Biomass for energy

Biomass is captured sunlight held in chemical bonds. A field converts on the order of one percent of the incident sunlight into plant matter — the molecular step of photosynthesis is efficient, but the plant spends most of its energy budget on its own upkeep. What it builds is mostly three polymers: cellulose, hemicellulose and lignin. Dry wood is roughly half carbon by mass and releases about 18–20 MJ per kilogram when burned. These numbers define the whole sector: the energy is real and large, but it arrives dispersed, wet and locked inside polymer walls.

Two route families

Every conversion route in this cluster belongs to one of two families, divided by how bonds are broken.

Thermochemical routes — combustion, pyrolysis, gasification — use heat to break the polymers non-selectively. Bonds fail statistically at high temperature, and the product is whatever the temperature, oxygen supply and residence time allow: heat and flue gas from combustion, tarry oils from pyrolysis, CO and H2 from gasification. Heat is indiscriminate; that is the strength (almost any feedstock works) and the weakness (energy is spent breaking good bonds along with bad).

Biochemical routes — anaerobic digestion, enzymatic hydrolysis followed by fermentation — let microbes and enzymes do the cutting, selectively and near ambient temperature. Selective chemistry is slow chemistry, and it only works on polymers the enzymes can actually reach.

Which family fits a feedstock is decided largely by water. Evaporating one kilogram of water costs about 2.26 MJ — a noticeable fraction of the energy in the dry matter it rides with. Wet substrates like slurry, manure and food waste are hopeless to burn but pump readily into digesters; dry wood and straw suit thermal routes. Water, not chemistry, draws the sector’s main dividing line.

The recalcitrance toll

The limiting quantity for every downstream route is the cost of opening the lignocellulose matrix. Evolution spent hundreds of millions of years making wood hard to eat. Cellulose chains are crystalline and hydrogen-bonded into microfibrils that enzymes struggle to enter, and lignin — a cross-linked aromatic network with no regular repeat unit — is deposited around them as a seal. Lignin is not food for anything; it is why trees stand up.

To reach the sugars, a process must pay first: milling, steam explosion, acid treatment, or enzymes in industrial quantity. To ignore the sugars, a process must go much hotter — a gasifier tears the matrix apart wholesale at 800–1200 °C. Either way the toll is paid before the first fuel molecule appears, and it is among the largest cost terms in any cellulosic flowsheet. Herbaceous biomass adds a second charge: potassium and chlorine, which at boiler temperature form sticky alkali salts that slag and corrode heat-transfer surfaces — the reason straw-fired boilers run derated.

What this frames

The rest of the cluster hangs off these axes. The digestion pages — biogas-anaerobic-fermentation and anaerobic-digestion-biogas-from-msw — are the biochemical family applied to wet carbon; the syngas pages, gasification-fischer-tropsch-saf and hydrogen-fuel-from-biomass, are the thermochemical family applied to dry carbon; the drop-in page, advanced-biofuels, is what happens when a second, catalytic stage rebuilds the molecules. Where a route appears to waive the recalcitrance toll, look closer: usually the toll has been moved, not removed.

One caveat completes the framework: biomass carbon is climate-neutral only if the harvest is regrown. The neutrality is an accounting property of the rotation, not of combustion.