Alkali Effects on Carbon Conversion in Biomass Gasification

Center for Sustainable Environmental Technologies

Alkali Effects on Carbon Conversion in Biomass Gasification

Gasification is the conversion of carbonaceous matter (such as coal or biomass) into gaseous fuel in a low oxygen environment.  Our gasifier located in 1056 H.M. Black Engineering Building gasifies between 1-2 kg fuel/hour.  Fuel varies from traditional feedstocks such as ground seed corn, switchgrass, or cornstover to more inventive feedstocks such as refused derived fuel (domestic garbage) or cornstarch.

The Black gasifier is a fluidized bed gasifier meaning that the reactor is partially filled with silica sand and fluidized by passing air in through the bottom.  Fluidized bed reactors are commonly chosen for gasification due to excellent mixing and heat transfer characteristics.  The bed is heated to 700-900 °C causing the incoming biomass to react with the fluidizing air.  The result is a series of reactions occurring in the bed that result in a mixture of gases (CO, CO2, CH4, H2, and light hydrocarbons), char (solid carbon and trace minerals from the biomass), water vapor, and tar (heavy hydrocarbons) which as a whole is called “producer gas.”

Figure 1: Gasification can be dirty work. Here I sample char to be later analyzed for potassium.

A new research project is underway which focuses on alkali metals effect on carbon conversion in biomass gasification.  It has been shown in the literature that alkali metals can help to catalyze the reactions in coal gasification that convert solid carbon to carbonaceous gas species.  This project investigates the effects of these same alkali metals on carbon conversion in biomass gasification. 

Biomass typically contains much higher alkali content than coal potentially allowing for a higher conversion of solid carbon to gaseous fuel.  A statistically designed set of experiments is underway to study the interconnected effects of temperature and biomass alkali content on carbon conversion.  Corn fiber metered by an auger system is mixed with a solution of potassium chloride metered by a peristaltic pump system.  Different biomass alkali contents can be simulated by changing potassium chloride solution concentration and flow rate.  Potassium is tracked by analyzing samples of bed material and char with Atomic Absorption Spectroscopy and Inductively Coupled Plasma Spectroscopy.

Once complete, the experimental set can be used to generate an empirical model which can then be applied to other experimental conditions and varieties of biomass.  If successful, the project will help researchers better understand how to increase the energy value of producer gas (more gas with higher energy content) while simultaneously decreasing the amount of time and effort required in cleaning it (less char).

Figure 2: Complete mass balance schematic of an experiment performed on 3 February, 2009. An overall mass balance allows us to quantify how well the system is operating. Perfect mass balance closure is 100% meaning that all of the mass into the system (biomass, salt, water, air, and helium) is accounted for in the exit stream. However, due to error in the system and sample analysis mass balances rarely close to exactly 100%. In this case the mass balance closed to 99.5%.

 

Principal Investigators:

Dr. Robert C. Brown, Iowa State University, rcbrown@iastate.edu
Dr. Samuel T. Jones, Iowa State University, sjones@iastate.edu

Research by:

Patrick M. Meehan, Iowa State University, pmeehan@iastate.edu