Energy from the Wind - Wind Turbines

Renewable Energy: Wind Power

GAS CONDITIONING FOR POWER GENERATION USING GASIFIER

Need for gas cleaning
Biomass gasification is a process of converting biomass to a combustible gas in a reactor, known as gasifier, under controlled conditions. During gasification, biomass is subjected to partial pyrolysis under sub-stoichiometric conditions. A typical gasifier plant consists of a reactor, which receives air and solid fuel and converts them into gas, followed by a cooling and washing train where the impurities are removed. The clean combustible gas at a nearly ambient temperature is a must for running diesel-generator sets in dual fuel mode or gas engine generator sets suitable for running on producer gas alone. In thermal applications, the cooling and cleaning of the raw gas is limited to the requirements of the thermal process.
The gasification system development has also included extensive research and development on product gas cleanup. Raw gases contain particulates, tars, ammonia, and other impurities that interfere with equipments and downstream processes and components or create emission problems. For low-technology thermal applications where the product gas is simply burned to provide heat, such as a cement kiln, the gas clean-up requirements may be minimal. However, high technology systems such as fuel cells and gas turbines or systems using synthesis gases require clean fuel gases. Extensive clean up may also be required to meet the environmental regulations even in relatively undemanding applications. There are examples of failure due to non-adherence of critical fuel quality requirements with end-use.
In 1993, the International Energy Agency (IEA), Bioenergy Gasification Activity repared a document summarizing the status of gas clean-up systems. This document identified primary gas contaminants, discussed unit operations that could potentially remove such contaminants, and discussed gas-cleaning strategies in relation to the end-use of the product gas. Since then, progress has been made in developing a better understanding of the chemistry and mechanisms of hot gas cleanup and conditioning as evinced by recent gasification systems which have incorporated technologies to improve gas cleanup.
Gas ontaminants
Gasification processes, particularly updraft gasification produces a dirty gas that has a relatively low level of purity and includes toxic chemicals and other contaminants besides carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen and water vapor. Pollutants such as tars, dust, and ash are also present, in relatively high concentrations, in these crude gases. Following a gasification process the crude gas must normally be cleaned of tar, filtered, purified and cooled before it is used in many energy-producing applications.
Cleanup systems
Gas cleanup systems may contain several components individually or in combination viz., cyclones, scrubbers, or filters; each of which removes one or more contaminants. Systems producing gas must deal with the cleanup of five primary contaminants including:
· Particulates
· Alkali compounds
· tars
· nitrogen-containing components
· Sulphur
Various gas-conditioning systems employed are:
Barrier Filters – Rigid, packed bed filters, bag filters, sieve-plate scrubbers, wet scrubbers, cyclone filters, electrostatic precipitators, sand bed filters, water wash towers and catalytic thermal tar destruction beds.
The application of particulate removal technologies to biomass gasification systems in general has matured over the past decade. Cyclones are routinely being used for bulk particulate removal, and technologies including bag filters and wet scrubbers are being used in large-scale systems. While this progress is incremental in nature, it is required for wide- spread deployment of gasification technologies. Based on the available data except for the catalytic tar crackers, none of the gas cleaning systems can meet a targeted cleaning objective exceeding 90 %.

Biomass gasifier

Biomass gasification is a process of converting biomass to a combustible gas in a reactor vessel, known as gasifier under controlled conditions. The combustible gas, known as producer gas has a composition of approx. 19 % CO, 10 % CO2, 50% N2, 18% H2 and 3 % CH4. This gas has a calorific value of 4.5 - 5.0 MJ/cubic metre. It has to be cooled and cleaned before using in internal combustion engines for power generation purposes.
Biomass fuel
A wide range of woody biomass in the form of wood or agro residue can be used in the gasifier. In general any biomass that has a density of more than 250 kgs per cubic meter can be used for gasification. Biomass from trees of Eucalyptus, Casuarina, Acacia, Albizzias, Cassia siamea and many other species and agricultural residues such as coconut shell, husk, fronds, corn cobs, corn stalks, mulberry stalks, briquettes of biomass or saw dust, coffee husk, groundnut husk, rice husk etc. can be used as fuel in fuel specific gasifiers.
Conversion efficiency
High Conversion efficiency from solid Biomass to gaseous fuel of upto 85% in hot gas mode, 75% in cold gas mode and 65% with rice husk is possible. Each Kg of biomass can producer 2.5 to 3.0 cubic meters of gas having a average calorific value of 4.5 - 5.0 MJ/cubic metre. During power generation through gen set upto 85% diesel saving is possible in duel fuel mode.

