The carbon footprint of hydrogen production via biomass gasification is a complex topic that requires a detailed examination of feedstock sourcing, processing emissions, and broader environmental trade-offs. Biomass gasification converts organic materials into hydrogen through high-temperature reactions with controlled oxygen, but the sustainability of this method depends heavily on feedstock type, supply chain efficiency, and system design.

Feedstock cultivation and sourcing play a significant role in determining the carbon footprint. Woody biomass, such as forest residues or dedicated energy crops, often requires land, water, and fertilizers, contributing to upstream emissions. Agricultural waste, such as straw or husks, avoids direct cultivation impacts but may still incur emissions from collection and transport. The carbon intensity of feedstock transportation varies with distance and logistics. For example, transporting woody biomass over long distances increases emissions due to fuel consumption, while localized agricultural waste systems can minimize this impact.

Biomass gasification emits biogenic CO2, which is often considered carbon-neutral under the assumption that regrowth reabsorbs the released CO2. However, this neutrality is debated. If biomass is harvested faster than ecosystems can regenerate, or if land-use changes reduce carbon sequestration capacity, the net emissions may not balance. Deforestation for energy crops, for instance, releases stored carbon and diminishes future sequestration potential, negating the theoretical neutrality. Conversely, using residues from sustainable forestry or agriculture may align closer to carbon neutrality, as these feedstocks would otherwise decompose, releasing CO2 or methane.

Land-use change is another critical factor. Converting forests or grasslands to biomass plantations reduces biodiversity and carbon sinks. Indirect land-use change, where food production displaces natural ecosystems due to biomass competition, further complicates the footprint. Agricultural waste feedstocks generally avoid direct land-use impacts but must be managed carefully to avoid soil degradation or nutrient loss when residues are removed.

Processing emissions from gasification include energy inputs for drying, grinding, and heating biomass, often derived from fossil fuels in current systems. Advanced gasification with renewable energy or waste heat recovery can lower these emissions. Syngas cleanup, necessary to remove tars and impurities before hydrogen separation, also consumes energy and chemicals, adding to the footprint. The choice of gasifying agent—air, oxygen, or steam—affects efficiency and emissions, with oxygen-blown systems reducing nitrogen dilution but requiring energy-intensive air separation.

Air pollutants such as particulate matter, NOx, and CO are byproducts of incomplete gasification or combustion. While modern gasifiers with proper filtration minimize these releases, small-scale or poorly managed systems may exacerbate local air quality issues. Woody biomass tends to produce more particulates than agricultural waste, which often has higher alkali content, leading to different pollution control challenges.

Comparing feedstocks, woody biomass generally has higher energy density and consistent composition, improving gasification efficiency but often requiring more intensive processing. Agricultural waste is more variable in moisture and ash content, leading to operational challenges but avoiding dedicated cultivation emissions. The net carbon footprint varies significantly based on these factors:

- Woody biomass from sustainable forestry with short transport distances and efficient processing may achieve near-neutral or low-carbon hydrogen.
- Agricultural waste systems avoid cultivation emissions but may have higher processing costs or lower yields.
- Dedicated energy crops risk higher land-use impacts unless grown on marginal lands with minimal inputs.

Quantitative studies indicate a wide range of carbon footprints for biomass-derived hydrogen, from 5 to 30 kg CO2 per kg H2, depending on system boundaries and assumptions. The lower end assumes optimal conditions: sustainable residues, minimal transport, and high-efficiency gasification with renewable energy integration. The higher end reflects inefficient systems with fossil-dependent processing or significant land-use change.

In summary, biomass gasification can produce low-carbon hydrogen if carefully managed, but its environmental benefits are not automatic. Feedstock choice, supply chain design, and gasification technology all influence the carbon footprint. While agricultural waste may offer advantages in avoiding cultivation emissions, woody biomass can be sustainable if sourced responsibly. Policymakers and operators must account for biogenic CO2 debates, land-use impacts, and air pollution trade-offs to ensure biomass hydrogen contributes meaningfully to decarbonization goals.