can improve its transport properties as a feedstock and also increase the calorific value of the overall fuel since lipids have higher calorific value than carbohydrates. The off gasses coming of the combustion chamber of the gasifier are mainly composed of carbon di-oxide which can be fed into a photo bioreactor to cultivate algae thereby making the system totally carbon neutral. In an effort to materialize this claim Clemson University under the leadership of Dr. Terry Walker is trying to partner with the Pee Dee Research centre thereby bringing about a change in using renewable sources of energy and also tethering to Dr. Walker’s dream of living in a sustainable world.

References 1.) Overview of biomass and waste fuel resources for power production- Ames L.Easterly and Margo Burnham2.) Biomass gasification“Alternative energy in agrilucture” Volume.II, Ed.D. yogi Goswami CRC press, 1986 pgs 83-1023.) Schapfer, P., and Tobler, J., Theoretical and Practical Investigations Upon the Driving of Motor Vehicles with Wood Gas, Bern 19374.) http://www.fao.org/docrep/T0269e/t0269e0c.htm5.) http://www.switchgrass.nl/pdf/Sw-FinalRep-chapter7.pdf

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