Energy studies of different cogeneration systems for black liquor gasification

Abstract

The prior goal for a pulp and paper mill is to achieve a chemical recovery system with high availability and good reliability. It is also important that the heat recovery system produces sufficient process heat for the heat-demanding processes in the mill. Black liquor gasification promises to be a future alternative or a complement to the conventional Tomlinson recovery boilers in the pulp industry. Black liquor gasification should offer several advantages from both a chemical recovery and an energy point of view. The gasification process produces a combustible gas which can be used for steam and power generation in many ways. Several studies have shown that the power output from a heat and power cycle integrated with black liquor gasification has potential to get a higher energy efficiency than today's heat recovery technology. Especially with pressurized gasification and a gas-turbine-based cogeneration system, the power output will be significantly improved. However, the gas turbine is very sensitive to corrosive elements. The gas cleaning equipment in the gasification system must therefore have a high removal efficiency and a high reliability. The ambition is to keep the gasification capacity constant during the year, but the fuel consumption for the gas turbine increases at low ambient temperatures. The seasonal variation will cause the gas turbine to operate at partial load and, consequently, there will be a decrease in the gas turbine efficiency. An alternative combined cycle with high power efficiency is given by a hybrid plant in which the gas turbine is fired with natural gas while the fuel gas from the pressurized gasifier is used as a supplementary fuel for the Rankine cycle. A major advantage of the hybrid system is the use of a high purity fuel gas in the gas turbine. The system also allows for variations in pulp capacity which is difficult in an integrated gasification combined cycle (IGCC). This study compares the effect of using an IGCC system and a hybrid combined plant instead of a conventional recovery system. The results indicate a potential to double the power output if the conventional system is replaced by an IGCC system. A hybrid combined cycle system has also a higher net power efficiency than the conventional recovery system. Furthermore, the hybrid system can make the mill self-supporting on power and allows an increase in the heat consumption.