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Integrating advanced biomass gasifiers into the New Zealand wood industry.
| Content Provider | Semantic Scholar |
|---|---|
| Author | Rutherford, Jillian P. Williamson, Christopher J. |
| Copyright Year | 2006 |
| Abstract | Biomass gasification offers an appealing cogeneration option for the energy intensive wood industry. The appeal of biomass gasification stems from the fact that gasification transforms a solid fuel, often waste, into a gaseous fuel which retains 75-88% of the heating value of the original (Higman and Burgt, 2003). A gaseous fuel offers easier handling and the ability to be utilized in either a gas engine or a gas turbine. Conventional biomass cogeneration plants utilize steam turbines and manage an electrical efficiency of 15-28%, while integration of a gasifier with a gas turbine or engine allow efficiencies of 25-40% (Franco and Giannini, 2005). This paper presents a model for assessing the feasibility of gasification based cogeneration plants. Firstly a chemical equilibrium model for a fast internally circulating fluid bed biomass gasifier is presented allowing estimation of the product gas composition. Secondly basic process flowsheets for two heat and power applications are considered; these are integration with a gas engine and integration with a gas turbine combined cycle. * Department of Chemical and Process Engineering, University of Canterbury. jpr62@student.canterbury.ac.nz. Introduction This work is undertaken to provide a modeling tool to evaluate the economic feasibility of an advanced gasification technology for woody biomass energy plant as part of a larger programme to develop biomass gasification systems for electricity production. There are a number of biomass energy plants constructed in the world but most of these plants are limited to demonstration scale. One of the issues is high capital costs and operation costs. However, these costs can be significantly reduced by optimizing the scale, location and the level of integration of the system. For example at small scales simple system may be more feasible whereas at large scales integrated systems may be preferred. The decision on these options will need both technology evaluation and economic analysis. The approach taken has been to model the gasifier using chemical equilibrium so that a product gas composition and heating value can be estimated. The gasification technology used in this work is fast internal circulating fluidized bed (FICFB) as reported by Brown et al. (2006). In conjunction process flow-sheets and costing models have been created for two possible energy plant concepts. FICFB gasification The FICFB gasifier produces a high hydrogen gas yield due to the use of steam as the gasifying agent. The endothermic nature of the gasification reactions combined with the use of steam as a gasifying agent requires that there is heat transfer to the gasification reactor in order for the gasification to take place. This is achieved through a twin bed system. The bubbling fluid bed (BFB) gasification reactor is combined with a circulating fluid bed (CFB) combustor. The CFB heats an inert heat carrying medium (sand) which flows from the CFB to the BFB providing the heat of reaction. A diagram of the system is shown below. The BFB reactor is screw-fed woody biomass accompanied by a nitrogen purge gas. The nitrogen purge gas is used to ensure positive gas flow into the gasifier. The biomass is fed in above the fluid bed. Drying and devolatilzation of the biomass occur immediately upon the biomass entering the reactor. The heterogeneous chargasification reactions have longer reaction rates (Kinoshita and Wang, 1993, Fiaschi and Michelini, 2001) and will occur throughout the BFB. The BFB has a sand bed fluidized with steam. During gasifying the bed will also contain significant amounts of char. The sand and char bed material flow from the BFB through a chute fluidized with either air or steam into the CFB. Inside the CFB, the char and any additional fuel in the form of LPG is combusted. The CFB is a sand bed fluidized with air. Air rates are maintained to provide excess air conditions. The CFB air velocity is significantly greater than the steam velocity in the BFB and hence the sand is entrained up and out of the CFB. The sand entrained out of the CFB is separated from the flue gases by a cyclone and fed back through a siphon into the BFB. The hot sand settles at the bottom of the siphon preventing flow of the BFB product gas out through the siphon. The sand is then fluidized with either air or steam up and over into the BFB. The sand, having passed through the combustion reactor, is hotter than the BFB bed and cools providing the heat for the gasification reactions. The product gas from the BFB flows out of the top of the BFB and through a cyclone, to separate particulates, before being burnt in an afterburner. When the FICFB is integrated into a process the afterburner would Figure 1: Diagram of FICFB gasifier |
| Starting Page | 35 |
| Ending Page | 41 |
| Page Count | 7 |
| File Format | PDF HTM / HTML |
| Volume Number | 51 |
| Alternate Webpage(s) | http://www.nzjf.org.nz/free_issues/NZJF51_3_2006/ACCE85B9-462C-484D-92FD-B5945802FE5C.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
