Magazines/Newspapers

Production of Hydrogen Rich Syngas by Thermo Chemical Gasification of Biomass in a Fluidized Bed Reactor

Description
Production of Hydrogen Rich Syngas by Thermo Chemical Gasification of Biomass in a Fluidized Bed Reactor A B Datta1, P K Chatterjee1, M K Karmakar1, K Sen2 1 Heat Power Engineering Group, Central Mechanical Engineering Research Institute, Durgapur-713209, India. Email- abdatta@cmeri.res.in, pradip@cmeri.res.in, malay@cmeri.res.in Phone – 0343 6510236 2 Project Assistant, Central Mechanical Engineering Research Institute, Durgapur-713209. Abstract: Hydrogen is considered as a novel fuel for the
Published
of 6
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  Production of Hydrogen Rich Syngas by Thermo Chemical Gasification of Biomass in a Fluidized Bed Reactor A B Datta 1 , P K Chatterjee 1 , M K Karmakar  1 , K Sen 2   1   Heat Power Engineering Group, Central Mechanical Engineering Research Institute, Durgapur-713209, India.Email- abdatta@cmeri.res.in, pradip@cmeri.res.in, malay@cmeri.res.in Phone – 0343 6510236 2 Project Assistant, Central Mechanical Engineering Research Institute, Durgapur-713209. Abstract: Hydrogen is considered as a novel fuel for the twenty first century, mainly due to its environmentally benigncharacter. Generation of hydrogen from renewable resources like biomass has several advantages compared tothat of fossil fuel hydrocarbons. A number of processes are being practiced for efficient and economicconversion and utilization of biomass to hydrogen.The present study includes the development of a fluidized bed system for gasification of biomass in a thermochemical reactor using steam as an oxidizing agent. Steam has been used to increase the hydrogen percentage inthe product gas having medium range heating value of 11.0 to 13.5 MJ/Nm 3 . Rice husk has been taken as the primary fuel. The system is also suitable for other biomass like wood-dust, sawdust, bagasse etc.The objective of this study is to investigate the process of syngas generation through thermo-chemical fluidized bed gasification of biomass in a laboratory scale pilot unit. The influence of different input parameters such assteam flow rate, reactor temperature on the percentage of hydrogen in the syngas has been studied.  Keywords: Biomass, Hydrogen, Fluidized bed, Steam, Gasification 1. Introduction: Climate change and the increasing concern over global warming have prompted a search for new and cleaner methods of power generation particularly from renewable energy sources. Out of different alternatives, biomassenergy has the huge potential in India where more than 70% of populations live in villages. Biomass energy,which is abundantly available, needs to be properly utilized for economic and social development of thesevillages. In India, the potential of biomass is around 19,500 MW (3,500 MW from co-generation and 16,000MW from other surplus bio-material).Thermo chemical gasification of biomass may be identified as a possible system for producing renewableenergy. Gasification is a thermal process that converts solid fuel into a fuel gas mixture of medium to lowheating value. The gasification reaction being endothermic requires heat. Heat addition is done directly by partialoxidation of the fuel or indirectly using some means of high rate indirect heat transfer.Since biomasses differ greatly in their chemical, physical and morphological properties, they make differentdemands on the method of gasification and consequently require different reactor designs or even gasificationtechnologies. For this reason that, during a century of gasification experience, a large number of differentgasifiers has been developed and marketed, all types geared towards handling the specific properties of a typicalfuel or range of fuels. Thus, the universal biomass gasifier, able to handle all or most biomass fuels or fueltypes, does not exist.The present work aims at the development of fluidized bed biomass steam gasifier. For steam being the gasifyingagent, steam gasification reactions are   the main proceeding reactions in biomass gasification in the temperaturerange normally used in fluidized beds.The initial decomposition of biomass fuel into combustible matter, water and ash can be expressed according tothe following equation. The biomass fuel containing carbon (C), hydrogen (H), oxygen (O), sulphur (S) andnitrogen (N), and water (W) and an ash (A) decomposes into combustible matter containing all these elements,water vapor and ash.C xc H xh O xo S xs N xn W xw A xa = C xc H xh O xo S xs N xn + xw .W + xa .A(1)  The biomass (dry and ash free) can also be represented as CH h O o S s  N n where the variables h, o, s and n aredetermined from the ultimate analysis of the biomass. For simplification, it is assumed that the char consists of  pure carbon and the volatile concentration of the components equals the combustible matter composition (vh = h,vo = o, vs = s, and vn = n). Therefore, the devolatilization