RESEARCH GAS CONDITIONING The use of wood gas to produce electricity in gas engines or turbines is an economically viable possibility these days. Modern turbo charged engines and gas turbines, however, are very sensitive to foreign materials found in wood gas in its raw state. This typically makes for additional complications and maintenance tasks. These materials include tars both in liquid and gaseous forms as well as particulate matter like soot, char and ash. Tars are not, however, problematic in applications where the gas is only burned to produce heat: tars possess a high heating value and are a valuable fuel. Tar substances (condensible hydrocarbons) become a problem below certain temperatures as they will condensate on sensors, valves and other process equipment like heat exchangers and compressors. This poses a challenge for the tar removal process as well as for cooling equipment that tends to get blocked. THERE ARE NUMEROUS WAYS TO REMOVE UNDESIRABLE SUBSTANCES: 1. BAG HOUSE FILTERS Bag house filters are good for particles, but are very sensitive to tars, as the filter fabric tends to get blocked by the sticky tar substances. The fabric filter elements require replacement every 1- 3 years even under normal operating conditions. The filter media resists the gas flow, and causes a large pressure drop. This again increases the power consumption as the gas has to be forced trough the filter. 2. SAWDUST / WOOD CHIP FILTERS Sawdust and wood chips can also be used as a filtering media, but the sawdust needs to be replaced often, and once used is difficult to get rid off. Sawdust filters are good for mobile and emergency power applications, but not really feasible for unmanned 24/7 power generation applications. Be aware that packed bed scrubbers and venturi scrubbers (water scrubbers) are not very effective for tar removal as tars are not water-soluble. Scrubbers consume a lot of feed water and produce a lot of difficult waste water. They also consume a lot of energy. HOW THE PUHDAS OPTIMIZED GAS CONDITIONING DIFFERS The Puhdas approach has been to include an electrostatic precipitator (ESP) for particulate and tar droplet removal. This is standard practice for online gas cleaning in large- scale industrial processes, yet only Puhdas Energia among manufacturers in the small-scale gasification business builds an ESP system into the design of all gasifier units. ACTIVE OR PASSIVE FILTERS Electrostatic precipitation is the only active gas cleaning method as the process charges the liquid and solid components in the gas stream and then collects them by attracting them to an oppositely charged collecting system. All other types of filters are passive in the sense that they just capture the droplet in their structure and eventually get choked. The ESP gives the droplets and particulates a positive (+) electrical charge and they become attracted by the collector plates that have an opposing negative (-) charge. The collection chamber is like a large hollow pipe that allows the gas to pass freely through without causing a substantial pressure drop. The actual power consumed in the separation process itself is less than a typical hair dryer would use (1000 w). Puhdas Retrofit gasifiers produce very little or no tars at optimum operating conditions. Tar formation is increased by excess humidity in the fuel, as well as low reaction temperatures. Low temperatures occur at part loads, for example, when the plant is not operated at full capacity. If the system is required to operate at part loads, or if the quality of the fuel cannot be controlled, excess tars might also be present.