Livestock manure represents a significant organic waste stream with potential for sustainable hydrogen production through thermochemical or biological pathways. Two primary methods for converting manure into hydrogen are gasification and anaerobic digestion, each with distinct process requirements, efficiency considerations, and byproduct management challenges. Compared to crop residues, manure presents unique handling complexities due to its high moisture content, variable composition, and the necessity for nutrient recovery to close agricultural loops.

Gasification of livestock manure involves thermochemical conversion at elevated temperatures (typically 700–1200°C) in a controlled oxygen environment. The process breaks down organic matter into syngas, a mixture of hydrogen, carbon monoxide, methane, and carbon dioxide. Manure must first be dried to reduce moisture content below 20–30% to ensure efficient gasification, as excess water lowers the reaction temperature and increases energy input. Fixed-bed, fluidized-bed, or entrained-flow gasifiers are commonly used configurations. The hydrogen yield depends on manure composition, with poultry manure typically exhibiting higher volatile matter (60–75%) than cattle or swine manure, leading to greater syngas production. Gasification also generates biochar, a carbon-rich solid byproduct that can be returned to soils to improve fertility and sequester carbon. Challenges include ash fouling due to manure’s high inorganic content (10–25% by weight) and the need for gas cleaning to remove contaminants like hydrogen sulfide and ammonia.

Anaerobic digestion offers a lower-temperature biological alternative, where microorganisms decompose manure in oxygen-free conditions to produce biogas (50–70% methane, 25–45% carbon dioxide, and trace hydrogen). While direct hydrogen yields are modest, a two-stage process can enhance output. In the first stage, hydrolytic and acidogenic bacteria convert manure into volatile fatty acids and small amounts of hydrogen. The second stage employs methanogens to produce methane, which can then undergo steam methane reforming to yield additional hydrogen. Anaerobic digestion operates optimally at 30–40°C (mesophilic) or 50–60°C (thermophilic), with thermophilic conditions favoring higher hydrogen partial pressures. Digestion handles high-moisture manure (up to 90% water) without pretreatment, but retention times are lengthy (15–30 days), and nutrient-rich digestate requires careful management to prevent eutrophication.

Moisture content is a critical differentiator between manure and crop residues. While crop residues like straw or husks have moisture levels below 15–20%, manure often exceeds 70–80%, necessitating energy-intensive drying for gasification. Anaerobic digestion avoids this drawback but faces viscosity challenges in continuous stirred-tank reactors. Nutrient recovery is another key contrast. Manure contains higher concentrations of nitrogen (1.5–6% by weight) and phosphorus (0.3–2.5%) than most crop residues, making digestate or biochar valuable as organic fertilizers. However, ammonia inhibition in digesters and phosphorus precipitation in gasification systems require mitigation.

Hydrogen production metrics vary by technology and feedstock. Gasification of dried dairy manure can achieve hydrogen yields of 50–100 g per kg of dry feedstock, with cold gas efficiency (energy in syngas/energy in feedstock) ranging from 50–70%. Anaerobic digestion followed by biogas reforming yields 30–60 g hydrogen per kg of volatile solids, with overall energy conversion efficiencies of 35–55%. System scalability differs as well; farm-scale digesters are widespread, while gasification plants typically require centralized operation due to higher capital costs.

Environmental trade-offs exist between the two pathways. Gasification emits less methane but produces tar and particulate matter requiring scrubbing. Anaerobic digestion minimizes airborne pollutants but risks methane leakage, a potent greenhouse gas. Life cycle assessments indicate that both methods can reduce greenhouse gas emissions by 60–80% compared to fossil-derived hydrogen when coupled with carbon capture or digestate management.

Integration with agricultural practices influences adoption. Co-digesting manure with energy crops or food waste boosts hydrogen output but may compete with land use. Gasification biochar improves soil water retention and crop yields, though transportation costs limit distribution. Regulatory frameworks often favor anaerobic digestion due to established waste management policies, while gasification faces permitting hurdles for syngas handling.

Technological advancements aim to address these limitations. Hydrothermal gasification, which processes wet manure without drying, shows promise with hydrogen yields exceeding 80 g/kg at subcritical water conditions. Microbial electrolysis cells, a nascent technology, combine anaerobic digestion with electrochemical hydrogen production, achieving efficiencies above 60%. Nutrient recovery techniques like struvite precipitation or membrane filtration are being optimized to extract value from process byproducts.

Economic viability hinges on localized factors. Proximity to hydrogen markets, manure collection logistics, and policy incentives determine the preferred method. Regions with high natural gas prices or carbon taxes may favor anaerobic digestion, while areas with biomass energy subsidies could prioritize gasification. Capital costs for manure-to-hydrogen systems range from $1.5–4 million per ton of daily hydrogen capacity, with operational costs dominated by feedstock preparation and labor.

In contrast to crop residues, manure’s heterogeneity demands robust preprocessing. Separation technologies like screw presses or centrifuges recover fibers for gasification while concentrating nutrients in liquid fractions for digestion. Pathogen reduction is another consideration, achieved through thermophilic digestion or high-temperature gasification, ensuring safe byproduct reuse.

Future development must address system synergies. Coupling manure hydrogen production with on-farm fuel cell power or ammonia synthesis could improve economics. Standardization of manure characteristics and automation of feeding systems would enhance process stability. Research priorities include optimizing catalysts for manure-derived syngas upgrading and developing low-cost anti-fouling materials for digester components.

The choice between gasification and anaerobic digestion depends on resource availability and end-use objectives. Gasification suits decentralized hydrogen hubs with access to drying infrastructure, while anaerobic digestion aligns with circular agriculture models prioritizing nutrient recycling. Both pathways contribute to decarbonizing agriculture while managing a pervasive waste stream, though further scale-up is needed to achieve cost parity with conventional hydrogen production.