Improving Methane Yields from Manure Solids through Pretreatment

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This paper presents a description of the ABFX (Ammonium Bicarbonate Fiber Explosion) pretreatment process. The ABFX process is an extremely simple and inexpensive process that possesses the attributes of the Ammonia Fiber Explosion Process (AFEX) and CO2 explosion process while eliminating the cost associated with high temperature, high pressure and ammonia recovery. The process uses ammonia bicarbonate (ABC) recovered from anaerobic digestate to pretreat the substrate. The ABC is simply added to the substrate, pumped to a reactor, heated to temperatures less than 100°C, for a short duration. The pressure created by ABC volatilization is then released and the gases (CO2, NH3, H2O) condensed at ambient temperature to produce ABC that is then reused in the process. The process can operate with low temperature waste heat.

This paper presents a description of the process and the results of a National Science Foundation Small Business Innovative Research investigation that compared the methane gas yields from both pretreated and untreated grass silage and pretreated and untreated screened (screw press) dairy manure solids. The ABFX pretreated manure solids produced 38% more methane gas than the untreated while the ABFX pretreated grass silage produced 14% more methane gas than the untreated. The economic benefits of the process will be discussed.

Is There Potential to Improve Methane Yields from Manure?

A large fraction of municipal solid waste (MSW), crop residues, animal manures, forest residues, or dedicated energy crops are composed of lignocellulouse. Lignocellulosic substrates consist of a tightly woven matrix of cellulose, hemicellulose, and lignin polymers. Biological degradation of these polymers are carried out by a variety of enzymes. Pretreatment can enhance the bioconversion of the wastes or cop residues for ethanol or biogas production by increasing the accessibility of the enzymes to the substrate. Thus, pretreatment can increase the energy yield (biogas or ethanol) while decreasing the residual waste to be disposed.

Anaerobic bacteria easily convert the hemicellulose and amorphous cellulose to gas. However, conversion of the crystalline cellulose and lignin is far more difficult. Lignin is not converted to gas by anaerobic organisms. Only a fraction of the crystalline cellulose is converted to gas within the detention times commonly used (20 days) in anaerobic digestion. Pretreatment is required to rupture the crystalline cellulose for enzymatic hydrolysis. A wide variety of pretreatment technologies have been developed. Dilute acid pretreatment solubilizes the hemicellulose. Alkali, lime or sodium hydroxide pretreatment solubilize the lignin thus exposing the hemicellulose and cellulose for enzymatic attack. A variety of explosion processes such as steam, carbon dioxide, and liquid ammonia (AFEX) have also been developed that disrupt the crystalline cellulose and hemicellulose. Ammonia soaking, over prolonged periods of time, has also been used to pretreat straw for animal feed and thereby improve rumen digestibility and animal weight gain. All of the processes use high pressure and temperature, or toxic chemicals. The commonly used, conventional processes are not suitable for on-farm use.

What Did We Do?

Figure 1: ABFX Process

We substantiated the feasibility of a breakthrough pretreatment technology under a National Science Foundation Small Business Innovative Research (SBIR) grant that used the non-toxic Ammonium Bicarbonate (ABC) recovered from the anaerobic digestate. The pretreatment was accomplished with a simple device, shown in Figure 4, composed of a pump, that pumps the solid biomass substrate, mixed with a small amount of ABC, into a reactor. The reactor is closed and heated to temperatures below the boiling point of water. Once heated the ABC breaks down to its water, ammonia, and carbon dioxide components putting the contents under significant pressure. The pressure is then rapidly released causing the explosion or disruption of the lignocellulosic substrate and the breakdown of the crystalline cellulose. The gases (H2O, NH3, and CO2), are then condensed in a separate chamber to produce ABC that is reused in the next cycle. Nothing is wasted. The ABC is recovered and reused. The applied heat and detention time provided is sufficient to pasteurize the biomass and meet the temperature requirements of the downstream anaerobic reactor. It is a simple process composed of a solids pump, heat pump, and two low detention time (10± minutes) reactors.

The SBIR research consisted of pretreating both grass silage and concentrated, screw press, manure solids and digesting both pretreated and untreated silage and manure solids. The pretreated and untreated solids were digested in 10 reactors at a 12.5 day HRT and 35°C.

What Have We Learned?

Pretreatment of the grass silage increased the methane yield 16% over several months of operation. Pretreatment increased the methane yield from the pretreated manure solids by 35% over the same period. The increased gas yield was approximately equal to the methane yield from the crystalline cellulose present in the substrate that is normally not converted to gas. The research demonstrated the feasibility of pretreating lignocellulosic substrates in a simple, short detention time, low temperature process that does not dilute the substrate stream or use toxic chemicals such as liquid or gaseous ammonia, acids, or caustic.

Future Plans

The current plan is to build a prototype facility to pretreat a variety of crop residuals (corn stover, rice straw, wheat straw), dry feedlot manure and poultry litter.


Dennis A. Burke, CEO, Environmental Energy & Engineering Company

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