What is Gasification?

Gasification is a flexible, reliable, and clean energy technology that can turn a variety of low-value feedstocks into high-value products, help reduce our dependence on foreign oil and natural gas, and can provide a clean alternative source of baseload electricity, fertilizers, fuels, and chemicals.

It is a manufacturing process that converts any material containing carbon—such as coal, petroleum coke (petcoke), or biomass—into synthesis gas (syngas). The syngas can be burned to produce electricity or further processed to manufacture chemicals, fertilizers, liquid fuels, substitute natural gas (SNG), or hydrogen.

Gasification has been reliably used on a commercial scale worldwide for more than 50 years in the refining, fertilizer, and chemical industries, and for more than 35 years in the electric power industry.

There are more than 140 gasification plants operating worldwide. Nineteen of those plants are located in the United States. Worldwide gasification capacity is projected to grow 70 percent by 2015, with 80 percent of the growth occurring in Asia.

Gasification can compete effectively in high-price energy environments to provide power and products.

Chemistry

In a gasifier, the carbonaceous material undergoes several different processes:

Pyrolysis of carbonaceous fuels

Gasification of char

1. The pyrolysis (or devolatilization) process occurs as the carbonaceous particle heats up. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal. The process is dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.
2. The combustion process occurs as the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions. Letting C represent a carbon-containing organic compound, the basic reaction here is
3. The gasification process occurs as the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen, via the reaction
4. In addition, the reversible gas phase water gas shift reaction reaches equilibrium very fast at the temperatures in a gasifier. This balances the concentrations of carbon monoxide, steam, carbon dioxide and hydrogen.

In essence, a limited amount of oxygen or air is introduced into the reactor to allow some of the organic material to be "burned" to produce carbon monoxide and energy, which drives a second reaction that converts further organic material to hydrogen and additional carbon dioxide.

How Coal Gasification Power Plants Work

The heart of a gasification-based system is the gasifier. A gasifier converts hydrocarbon feedstock into gaseous components by applying heat under pressure in the presence of steam.

A gasifier differs from a combustor in that the amount of air or oxygen available inside the gasifier is carefully controlled so that only a relatively small portion of the fuel burns completely. This "partial oxidation" process provides the heat. Rather than burning, most of the carbon-containing feedstock is chemically broken apart by the gasifier's heat and pressure, setting into motion chemical reactions that produce "syngas." Syngas is primarily hydrogen and carbon monoxide, but can include other gaseous constituents; the composition of which can vary depending upon the conditions in the gasifier and the type of feedstock.

Minerals components in the fuel, which don't gasify like carbon-based constituents leave the gasifier either as an inert glass-like slag or in a form useful to marketable solid products. A small fraction of the mineral matter is blown out of the gasifier as fly ash and requires removal downstream.

Sulfur impurities in the feedstock are converted to hydrogen sulfide and carbonyl sulfide, from which sulfur can be easily extracted, typically as elemental sulfur or sulfuric acid, both valuable byproducts. Nitrogen oxides, another potential pollutant, are not formed in the oxygen-deficient (reducing) environment of the gasifier; instead, ammonia is created by nitrogen-hydrogen reactions. The ammonia can be easily stripped out of the gas stream.

In Integrated Gasification Combined-Cycle (IGCC) systems, the syngas is cleaned of its hydrogen sulfide, ammonia and particulate matter and is burned as fuel in a combustion turbine (much like natural gas is burned in a turbine). The combustion turbine drives an electric generator. Exhaust heat from the combustion turbine is recovered and used to boil water, creating steam for a steam turbine-generator.

The use of these two types of turbines - a combustion turbine and a steam turbine - in combination, known as a "combined cycle," is one reason why gasification-based power systems can achieve high power generation efficiencies. Currently, commercially available gasification-based systems can operate at around 40% efficiencies; in the future, some IGCC systems may be able to achieve efficiencies approaching 60% with the deployment of advanced high pressure solid oxide fuel cells. (A conventional coal-based boiler plant, by contrast, employs only a steam turbine-generator and is typically limited to 33-40% efficiencies.)