Alkaline fuel cell stack design

Alkaline fuel cell stack design involves working out the details of optimum configuration of the individual module, array or pack to develop the required output. The basic concept in the design being the assembly of pair of electrodes, provision of hydrogen and oxygen gases to the appropriate electrodes and supply of electrolyte into the individual cell assemblies. Cascading the electrode pairs is possible in many different ways. The basic configuration can be either mono or bipolar with either internal or external manifolding. Also electrolyle supply by matrixing or stagnant boundary or by circulation are the various possibilities. The technique to be adopted in the stack boundary or by circulation are the various possibilities. The technique to be adopted in the stack fabrication mainly depends on the limitation in materials to be used.
Stack design concepts
Various possibilities of stack disign are possible, The options for air electrode of hydrophilic and hydrophobic nature with electrolyte circulation by pump / fan or by natural respiration are possible.
Internal manifolding where electrolyte and gas manifolds arranged internally or external manifolding is possible..
The choice of internal or external manifolding have their inherent problems. The external type has all plumbing works done externally which are amenable for easy maintenance, while the choice of polarity and coupling depends on the capacity. In the internal type cell reliability of fluid transfer depends on seals and O rings and the maintenance requires dismantling of cell assembly.
Stack designs
The stack design adopted by various investigators are described below:
Filter press type: Hydrogen - oxygen fuel cell have circular frame and electrodes. The frame consists of rooms for KOH, H2 and O2. There are six passages two each for entry and exit of electrolyte, hydrogen and oxygen and the frame was clamped by six bolt holes. The electrolyte and gas diffuse radially inwards and leave radially out. This design akins to Grove's design concept.
Monocell construction: The monocell of the actual design contains seven structural components. The central matrix of 20.4 x 20.4 x 0.025cm is connected by means of film gluing with an epoxy frame layer, with the later supported with Teflon mesh screen. Adjacent nickel collectors of the electrodes consist of galvanised screens which are soldered and filled with tin at two opposite edges and sealed in the frame zone after having been charged with catalyst.
Bipolar alkaline fuel cell battery: This consists of bipolar cells, separator packages, electrolyte frame and end plate assembly. The electrodes are rectangular size and framed with suitable slots for gas and electrolyte entry.

Partial oxidation of biogas to hydrogen

The conversion of gaseous hydrocarbons to hydrogen can be achieved in many ways. Partial oxidation with air is one of the options. In this process methane in the biogas with oxygen in air to form hydrogen in a bed of catalyst.
Principle
This commercial process route is basically the result of sequential combustion reactions in which the gas is burnt with deficit oxygen (nearly 30 % of stoichiometric requirement). Depending on the feed -stock the product gas may require purification of sulphur compounds and CO2 - removal. The advantage of dispensing with an external heat source favours the partial oxidation step, since the oxidation of CO supplies the necessary heat.
Process flow
The biogas containing 55-60 % methane , 40 % CO2 and traces of H2 S is first dehydrated and then purified from H2 S. The gas is then sent to the partial oxidizers to form hydrogen. Depending on the composition of the outlet gas from the oxidizer methanation of residual carbon oxides can be incorporated. Finally a Co2 scrubber may be added depending on the type of fuel cell to be used in the power plant.
Process
The process occuring in the partial oxidizer is
CH4 + 0.5 O2 = Co + 2H2
The process requires oxygen, which may be separated and supplied from air and is favoured by moderately high pressure. It is understood that due to residence time limitations, the process approaches equilibrium leaving some residual methane and carbon in the product gas. Hence CO2 needs to be scrubbed and recycled in the plant (Balthasar).
Need for pure oxygen
If air is used instead of oxygen the separation of hydrogen from nitrogen is difficult. Hence oxygen at a purity of at lease 95 % and has to be used in large scale systems. Pressure of notrogen could be accepted in small scale fuel cell applications. After all the cathode gas (air) contains 80% nitrogen. The process has thus to be modified for operation on air in this particuler study.
Factors controlling the process
As the partial oxidation proceeds through a flame reaction, it is necessary to moderate the flame temperature, preferably by means of steam. The raw gas composition is controlled by the oxygen to methane ratio and by the steam addition. In order to reduce the oxygen consumption for the oxidation step the biogas and steam has to be preheated and have to be metered precisely to the reactor.
The operating conditions vary with the non-catalytic reactors.
Pressure: 60 to 90 bar. Reaction temperature: 1200 to 1370 C.
Under catalytic partial oxidation a commercial process, topsoe SBA has specified the following conditions.
Pressure: up to 30 bar or more. Temperature: 90c.
Catalyst: Ni
Feed-stock gas and super heated steam are mixed and preheated to 60c and mixed with oxygen
Reactor
The reactor may be a refractory lined stainless steel vessel with one or more burners. In order to initiate the reaction part of the gas has to be burnt inside the reactor. Johnson Matthey Ltd. report in their patent a reactor made of SS tube with entry for gas at the middle of the reactor and exit for product gas from the end. As gas entry is made near the middle of catalyst bed, they have observed a higher temperature of 450 c at the tip and a uniform temperature of 280 c surrounding the hot zone. This is claimed to be superior to at once through reactor. The reactor may then have multiple (4 or 5 entries from the sides).
A theoretical model developed by Opris et. al describes the temperature and product distribution profiles along the length of the reactor. This fits well with the commercial data. The reactor requires a minimum length for complete partial oxiation of methane leading to a continuous supply of hydrogen at a fixed concentration.
Catalysts
The catalyst used by Johnson Matthey let. in their Hot Spot TM reactor contained 0.01 to 5 wt % platinum and from 1 to 15 wt % chromium oxide supported on a refractory solid such as silica. The support may be monolith honeycomb or particles with a maximum size of 1.5 mm.
Catalyst deactivation
The successful economic and technical utilization of the process depends on the avoidance of free carbon deposition which decreases the catalyst surface area resulting in lower reaction rates. It is suggested that carbon deposition shall be avoided by operational techniques rather than by inhibition.
Product gas
The hot product gas is expected to have the following composition on a dry basis.
Hydrogen and CO, 93 % by volume
Carbon dioxide, 5 % by volume
Nitrogen and argon, 1.5 % by volume
Methane, 0.6 % by Volume
The reactor effluent needs rapid cooling to freeze the gaseous equilibria established at the high temperature reactor by a direct water quench or by heat exchange. The product gas then has to be washed free of carbon-sulphur compounds, carbon dioxide and inert gases.

