Technology

Below is a general description of the Pressurised Entrained flow Biomass Gasification (PEBG) process at ETC that will be constructed during 2009 within the Solander Research project 'Direct gasification of biomass for fuel production':

PEBG Process

Overview

Dry biomass powder is fed to a solids feed system of interacting lock hoppers. The lock hoppers are pressurised with CO2, primarily coming from the acid gas removal system. From the lock hoppers the powder is fed into the gasifier, by another stream of CO2, where pyrolysis and gasification takes place. The hot raw gas from the gasifier is quenched by spraying with water and/or condensate. Most of the ash that is entering the quench together with the raw gas is separated from the gas and captured in a water suspension at the bottom of the quench vessel. The ash/water/condensate slurry is pumped to an external water treatment system where the ash is removed before the cleaned water is pumped back to be used in the quench again.

The raw gas is, after being quenched and saturated with steam, cooled and dried in a counter current condenser (CCC). The CCC also acts as a first gas cleaning step where most of the particles in the gas are removed together with some condensable gas species. The condensate from the CCC is recirculated to the quench and used for quenching the hot raw gas, possibly after being mixed with water. The dry raw gas is cleaned and conditioned in a separate gas cleaning step to produce a syngas that is still contaminated with acid gas (CO2 and H2S). In the following acid gas removal some of the CO2 is recirculated via a compressor to the lock hopper and transport system. After acid gas removal the gas is passed through a guard bed before it is sent to a fuel synthesis plant (e.g. methanol).

Upstream processes

There are two important constraints in the upstream processes of the plant: The energy cost for preparation of the bio-fuel and feeding must be low and the amount of inert gases in the syngas resulting from pre-treatment and feeding must be minimised.

Bio-fuel preparation

In order for any fuel to be properly converted in entrained flow gasifiers it has to be efficiently reduced in size. In the ETC PBG plant it will be possible for the bio-fuel to be grinded and prepared either as received, after torrefication or after pre-pyrolysis. The final powder will then be fed batch-wise to a lock hopper system consisting of 2 silos.

Feeding

The lock hopper system is pressurized with CO2 (coming from the fuel synthesis plant in a full scale application) and work as a sequential batch feeder to the fuel injection part, consisting of a pneumatic ejector and a burner. A full loaded hopper is first sealed and pressurized before it is connected to the injection and the other hopper is disconnected for reload. The ejector in the injection system then pneumatically feed the fluidized powder with the transport gas to the burner, which distribute the powder into the reactor.

Gasification

The gasifier in the ETC PBG plant is of a two stage type where the bio-fuel is thermally converted in the first stage (reactor) and the products from the conversion is cooled and separated in the following stage (quench). The gasification reactopr is of an entrained flow type operating at elevated pressures. The biomass powder is pneumatically fed from the lock hopper system by some carrier gas (e.g. CO2). The powder particles and carrier gas are distributed into the reactor with a powder burner assembly at the top of the reactor. The reactor is refractory lined both for thermal and chemical protection of the structural parts of the pressure vessel. The reactor operates at 1-5 MPa (depending on the requirements of the downstream sub processes) and about 1100-1300ºC (depending on the fuel ash characteristics).

The quench operates by spraying a mixture of water and condensate from the gas cooler (Counter Current Condenser) on the product gas that exits the refractory lined (hot) part of the gasifier. The flow rate of the primary quench is adjusted so the gas is saturated with steam. Any additional quench liquid will settle to the bottom of the quench vessel where it will form a pool of liquid with ash from the gas suspended in it.