can be rewritten as:C H h O o S s N n = C vc H vh O vo S vs N vn + C (1 - vc) (2)where, vc can be calculated as per the following equationvc = x vol, waf  + (x vol, waf  – 1) [h. M H /M C + o. M o /M C + s. M s /M C + n. M n /M C ](3)where, M C , M H, M o , M s , M n are molar mass of carbon, hydrogen, oxygen, sulphur and nitrogen. The factor x vol,waf  is the volatile content of the fuel as determined in the proximate analysis (on dry and ash-free basis).The following equation represents the overall reaction for the devolatilization:C vc H vh O vo S vs N vn = V s . H 2 S + ½. V n . N 2 + ξ CO . V o . CO + ((1 - ξ CO )/2). V o . CO 2 +( (1 – 2. ξ C2H4  – 6. ξ tar  ). V c – ((1+ ξ CO )/2). V o ). CH 4 +[½ . V h – 2. (1 - ξ C2H4  – 4.5 ξ tar  ). V c + ( ξ CO + 1). V o – V s ]. H 2 + ξ C2H4 . V c . C 2 H 4 + ξ tar  . V c . C 6 H 6 (4)The splitting factors ξ CO , ξ C2H4 and ξ tar  are taken as 0.3, 0.1 and 0.005 [Peterson Inga, 2003].Ultimate analysis of rice husk (dry and ash free basis) yields a typical mass composition of 49% carbon, 46.4%oxygen, and 3.8% hydrogen with the balance comprised of traces of nitrogen and sulfur. Considering the major elements, the fuel may be represented on a molar basis as CH 0.93 O 0.71 . From this, a theoretical yield of molecular hydrogen from biomass can be calculated based on assumptions about the reaction pathway of the conversion process. If the fuel is reacted with a hydrogen bearing species, the hydrogen yield potential from the process may be increased by fuel composition. Using steam as an oxidizer, the following balanced chemical equations can bewritten:C H 0.93 O 0.71 + 0.29 H 2 O = CO + 0.755 H 2 (5)CO + H 2 O (g) → CO 2 + H 2 (6) Under these assumptions, a theoretical maximum yield of 145 g H 2 per kg of biomass is computed, a more thantripling of the 38 g H 2 per kg of biomass possible from the fuel alone. This yield is not a practical goal due to themolecular structure of biomass, the uncontrolled and complex decomposition the fuel undergoes upon heating,and system losses and irreversibility.The products of the heterogeneous reactions of solid carbon with steam are hydrogen and carbon monoxide. To asmaller extent also carbon dioxide can be produced, but in the reducing atmosphere of a gasifier this is not morethan 20%.C (s) + H 2 O (g) → CO + H 2 (7)   C (s) + 2H 2 O (g) → CO 2 + 2H 2 (8)Some other reactions also occur during the gasification processC (s) + CO 2 (g) → 2CO + H 2 (9) The most interesting homogeneous gas phase reaction with steam is the one with methane:CH 4 + H 2 O (g) → CO + 3H 2 (10)CO + H 2 O (g) → CO 2 + H 2 (11)In addition, homogeneous gas phase reactions with steam are reactions with higher hydrocarbons and tar. Thesteam biomass gasification reactions, where steam is used as gasifying agent, are strongly endothermic; thegasifier needs external heat from the outside to maintain the desired and necessary reactor temperature.  2.0 Methods2.1 Feed Materials:  Rice husk as primary fuel: In this study, rice husk is taken as the raw material since rice is cultivated in morethan 75 countries in the world and it is one of the important biomass resources. The worldwide annual husk output is about 115 million tonnes with an annual energy potential of 1.5 x 10 9 GJ. In India, approximately 25million tonnes of rice husk are produced every year.Steam as Gasification agent: Steam, the fluidizing media, has been used as the gasification agent. It prevents thedilution of product gases, which happens during air gasification, and also increases the hydrogen content of the product gas. 2.2 Experimental set-up: A laboratory scale fluidized bed gasifier (shown in Picture-1) has been set up in the laboratory shed. The flowdiagram of this set up is shown in sketch-1. Details of components are explained below.Gasifier: The gasifier is a 50 mm inside diameter tube with a length of 900 mm. It is fitted with a distributor  plate at the bottom and the product gas outlet pipe comes out from the top. There is an opening for under-bedfeeding of rice husk inside the vessel. This gasifier reactor is placed inside an electric furnace, which providesthe heat for gasification reaction. The gasifier temperature is controlled in the temperature range of 650-800 o Cusing a thermocouple with a control panel system. The ash from the reactor is disposed off periodically through alock hopper arrangement.Biomass Feeding System: The feeding system consists of two numbers screw feeders with a hopper, which arefitted to feed the biomass inside the gasifier tube. The upper feeder connected to a variable speed drive systemcontrols the fuel feed rate and the lower high-speed screw feeder feeds the fuel inside the vessel instantaneously.The lower screw feeder is maintained at high speed to avoid pyrolysis of biomass outside the gasifier vessel.Boiler: Steam for gasification is obtained from a small boiler of generation capacity 10 kg per hour, which ismaintained at 5 kg/cm 2 (g). The steam