Higher efficiencies mean that less fuel is used to generate the rated power, resulting in better economics (which can mean lower costs to ratepayers) and the formation of fewer greenhouse gases (a 60%-efficient gasification power plant can cut the formation of carbon dioxide by 40% compared to a typical coal combustion plant).

All or part of the clean syngas can also be used in other ways:

·         As chemical "building blocks" to produce a broad range of higher-value liquid or gaseous fuels and chemicals (using processes well established in today's chemical industry);

·         As a fuel producer for highly efficient fuel cells or perhaps in the future, hydrogen turbines and fuel cell-turbine hybrid systems;

·         As a source of hydrogen that can be separated from the gas stream and used as a fuel (for example, in the hydrogen-powered Freedom Car initiative) or as a feedstock for refineries (which use the hydrogen to upgrade petroleum products).

Another advantage of gasification-based energy systems is that when oxygen is used in the gasifier (rather than air), the carbon dioxide produced by the process is in a concentrated gas stream, making it easier and less expensive to separate and capture. Once the carbon dioxide is captured, it can be sequestered - that is, prevented from escaping to the atmosphere, where it could otherwise potentially contribute to the "greenhouse effect."

The gasification process

Because of the limitations of state-of-the-art biomass technology, a quest to improve the efficiency and range of applications has been underway for several decades.

The point of departure was the recognition that the combustion process actually comprises several separate thermal processes which, if conducted in a controlled manner, may considerably improve the result. These processes are:

Drying, where free moisture and cell-bound water are removed from the biomass by evaporation. These processes should ideally take place at a temperature of up to about 160ºC using waste heat from the conversion process.

Pyrolysis, where volatile gases are released from the dry biomass at temperatures ranging up to about 700ºC. These gases are non-condensable vapours (e.g. methane, carbon-monoxide) and condensable vapours (various tar compounds) and the residuum from this process will be mainly activated carbon.

Reduction, where the activated carbon reacts with water vapour and carbon dioxide to form combustible gases such as hydrogen and carbon oxide. The reduction (or gasification) process is carried out in the temperature ranging up to about 1100ºC.

Oxidation, where part of the carbon is burned to provide heat for the previusly described processes.

 

The updraft gasification

In the updraft gasifier, moist biomass fuel is fed at the top and descends though gases rising through the reactor. In the upper zone a drying process occurs, below which pyrolysis is taking place. Following this, the material passes through a reduction zone (gasification) and in the zone above the grate an oxidation process is carried out (combustion).   

 To supply air for the combustion process and steam for the gasification process, moist hot air is supplied at the bottom of the reactor. Combustible gas at a low temperature (because of the evaporation of moisture in the drying zone) is discharged at the top of the reactor, and inert ash from the heat-generating combustion process is extracted from the reactor bottom through a water lock.

Woody Biomass Conversion Technologies

By Salman Zafar on October 6, 2008

Biomass Gasification

There are many ways to generate electricity from biomass using thermo-chemical pathway. These include directly-fired or conventional steam approach, co-firing, pyrolysis and gasification.

1. Direct Fired or Conventional Steam Boiler
Most of the woody biomass-to-energy plants use direct-fired system or conventional steam boiler, whereby biomass feedstock is directly burned to produce steam leading to generation of electricity. In a direct-fired system, biomass is fed from the bottom of the boiler and air is supplied at the base. Hot combustion gases are passed through a heat exchanger in which water is boiled to create steam.

Biomass is dried, sized into smaller pieces and then pelletized or briquetted before firing. Pelletization is a process of reducing the bulk volume of biomass feedstock by mechanical means to improve handling and combustion characteristics of biomass. Wood pellets are normally produced from dry industrial wood waste, as e.g. shavings, sawdust and sander dust. Pelletization results in:

1. Concentration of energy in the biomass feedstock.
2. Easy handling, reduced transportation cost and hassle-free storage.
3. Low-moisture fuel with good burning characteristics.
4. Well-defined, good quality fuel for commercial and domestic use.

