Gas Lamp

Biogas is the methane-rich gas that comes off a digester, a sealed tank where manure and other wet waste break down without air. Most of it goes to cooking. A gas lamp is the other thing it can do: the gas feeds a small burner inside a mantle, a mesh sleeve that glows white hot in the flame, and the room is lit straight from the fuel. Nothing is turned into electricity on the way, and no battery is drained.

Two properties of the gas decide whether the light is good. The first is pressure. The brightest output needs about 40 cm (16 in) of water column - a way of measuring gentle gas pressure by how far it pushes water up a tube - and only a fixed-dome digester, a sealed masonry vault, reaches it. A floating-drum or flexible-bag digester delivers nearer 2 to 4 cm (1 to 1.5 in), and lamps for it are built differently. A lamp and digester chosen separately are the usual reason a working digester gives poor light. The second is cleanliness: raw biogas carries hydrogen sulphide, which burns to something that eats the burner away from the exhaust side, so it is passed through a water scrubber first.

Lighting uses about 0.15 cubic metres an hour per 100 candlepower, against a published household figure of roughly 2.9 cubic metres a day for combined cooking and lighting, so an evening of light competes with the cooking burner rather than running on surplus. The lamp is also an open flame burning indoors with no flue. A window stays open while it runs, it is never left burning while anyone sleeps, and carbon monoxide alarms belong near every sleeping area and on each floor. That constant air exchange is also why the lamp gives no usable warmth: the airflow that makes it safe carries the heat away. The mantle is fragile and replaced about once a year, so spares are kept ahead. The gas connection and venting are licensed-trade work in most places.