Development of the MILENA gasification technology for the production of Bio-SNG

The production of Substitute Natural Gas from biomass (Bio-SNG) is an attractive option to reduce CO2 emissions and replace declining fossil natural gas reserves. The Energy research Center of the Netherlands (ECN) is working on the development of the MILENA gasification technology that is ideally suited to convert a wide range of biomass fuels into a gas that can be upgraded into Bio-SNG. The MILENA gasification process is a new biomass gasification process designed to produce a medium calorific value gas (approximately 16 MJ nm-3 on dry basis) with a high content of hydrocarbons like methane and ethylene. A high hydrocarbon content is beneficial if the gas is going to be used in a prime mover or will be upgraded to Bio-SNG. The MILENA is best described as an Indirect or Allothermal fluidized bed gasifier. One of the major advantages of Indirect gasifiers is the near 100% conversion of the fuel into a combustible gas and sensible heat. The residual ash is virtually carbon free (<1 wt.% C), which means that the loss in heating value of the remains including the ash is close to zero.. The overall efficiency of the MILENA gasifier is relatively high, compared to the alternatives, because of the complete fuel conversion and the relatively low amount of steam required in the process. Chapter 4 presents an analysis of the obtainable Bio-SNG process efficiency using three different, more or less suitable, gasification technologies and associated gas cleaning and methanation equipment. These technologies are: 1) Entrained Flow, 2) Circulating Fluidized Bed and 3) Indirect gasification. Overall efficiency to SNG is highest for Indirect gasification. The net overall efficiencies on an LHV basis, including electricity consumption and pretreatment, but excluding transport of biomass, are 54% for Entrained Flow, 58% for CFB and 67% for Indirect gasification. Because of the significant differences in overall efficiencies to SNG for the different gasifiers, ECN has selected the Indirect gasification as the preferred technology for the production of SNG. A pseudo-equilibrium model is made to describe the MILENA gasification process. This MILENA model was used to design the lab-scale and pilot-scale installations. The model will be described in Chapter 5. The model is also usable for other fluidized bed biomass gasification processes operating at atmospheric pressure between 770 and 880°C. In 2004 the 30 kWth lab-scale MILENA gasifier was build. After successful operation of the MILENA lab-scale gasifier for some years it was, at the end of 2006, decided to start the realization of a pilot-scale gasifier. Construction started in 2007 and the 800 kWth pilot plant was taken into operation in 2008. First tests with the complete system (gasifier and gas cleaning) were done in 2009. The MILENA process and the lab-scale and pilot-scale installation are described in Chapter 6. An extensive test program was done in the lab-scale and pilot-scale MILENA installations. Different fuels, such as clean wood, demolition wood, sewage sludge and lignite were tested. Test results were used to verify the MILENA model. Only minor adjustments were made to the model. Tests with demolition wood were done to produce data for the engineering of a MILENA demonstration plant. Results of these tests are described in Chapter 7. A MILENA demonstration gasifier to be operated with demolition wood B (painted wood), in combination with OLGA gas cleaning, is planned for 2012. The cleaned gas will be used in a gas engine to produce heat and electricity. The MILENA demonstration is designed for a net electrical output of approximately 3 – 3.5 MWe. Residual heat will be used in the local heat grid. A 50 MWth Bio-SNG demonstration plant is scheduled to be started in 2015. Experimental work will focus on testing of the required catalysts as well as final gas conditioning steps that are required for upgrading the gas into Bio-SNG.