The processed biomass is added to a furnace or a boiler to generate heat which is then run through a turbine which drives an electrical generator. The heat generated by the exothermic process of combustion to power the generator can also be used to regulate temperature of the plant and other buildings, making the whole process much more efficient. Cogeneration of heat and electricity provides an economical option, particularly at sawmills or other sites where a source of biomass waste is already available. For example, wood waste is used to produce both electricity and steam at paper mills.

2. Co-firing
Co-firing is the simplest way to use biomass with energy systems based on fossil fuels. Small portions (upto 15%) of woody and herbaceous biomass such as poplar, willow and switch grass can be used as fuel in an existing coal power plant. Like coal, biomass is placed into the boilers and burned in such systems. The only cost associated with upgrading the system is incurred in buying a boiler capable of burning both the fuels, which is a more cost-effective than building a new plant.

The environmental benefits of adding biomass to coal includes decrease in nitrogen and sulphur oxides which are responsible for causing smog, acid rain and ozone pollution. In addition, relatively lower amount of carbon dioxide is released into the atmospheres. Co-firing provides a good platform for transition to more viable and sustainable renewable energy practices.

3. Pyrolysis
Pyrolysis offers a flexible and attractive way of converting solid biomass into an easily stored and transportable fuel, which can be successfully used for the production of heat, power and chemicals. In pyrolysis, biomass is subjected to high temperatures in the absence of oxygen resulting in the production of pyrolysis oil (or bio-oil), char or syngas which can then be used to generate electricity. The process transforms the biomass into high quality fuel without creating ash or energy directly.

Wood residues, forest residues and bagasse are important short term feed materials for pyrolysis being aplenty, low-cost and good energy source. Straw and agro residues are important in the longer term; however straw has high ash content which might cause problems in pyrolysis. Sewage sludge is a significant resource that requires new disposal methods and can be pyrolysed to give liquids.

Pyrolysis oil can offer major advantages over solid biomass and gasification due to the ease of handling, storage and combustion in an existing power station when special start-up procedures are not necessary.

4. Biomass gasification
Gasification processes convert biomass into combustible gases that ideally contain all the energy originally present in the biomass. In practice, conversion efficiencies ranging from 60% to 90% are achieved. Gasification processes can be either direct (using air or oxygen to generate heat through exothermic reactions) or indirect (transferring heat to the reactor from the outside). The gas can be burned to produce industrial or residential heat, to run engines for mechanical or electrical power, or to make synthetic fuels.

Biomass gasifiers are of two kinds – updraft and downdraft. In an updraft unit, biomass is fed in the top of the reactor and air is injected into the bottom of the fuel bed. The efficiency of updraft gasifiers ranges from 80 to 90 per cent on account of efficient counter-current heat exchange between the rising gases and descending solids. However, the tars produced by updraft gasifiers imply that the gas must be cooled before it can be used in internal combustion engines. Thus, in practical operation, updraft units are used for direct heat applications while downdraft ones are employed for operating internal combustion engines.

Large scale applications of gasifiers include comprehensive versions of the small scale updraft and downdraft technologies, and fluidized bed technologies. The superior heat and mass transfer of fluidized beds leads to relatively uniform temperatures throughout the bed, better fuel moisture utilization, and faster rate of reaction, resulting in higher throughput capabilities.

Gasification, An Overview of the Process and Products

YouTube:

http://www.youtube.com/watch?v=w5Y1w7708qc

Gasification Products and Applications

Chemicals and Fertilizers

Modern gasification has been used in the chemical industry since the 1950s. Typically, the chemical industry uses gasification to produce methanol as well as chemicals, such as ammonia and urea, which form the foundation of nitrogen-based fertilizers. The majority of the operating gasification plants worldwide produce chemicals and fertilizers. And, as natural gas and oil prices continue to increase, the chemical industry is developing additional coal gasification plants to generate these basic chemical building blocks.

