Of the methods we’ve discussed, there are pretreatment options that lead the others (some under commercialization). The current leading pretreatment options include dilute acid, AFEX, liquid hot water, lime, and aqueous ammonia, with dilute acid and water, AFEX, and lime under commercialization. The figure below shows switchgrass before pretreatment and after several pretreatment options, i.e., AFEX, dilute acid, liquid hot water, lime, and soaking in aqueous ammonia (SAA).
The Resulting Switchgrass Solids after Different Pretreatment Technologies.
The image displays six samples of switchgrass, each subjected to a different pretreatment method, arranged side by side for visual comparison. The samples are labeled from A to F, with each label corresponding to a specific treatment:
- A. Control – Untreated switchgrass, serving as the baseline for comparison.
- B. AFEX – Treated with Ammonia Fiber Expansion, a method used to enhance biomass digestibility.
- C. Dil. Acid – Treated with dilute acid, commonly used to break down hemicellulose and improve enzymatic access.
- D. LHW – Treated with Liquid Hot Water, a hydrothermal method that disrupts plant cell wall structure.
- E. Lime – Treated with calcium hydroxide (lime), which helps in delignification and cellulose exposure.
- F. SAA – Treated with Sulfite-Assisted Alkaline pretreatment, aimed at removing lignin and enhancing cellulose accessibility.
A scale bar in the bottom right corner indicates 5 mm, providing a reference for the size and physical changes in the switchgrass samples due to each treatment. The image highlights the visual differences in texture, color, and structure among the treated and untreated samples, which are important for evaluating the effectiveness of each pretreatment method in biofuel or biochemical production processes.
To summarize the methods of pretreatment, the table below shows some of these pretreatment methods and the major and minor effects on lignocellulosic biomass. All methods (AFEX, dilute acid, lime, liquid hot water, soaking aqueous ammonia, and treatment with SO2) affect increasing surface area, removing hemicellulose, and altering lignin structure. Only AFEX, lime, and SAA pretreatments remove lignin, and AFEX and SAA decrystallize cellulose.
| Pretreatment | Increases Accessible Surface Area | Decrystallizes Cellulose | Removes Hemicellulose | Removes Lignin | Alters Lignin Structure |
|---|---|---|---|---|---|
| AFEX | Major Effect | Major Effect | Minor Effect | Major Effect | Major Effect |
| Dilute Acid | Major Effect | - | Major Effect | - | Major Effect |
| Lime | Major Effect | Not Determined | Minor Effect | Major Effect | Major Effect |
| Liquid H2O | Major Effect | Not Determined | Major Effect | - | Minor Effect |
| SAA | Major Effect | Major Effect | Minor Effect | Major Effect | Major Effect |
| SO2 | Major Effect | - | Major Effect | - | Minor Effect |
This table shows the conditions for ideal pretreatment of lignocellulosic biomass for dilute acid, steam explosion, AFEX, and liquid hot water.
| Pretreatment Process | Dilute Acid | Steam Explosion | AFEX | Liquid Hot Water |
|---|---|---|---|---|
| Reactive Fiber | Yes | Yes | Yes | Yes |
| Particle Size Reduction Required | Yes | No | Nob | No |
| Hydrolyzate Inhibitory | Yes | Yes | No | Slightly |
| Pentose Recovery | Moderate | Low | High | High |
| Low-Cost Materials of Construction | No | Yes | Yes | Yes |
| Production of Process Residues | Yes | No | No | No |
| Potential for Process Simplicity | Moderate | High | Moderate | High |
| Effectiveness at Low Moisture Contents | Moderate | High | Very High | Not Known |
a Modified from (86); AFEX ratings from Bruce Dale (personal communication).
b For grasses, data for wood not available.
Credit: Lynd, 1996. Annual Rev. Energy Environ., 21: 403-465