GASIFIERS – Downdraft Biomass

PROCESS

In the downdraft gasification process, the flow of gases in the gasifier reactor is downwards. The fuel is fed through the top and then travels downwards. Ash /char is removed from the bottom. Gasifying agent enters into the fuel bed around the middle of the reactor. Both gases & fuel travel down in a co-current manner. Gas is extracted from the bottom of the reactor.

COSMO Biomass Downdraft gasifiers are air-blown and operate at near atmospheric pressure. The output gas contains 38-42% combustible constituents mainly in the form of CO, H2, CH4 and balance inert gases consisting of CO2 & N2.

The middle zone of reactor, where air enters, is called oxidation zone. The fuel, entering the reactor through the top, undergoes drying and pyrolysis stages before it participates in oxidation. Further down, gaseous products of oxidation react with char to produce producer gas. Since the vapours of pyrolysis pass through red hot oxidation zone, heavy hydrocarbons (tars) get cracked, oxidized and then participate in gasification reactions. Therefore, gas from a downdraft gasifier has negligible tar.

The downdraft gasifier can be used in ‘hot gas mode’, ‘hot-clean gas mode’ or ‘cold-clean gas mode’ as per the requirement of the application. Particulate matter (dust) is the major contaminant in gas, which is removed by appropriate sub-systems as per the need of the application.

The calorific value of cold gas is in the range of 1000 – 1200 kcal/Nm3. The gasifier efficiency can be expected to be 75-80% in ‘hot gas mode’ and 70-75% in ‘cold gas mode’.


ACCEPTABLE FUELS


Biomass fuels, such as wood chips, wood like agro-residues (stems, stalks), biomass briquettes, etc., having following properties are acceptable:

Bulk density: >250 kg/m3,             Ash: <10%,          Moisture: <20%,               Size: 40-150 mm (depending upon the size of gasifier)


CAPACITY RANGE


120 kWth to 2400 kWth in single reactor modules
(Equivalent to 10 to 200 litres/hour oil substitution)
With parallel installation of multiple gasifiers, any capacity is feasible


SPECIAL FEATURES

Automatic fuel feeding & ash removal
 Fully automatic computer controlled plants, appropriately integrated with the user’s equipment, are offered as per requirement
 Semi-automatic, low cost gasifiers are also offered for small-medium-scale applications
 Refractory lined reactors, maintenance-free design
 Hot gas mode’, ‘Hot-clean gas mode’ and ‘Cold-clean gas mode’ configurations are offered as per the requirement of the application. For most of the industrial heating applications, ‘hot gas mode’ or ‘hot-clean gas mode’ serve the purpose. ‘Cold-clean gas mode’ is required where precision controls are involved or for power generation through IC engines.
 Quick start-up and stoppage possible – within few minutes
 Long, trouble-free campaign life, some gasifiers have been operated for 24 X7, 3 months at a stretch without interruption
 Water requirement is minimal
 Environment friendly technology – invisible stack emissions, lower SO2 as compared to FO fired operation
 Downdraft gasifiers offer cost effective option for small / medium scale clean-gas applications. However, fuel flexibility is compromised as coal is not acceptable.