Eastman Chemical Company helped advance the use of coal gasification technology for chemicals production in the U.S. Eastman's coal-to-chemicals plant in Kingsport, Tennessee converts Appalachian coals to methanol and acetyl chemicals. The plant began operating in 1983 and has gasified approximately 10 million tons of coal with a 98 to 99 percent on-stream availability rate.

Power Generation with Gasification

Coal can be used as a feedstock to produce electricity via gasification, commonly referred to as Integrated Gasification Combined Cycle (IGCC). This particular coal-to-power technology allows the continued use of coal without the high level of air emissions associated with conventional coal-burning technologies. In gasification power plants, the pollutants in the syngas are removed before the syngas is combusted in the turbines. In contrast, conventional coal combustion technologies capture the pollutants after combustion, which requires cleaning a much larger volume of the exhaust gas. This increases costs, reduces reliability, and generates large volumes of sulfur-laden wastes that must be disposed of in landfills or lagoons.

Today, there are 15 gasification-based power plants operating successfully around the world. There are three such plants operating in the United States. Plants in Terre Haute, Indiana and Tampa, Florida provide baseload electric power, and the third, in Delaware City, Delaware provides electricity to a Valero refinery.

Substitute Natural Gas

Gasification can also be used to create substitute natural gas (SNG) from coal and other feedstocks, supplementing U.S. natural gas reserves. Using a "methanation" reaction, the coal-based syngas—chiefly carbon monoxide (CO) and hydrogen (H₂)—can be profitably converted to methane (CH₄). Nearly identical to conventional natural gas, the resulting SNG can be shipped in the U.S. natural gas pipeline system and used to generate electricity, produce chemicals/fertilizers, or heat homes and businesses. SNG will enhance domestic fuel security by displacing imported natural gas that is generally supplied in the form of Liquefied Natural Gas (LNG).

Hydrogen for Oil Refining

Hydrogen, one of the two major components of syngas, is used in the oil refining industry to strip impurities from gasoline, diesel fuel, and jet fuel, thereby producing the clean fuels required by state and federal clean air regulations. Hydrogen is also used to upgrade heavy crude oil. Historically, refineries have utilized natural gas to produce this hydrogen. Now, with the increasing price of natural gas, refineries are looking to alternative feedstocks to produce the needed hydrogen. Refineries can gasify low-value residuals, such as petroleum coke, asphalts, tars, and some oily wastes from the refining process, to generate both the required hydrogen and the power and steam needed to run the refinery.

How do Pyrolysis and Gasification Differ?

What is the difference between Pyrolysis/Gasification and Incineration?

Both gasification is the overall outcome term for processes which involve pyrolysis to turn wastes into energy rich fuels by heating the waste under controlled conditions.

Whereas incineration fully converts the input waste into energy and ash, these processes deliberately limit the conversion so that combustion does not take place directly.

Instead, they convert the waste into valuable intermediate materials that can be further processed for the prupose of materials recycling and/or energy recovery:

PYROLYSIS
Thermal degradation of waste in the absence of air to produce char, pyrolysis oil and syngas, eg the Conversion of wood to charcoal

GASIFICATION
Breakdown of hydrocarbons into a syngas by carefully controlling the amount of oxygen present, eg the conversion of coal into town gas.

Explanation of Terms

Char is created when an organic material—usually wood—is burned in a smothered environment. Char is the most common freshwater fish in Iceland. Char may also have the potential to sequester large amounts of carbon in the soil.

Charcoal is made by burning wood in the absence of oxygen, and lump charcoal is the product of that. One of the most important applications of wood charcoal is as a component of gunpowder .

Charcoal is a black substance that resembles coal and is used as a source of fuel. It is generally made from wood that has been burnt, or charred, while being deprived of oxygen so that what's left is an impure carbon residue.

There is a diagram which was published by Bridgwater which shows the nature of the difference between incineration and gasification and pyrolysis very clearly, and we have reporduced it below to show the differences, not only between gasification and incineration but with other combustion type processes.

by Steve Evans with assistance from the Juniper Gasification and Pyrolysis Fact Sheet

Pyrolysis and gasification – how it works

Like incineration, pyrolysis, gasification and plasma technologies are thermal processes that use high temperatures to break down waste. The main difference is that they use less oxygen than traditional mass-burn incineration.