Natural gas as a source of hydrogen

A common misconception about natural gas is that it is running out quickly. In fact, there is a vast amount of natural gas estimated to still be in the ground. According to latest estimate of Energy Information Administration (EIA) of proved natural gas reserves, the total available future supply can be available for more than about 100 years of supply at current rates of consumption in the case of US alone. Natural Gas is one of the principle sources of energy for many of our day-to-day needs and activities.
Chemical composition of natural Gas
Component Range (mole %)
Methane 87 - 96
Ethane 1.5 - 5.1
Propane 0.1 - 1.5
iso – Butane 0.01 - 0.3
normal – Butane 0.01 - 0.3
iso – Pentane trace - 0.14
normal – Pentane trace - 0.04
Hexanes plus trace - 0.06
Nitrogen 0.7 - 5.6
Carbon Dioxide 0.1 - 1
Oxygen 0.01 - 0.1
Hydrogen trace - 0.02
Specific Gravity 0.57 - 0.62
Gross Heating Value (MJ/m3), dry basis: 36.0 - 40.2
Hydrogen can be generated from natural gas with approximately 80% efficiency and the steam reforming of methane or natural gas usually produces bulk hydrogen. The primary methods in which natural gas is converted to hydrogen are:
(1) reaction with either steam (steam reforming),
(2) oxygen (partial oxidation), or
(3) both in sequence (auto thermal reforming).
Steam-Methane Reforming
Steam reforming is a process in which high-temperature steam (700°C–1000°C) is used to produce hydrogen from natural gas. In steam reforming, methane reacts with steam under high pressure in the presence of a catalyst to produce hydrogen, carbon monoxide, and a relatively small amount of carbon dioxide. Heat is supplied for the reaction to proceed. In the second stage carbon monoxide and steam are reacted using a catalyst to produce carbon dioxide and more hydrogen. Carbon dioxide and other impurities are removed from the gas stream, leaving essentially pure hydrogen using pressure-swing adsorption.
Partial Oxidation
In Partial oxidation process methane in natural gas reacts with oxygen in air to form hydrogen in a bed of catalyst. This commercial process route is basically the result of sequential combustion reactions in which the gas is burnt with deficit oxygen (nearly 30 % of stoichiometric requirement). Depending on the feed -stock the product gas may require purification of sulphur compounds and CO2 - removal. The advantage of dispensing with an external heat source favours the partial oxidation step, since the oxidation of CO supplies the necessary heat. Partial oxidation is an exothermic process—it gives off heat. The process is, typically, much faster than steam reforming and requires a smaller reactor vessel. As can be seen from the chemical reactions of partial oxidation (below), this process initially produces less hydrogen per unit of the input fuel than is obtained by steam reforming of the same fuel.
Auto thermal reforming (ATR)
Auto thermal reforming (ATR) uses oxygen and carbon dioxide or steam in a reaction with methane. Auto thermal reactor concepts for the heat integrated coupling of endothermic and exothermic reactions are required for an efficient on-site production of hydrogen from natural gas for use in fuel cells. The reaction takes place in a single chamber where the methane is partially oxidized. The reaction is exothermic due to the oxidation. When the ATR uses carbon dioxide the H2: CO ratio produced is 1:1; when the ATR uses steam the H2: CO ratio produced is 2.5:1 The advantage of ATR is that the H2: CO can be varied, this is particularly useful for producing certain second-generation biofuels.