Preparation of Synthesis Gas from Bio Slurry

The characteristic feature of the Karlsruhe BTL process is a hot pumpable bio slurry as feed for an entrained flow pressurized gasifier equipped with a cooling screen. Especially at high pressures above the synthesis pressure, slurries are easier to handle as fuel powder, allowing the complicated intermediate step of compression of the syngas to be omitted. Slurries or their process engineering precursors, such as pastes or tar-impregnated char crumbs, have a density ten times higher than, for example, straw bales. This is why they can be stored in more compact form and transported at low cost by electrified train in silo wagons even to a far away large-scale plant. After intensive studies of the available gas generation technologies, we selected the GSP-type (Gaskombinat Schwarze Pumpe) gasifier for gasification.

The selected entrained flow pressurized gasifier was developed about 25 years ago at the former German Fuel Institute (DBI) in the town of Freiberg for the former East German brown coal industry. Just like dry grass or crop straw, it contains 5 to 10% of ash and one to a few percent of alkali metal chlorides. A 130 MV gasifier of this GSP-type has been successfully in operation for 15 years at Schwarze Pumpe using different feedstocks. Its main characteristic is a pressure-resistant cylindrical steel jacket, into which a light-weight cylinder screen with SiC liner, cooled with pressurized water of 250°C, is inserted. A hot gasifier flame burns from top to bottom in the gasifier axis, and a thin slag coat (< 20 Pas) runs slowly down the inner wall of the screen as corrosion protection. The screen design has the following advantages: (1) Suitability for fuels high in ash containing corrosive slag, (2) Quick feed and discharge, due to the low heat capacity, (3) High throughput rates and high operating pressures and temperatures, guaranteeing complete C-conversion and tar-free gas after only a few seconds of residence time. High pressures above the synthesis pressure also improve slag separation, facilitate purification of the syngas and allow lower gasification temperatures. The latter saves O2 and improves the cold gas efficiency.

The gasification of concentrated bio slurries having solids contents of up to 39% by weight was tested in the 3-5 MW entrained flow pilot gasifier in the former DBI Freiberg, now the Siemens Fuel Gasification Technology. At operating conditions of 26 bar and 1,600 to 1,200°C, different slurries were pumped into the gasifier at throughputs of up to 0.6 t/h and atomized with pure oxygen (no addition of steam). Technical grade oxygen constitutes a significant cost factor, which, however, is compensated again by the small gas volumes to be purified for the subsequent synthesis. A small N2 flow is used as purge gas for safety reasons.

Gas
CO2
H2
N2
CH4
CO
Concentration (% by volume)
16
28
10
<0.1
46

Typical gas composition of the crude synthesis gas

 

The table shows a typical gas composition of the crude synthesis gas obtained in our campaigns. Owing to the high gasifier temperature, the composition of the crude synthesis gas can be estimated with useful accuracy from the equilibrium of the homogeneous shift reactionCO+H2 -> CO2+H2;, if the fuel composition and the gasifier temperature are known. For typical biomass slurries (~ C6H9O4) and a gasifier temperature of ~ 1200°C, one would expect ~ 50 % by volume of CO, ~ 30 % by volume of H2,10 to 15 % by volume of CO2 and a few percent of N2 in the dry synthesis gas. The unfavourable surface-to-volume ratio in a pilot gasifier leads to elevated heat losses, causing a higher CO2 content in the synthesis gas. For larger industrial scale gasifiers, these proportions are reduced considerably.

Solidified slag

Solidified slag from the GSP gasifier.

Process concept

Process concept of the preparation of synthesis gas from slurries