The operating principle of an atmospheric gasification reactor for lignite was developed in the 1920’s by Fritz Winkler. In the 1970’s ThyssenKrupp AG and Rheinische Braunkohlewerke AG extended this operating principle to a pressurized gasification high-temperature process: The High Temperature Winkler (HTW) gasification. This process constitutes the core of the LIG2LIQ project.
Even though the underlying operating principle of the HTW is not new, there is still potential for its further investigation and development. The most significant benefits of the HTW gasification are the wide range of usable feedstocks and the output of a high potential row product: the syngas.
Nowadays, it is necessary to search for alternative feedstocks that allow for a reduction in the use of fossil resources. For the HTW gasification the following feedstocks may be used: wood residuals, straw or other lignin-biomass, sludge, lignite and all kind of plastics. Even feedstocks of low higher heating value can be used for HTW-gasification, e.g. sludge, by mixing them with higher higher caloric feedstocks. Before their introduction into the reactor, the feedstock is just roughly grounded, to a particle size of about 1 to 5 mm. That makes this process cheap,even for feedstock with a bad grindability.
There are two zones inside the gasifier. A high particle loaded bed zone and an (almost) particle free post gasification zone (PGZ). The temperature in the bed zone is approximately 750-850°C, always below the ash-deformation temperature. The feedstock enters the reactor in the bed zone. Because of the high temperature, it is first devolatilized, forming char (C), ash, and volatiles, mainly CH4, CO, H2, CO2, H2O, H2S, NH3, and higher hydrocarbons (tars). Additionally, oxygen and steam are injected in the bed zone as gasification agents. The oxygen burns a part of the carbon and delivers carbon dioxide and heat.
Chemical Reactions within Gasification
An important characteristic of the gasification process is that not enough oxygen for a complete burning is injected; it is substoichiometric. Therefore, the carbon dioxide and the steam react with the char:
C + CO2 → 2 · CO
C + H2O → CO + H2
In addition, other gasification products like hydrogen can react with the char and build the unwanted methane:
C + H2 → CH4
All gaseous mediums flow into the the PGZ. Meanwhile, a cooled screw discharges ash, a small amount of unconverted carbon and other unreactive materials, like metals, from the bottom of the reactor. In the PGZ, a small amount of oxygen and steam are injected again to rise the temperature to 950-1000°C. With this high temperature, tars and other higher hydrocarbons are cracked into smaller molecules. Additionally, other gas phase reactions will go on. E.g. water gas shift reaction, in which carbon monoxide react with water to form hydrogen and carbon dioxide:
CO + H2O → CO2 + H2
The syngas leaves the reactor thorough a cyclone and with a composition of mainly CO, H2, CH4, CO2 and H2O. A syngas cooler reduces the gas temperature immediately down to 350°C. This quick temperature drop is necessary to prevent a rebuilding of higher hydrocarbons. Further, a following gas treatment plant cleans the syngas and remove the CO2 and water. At the end, a catalyst converts the syngas either to synthetic fuel or to the basic chemical methanol.
The HTW gasification can use a wide range of feedstocks. Even using challenging feedstocks like waste and sludge is possible. The feedstock pre-treatment is relatively easy and cheap; chlorine and sulphur contamination are acceptable, because the gas treatment plant separates the pollutants from the syngas. Therefore, this technology allows for the utilization and recovery of waste to produce syngas.
During the gasification, a part of the feedstock’s energy is used to heat the process and, therefore, external heating is not necessary (autothermal operation). The rest of the generated heat constitutes an useful product, e.g. for central heating.