Influence of Process Conditions on Properties of Ash From Biomass Gasification

Influence of process conditions on properties of ash from biomass gasification

Naomi Klinghoffer¹, Marco J. Castaldi¹, Ange Nzihou²

 

¹Columbia University Department of Earth and Environmental Engineering

²Ecole des Mines d'Albi Carmaux, Department of Chemical and Environmental Engineering

 

 

Abstract

Biomass is likely to be a significant energy resource in the future. A common way to recover energy from biomass is through gasification where synthesis gas is produced; a by-product of this process is ash. This research investigates the potential to use the ash as a catalyst by understanding the properties of ash generated under different gasification conditions. Specifically, it is desired to produce a porous ash which could be used as a catalyst or as a support for more catalytically active metals.

In this work, poplar wood was gasified under CO₂, steam, and air at different reaction temperatures. Experiments were done in a fluidized bed reactor at temperatures of 750^oC and 920^oC and ash was recovered. BET-surface area measurements showed that ash from gasification under CO₂ had a higher surface area than ash produced from steam gasification. TGA experiments showed that gasification under CO₂ resulted in a higher mass loss compared to gasification with steam. Gasification with steam/CO₂ mixtures yielded a mass loss similar to that of steam only which could be indicative of competitive reactions between steam and CO₂.

Introduction

Gasification is a promising technology to yield a usable gaseous product from biomass or waste with high efficiency.  There are three main components that are produced from gasification: light gases (such as H₂, CO, and CO₂) that can be used for synthesis of fuels or combustion applications, tars (heavier hydrocarbons), which need to be removed as they can cause problems with downstream equipment, and ash, which is made up of minerals and metals that do not enter into the gas phase. It has been shown that the characteristics of the ash (for example, composition and porosity) can be affected by the reactive gases that are used for gasification⁽¹⁾. The porosity of the ash, combined with high content of metals and minerals, makes ash a good candidate to be used as a catalyst⁽²⁾. It is common that ash generated from gasification of biomass or waste is either sent to a landfill or used in concrete. Use of ash in catalytic applications would increase its value as well as avoid the need for more expensive catalysts. This research investigates the potential to use the ash as a catalyst by understanding the properties of ash generated under different gasification conditions. Specifically, it is desired to produce a porous ash which could be used as a catalyst or as a support for more catalytically active metals.

 

 

 

Experimental

In this work poplar wood was gasified and analyzed at three different scales. Gasification was done in an in-house built fluidized bed reactor; various flow-through gases were used and results are presented here from experiments done under 10% CO₂ in N₂ and 90% H₂O in N₂. The reactor was heated up to a maximum temperature of 750^oC or 920^oC and held at the maximum temperature for 1 hour or 30 minutes. The top of the reactor was equipped with a porous frit therefore all ash was trapped inside the reactor and recovered after experiments. BET surface area of the ash samples was measured (Micrometrics Gemini). TGA/DSC (Setaram 111) experiments were done where the biomass samples were heated to 800^oC at 20^oC/min Experiments were done at different concentrations of CO₂ and H₂O in N₂. Gasification was also done in an environmental scanning electron microscope (FEI XL30) to observe the changes in physical properties of the sample during gasification. Gasification was done under air, CO₂ and H₂O and images were recorded throughout the experiments.

 

Results and Discussion

After fluidized bed experiments, the ash was recovered and BET-surface area was measured. Results are shown in Figure 1. When comparing surface area of ash generated under steam at 750^oC, it is clear that longer gasification time leads to a higher surface area ash.  However, mass recovery data shows that the mass of ash recovered is similar in both cases; for 30 minutes 5.60% of the initial mass is recovered as ash and at 1 hour 4.95% of the initial mass is recovered.  TGA tests were also done with steam at 800^oC (with 9%H₂O in N₂); after 30 minutes at 800^oC the mass loss is 80.5% and after 1 hours mass loss is 82.7%. Therefore, both TGA tests at low H₂O concentrations and fluidized bed experiments at high concentrations of H₂O both show that mass loss is very gradual at temperatures in the range of 750-800^oC. However, the BET surface area of the ash from fluidized bed experiments showed very different surface areas based on the time spent at maximum temperature. At 1 hour the surface area of the ash sample is 621m²/g and at 30 minutes the surface area is 429m²/g. This indicates that it is possible to produce ash with significantly different properties without necessarily influencing overall production of gas/tars. This is important because if it desired to produce ash with specific properties (for example, higher porosity), one would want to do this without impacting overall gas yield.