from the boiler is further superheated in an electric furnace to thetemperature range of 250-350 deg C. This superheated steam is supplied at the bottom of the fluidized bed tofluidized the bed of sand and rice husk.Gas Cleaner: The product gas from the gasifier is made dust-free and cleaned by passing it through a gas-cleaning column before it is put into the gas chromatograph for online analysis.Gas chromatograph (GC): The product gas from the gasification chamber is analyzed online in a gaschromatograph. Two detectors have been used in this equipment– One is Thermal Conductivity Detector (TCD)and the other is Flame Ionization Detector (FID). The standard gas mixtures have been used for calibrating theequipment. The GC shows the different picks for different constituents of product gas mixture. 2.3 Experimental Procedure Experiments were carried out with a constant biomass feed rate 1.0 kg per hour. The proximate and ultimateanalyses of rice husk are presented in the Table 2. Table 2- Ultimate and Proximate Analysis of Rice Husk  Ultimate Analysis PercentProximate AnalysisPercentCarbon38.43 Volatile matter55.54Hydrogen2.97 Fixed Carbon14.99Sulphur0.07 Moisture9.95 Nitrogen0.49 Ash19.52Oxygen36.36Ash21.68During the experiment, the temperature of gasifier has been varied in the range 650-800 deg C. The experimentsshowed that rice husk is difficult to be fluidized due to its non-granular and flaky nature. However, thisfluidization behavior of rice husk was improved due to presence of sand as solid particles.The steam temperatures were maintained in the range of 250–350 deg C. This steam is passed through a coiledtube, which is inserted in the gasification chamber at the bottom in order that the super heated steam may enable better fluidization of the biomass and thereby attaining better gasification results. The quantity of steam is veryimportant for maintaining fluidization conditions in the biomass gasifiers.  Sand particles with size ranging from 0.40 mm to 0.67mm diameter have been used as the fluidizing bedmaterial. The sand particles ensure proper fluidization inside the gasifier and maintain the uniform temperaturethrough out the gasification zone.The control of gasifier temperature is very important to avoid problems resulting from the agglomeration of theash and bed material, and subsequent blockage of flow through the system. At the high operating temperatures of gasifier system, sometimes a portion of the agglomeration of ash is the result of the partial melting of the ashconstituents. Thus utmost care is taken to control the reactor temperature.The process of biomass gasification occurs through three steps, which are: the initial devolatilization or pyrolysisstep, that produces volatile matter and a char residue, followed by secondary reactions involving the volatile products and finally, the gasification reactions of the remaining carbonaceous residue with steam and carbondioxide. The biomass devolatilization occurs instantaneously after its introduction to the reactor resulting involatiles and char. 3.0 Gas Analysis Result and Discussion: The gasifier has been operated at different process conditions during experimentation. After the gaseous productwas separated in a cyclone, the product gas passed through the cotton filter for cleaning before analysis. The dryand clean gas was then online analyzed on a Gas Chromatograph (Make- Chemito, model – GC 1000). Heliumgas is used as a carrier gas for detecting the gas composition. 3.1 Reactor Temperature The reactor temperatures were varied keeping other parameters unchanged and the product gases were analyzedindividually. Figure- 1 shows the composition of the product gas at different reactor temperatures. It is observedthat the percentage of hydrogen in the product gas increases with rise in the reactor temperature while theCO/CO 2 ratio decreases gradually. 3.2 Steam Rate: In these tests, steam rates were varied in the range 0.8 – 2.6 kg per hour maintaining all other conditionsconstant. The product gas analyses are presented in the Figure-2. With the increase of steam flow rate, thehydrogen concentration showed a gradual increased trend while those of CO and CH 4 have been decreased.Biomass feed rate was fixed at 1.0 kg per hour. The increased trend of hydrogen concentration may be explained by that there were more steam reforming reactions of CO, CH 4 and C 2 H 4 taking place because of increased steamquantity.The product gas composition based on process parameters reported in literature are different due to diversity of gasifiers, biomass, moisture and steam-biomass ratios used. The variation in gas composition is due to thedifferent operating conditions of the fluidized bed gasifier. Thus, for good result, a perfect combination of all parameters, especially the reactor bed temperature and the steam flow rate, should be maintained properly.Picture-1 Fluidized Bed Biomass Gasifier 
Search
Tags
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks