Integration of biomass gasification and DRI production
The requirements of the reducing gas for the DRI production are well known, both in terms of major and minor components .
The required characteristics of the syngas implies the adoption of purification actions before the use of the syngas in the shaft furnace, using technologies per- mitting hot gas cleaning, to save energy efficiency of the process.
Hot syngas purification is an important issue, largely studied [11,12]:
Taking into account the requirements of the reducing gas to be injected in the shaft furnace, and also the characteristics of the gas top gas to permit gas recy- cling and CO2 capture and storage, the following gas purification steps can be envisaged.
This operation can be performed at temperature (800-900 °C) and pressure (10-12 bar) conditions of the syngas, with ceramic filter candles or with cy- clone systems. Systems for alkalis removal can be used in sequence with dust abatement, using solid sorbents . The benefit is the reduction of erosion problems in the flow lines, limitation of accumulation of ashes in the shaft furnace, and the protection of syngas purification units based on catalyst and sor- bents, like reactors for desulphurisation and tar re- moval.
Tar is the product of condensation of hydrocarbons present in the syngas. In principle tar and hydrocar- bons could be entered into the shaft furnace, without significant effect on the performance of the reduction process. However two reasons suggest to adopt options for the removal of such compounds.
The first is that some hydrocarbons could not react in the shaft furnace, moving the problem of tar abatement downstream. Even if in this case the problem should be less heavy, the presence of hy- drocarbon compounds in the gas exiting from the shaft furnace could be detrimental for its re- circulation.
The second reason is that possible deposits of tar could be detrimental for the life cycle of components and lines.
The occurrence of this problem depends on the gen- eral architecture of the flow lines.
In any case a tar removal step at high temperature, immediately after the gasifier, can be performed by auto-thermal reforming. This is an oxidation of hy- drocarbons by means of a combination of oxygen and steam. The process requires no external energy
T. Buergler and A. Di Donato
source and do not change significantly the gas qual- ity.
From an engineering point of view it consists in a simple reactor, located just after the gasifier, with minor impact on cost and operation complexity.
The only key point is the use of appropriate catalysts to enhance the tar reforming.
Catalyst for tar abatement are currently used in the oil refining industry. Cheaper catalyst based on Ni/dolomite and Ni/Olivine have been also studied.
From industrial experience and literature data cata- lytic tar destruction systems with removal efficiency higher than 90% are available .
S compounds abatement
The sulphur has a negligible effect on the direct reduction process, however the control of sul- phur is essential for the commercial value of DRI and for the quality of the top gas, specially in view of CO2 capture and storage.
Various well assessed technologies are available for sulphur removal from gas deriving from combustion and gasification, working with cooled gas like wet- scrubber.
A possible useful option to eliminate or reduce the content of this gas is the use of economic earth alkaline sorbents as calcite or dolomite that have the advantage of working at high temperatures as 850ºC or 990 ºC[15,16].
Such materials can be used directly inside the gasi- fier or in separated reactors.
The first option implies no significant cost increase (except the cost of materials).
The adoption of separated reactor implies, also in the case of tar abatement, cost for installation and maintenance of a relatively simple reactor.
On the basis of the available information, hot desul- phurisation system should be effective to fulfil the requirements of sulphur content in the reducing gas, for which accepted values are comprised between 100 ppm and 3000 ppm, depending on the final destination of the DRI.
SP12 – ULCOS-4, October 2008