 A.

B.

Figure 1 Ash recovered from fluidized bed experiments: A. surface area B. mass recovery

Gasification was done in the fluidized bed at 750^oC for 30 minutes under CO₂ (10% CO₂ in N₂) and H₂O (90% H₂O in N₂) and BET-surface area was measured.  The mass recovered was 15.44% under CO₂ and 5.6% under H₂O; it is expected that the mass loss would be higher under steam since more reactant was introduced. However, the surface area of the two samples was the same. This indicates that the CO₂ produces a more porous ash per gram of biomass reacted. Therefore, if it desired to produce a more porous ash, CO₂ is likely a better candidate than H₂O.

TGA tests were done on biomass under 100% CO₂, 9% CO₂ in N₂, 9% H₂O in N₂, and H₂O/CO₂/N₂ mix (9%, 8%, balance), as shown in Table 1. The highest mass loss was 98.25% with 100% CO₂.  It is reported in the PHYLLIS database that poplar wood has an ash content of 1.5% (dry) therefore the results with pure CO₂ demonstrate almost complete reaction.  In the temperature range of 100-200^oC, there is dehydration of the sample and a higher mass loss is observed for the dry samples where there is a stronger driving force for water to leave the sample. When 200^oC is reached the mass loss under pure CO₂ is 7.11% while the mass loss under 9% CO₂ in N₂ is 5.85%. If dehydration is the only process taking place then one would expect equal mass loss in both cases, since both use dry gases. For example, TGA experiments were done at with air and N₂ at identical conditions (heating rate of 2^oC/min to 500^oC); both gases were dry. While the final mass loss was very different for the two gases (98.4% for air and 76.6% for N₂), the mass loss was similar until temperatures of 200^oC.  Therefore, it is possible that under CO₂ reaction begins to take place at temperatures as low as 150^oC. ESEM images also showed more physical changes at low temperatures under CO₂ compared to H₂O. For example, at 400^oC the pores in the biomass sample have expanded under CO₂ whereas the sample under steam has very similar physical properties to the raw biomass, as shown in Figure 2.

Table 1 Mass loss in TGA gasification experiments

Gas	Final mass loss (%)
100% CO2	98.25
9% CO2 in N2	86.66
9% H2O in N2	81.95
9% H2O, 8% CO2 in N2	81.27

Figure 2 ESEM images A. CO_2, raw biomass B. CO₂, 430^oC C. H₂O, raw biomass D. H₂O,  425^oC

Mass loss was 86.66% under 9% CO₂ in N₂ and 81.95% under 9% H₂O in N₂.  The higher mass loss under CO₂ could be partly due to the Boudouard reaction, shown in Equation 1. It was also interesting to note that mass loss under the mixture of CO₂ and H₂O was almost identical than mass loss under H₂O only (81.27% with CO₂ and H₂O and 81.95% with H₂O only).  This could be indicative of a competitive reaction between H₂O and CO₂.

 Equation 1  CO₂ + C ßą 2CO    Boudouard Reaction                    

Conclusion

If the ash generated from biomass gasification is to be used in catalytic applications it is important to understand the properties of the ash as well as the ash yield that one can expect from different reaction conditions. Overall, it is desirable to produce an ash with catalytic properties without significantly impacting overall gas recovery.  Gasification with CO₂ produces a more porous ash compared to gasification under steam. A higher mass loss is observed under CO₂ compared to steam and reactions under CO₂ begin at lower temperatures compared to steam.

References:  (1) Butterman, H.C.;  Castaldi, M. J. Environ. Sci. Technol. 2009, 43, 9030-9037

(2)   El-Rub, Z. A.; Bramer, E. A.;  Brem, G. Fuel. 2008, 87, 2243-2252