These technologies are sometimes are known as Advanced Thermal Technologies or Alternative Conversion Technologies. They typically rely on carbon-based waste such as paper, petroleum-based wastes like plastics, and organic materials such as food scraps.

The waste is broken down to create gas, solid and liquid residues. The gases can then be combusted in a secondary process. The pyrolysis process thermally degrades waste in the absence of air (and oxygen). Gasification is a process in which materials are exposed to some oxygen, but not enough to allow combustion to occur. Temperatures are usually above 750oC. In some systems the pyrolysis phase is followed by a second gasification stage, in order that more of the energy carrying gases are liberated from the waste.

The main product of gasification and pyrolysis is syngas, which is composed mainly of carbon monoxide and hydrogen (85 per cent), with smaller quantities of carbon dioxide, nitrogen, methane and various other hydrocarbon gases.

Syngas has a calorific value, so it can be used as a fuel to generate electricity or steam or as a basic chemical feedstock in the petrochemical and refining industries. The calorific value of this syngas will depend upon the composition of the input waste to the gasifier.

Most gasification and pyrolysis processes have four stages:

1) Preparation of the waste feedstock: The feedstock may be in the form of a refuse derived fuel, produced by a Mechanical Biological Treatment plant or an autoclave (see links to briefings on MBT and autoclaving on page 6). Alternatively, the plant may take mixed waste and process it first through some sort of materials recycling facility, to remove some recyclables and materials that have no calorific value (e.g. grit)

2) Heating the waste in a low-oxygen atmosphere to produce a gas, oils and char (ash)

3) ‘Scrubbing’ (cleaning) the gas to remove some of the particulates, hydrocarbons and soluble matter

4) Using the scrubbed gas to generate electricity and, in some cases, heat (through combined heat and power – CHP). There are different ways of generating the electricity from the scrubbed gas – steam turbine, gas engine and maybe some time in the future, hydrogen fuel cells (see page 4).

In plasma technologies the waste is heated with a plasma arc (6,000º to 10,000º Celsius) to create gases and vitrified slag. In some cases the plasma stage may follow on from a gasification stage.

AN INNOVATION FOR Clean- Alternate renewable energy.

GASIFIERS- (Pyrolyzers)  A non-conventional,  Co-Generation, Renewable energy source, a Green Project  receiving backing & Subsidies from various Governments the world over, utilizes any type of Biomass / Coal / Municipal Solid Waste as fuel, namely- Waste Wood, Saw dust, Furniture  waste wood, Bagasse, Rice husk, Coconut Shells, Poultry Litter, Thermocol, Waste Plastic, Rubber, Tyres, Leather, Coal etc.  Max.Output of a single Unit- 04 Mega Watts. For higher requirements multiple Units can be commissioned.                                                                  

GASIFICATION It is a thermo-chemical process of cracking that converts solid waste, Biomass or coal to a low heat value (LHV) gaseous fuel called “Producer Gas”. This producer gas is fuel for many different applications of shaft power, thermal power or electricity in the equipment like, Internal Combustion Diesel / Furn.Oil engines, furnaces, kilns, dryers, rolling mills and heat treatment equipment.

The equipment to be utilized is the new generation modified version using Fluidised Circulating Bed Updraft Technology Gasifier developed by consistent R&D at Bijendra to get better viability of Gasifiers & to obtain clean, rich and consistent supply of Producer Gas which has better calorific values (1000 -1400K.Cal/NM³) with minimum contents of soots and smoke formed by cracking of Tar with higher gasification efficiency and low coal consumption rate. Further Volatile Matter contents of the coal are converted into fix carbon which on gasification increases the Calorific Value of Gas & reduces fuel consumption and increases the overall efficiency of gasification. Further, steam is cogenerated & injected with the air into the Pyrolyzer  which dissociates to form more of Carbon Monoxide, resulting in increase of the calorific value of the Producer Gas. It is then washed by venturi cyclones & multiple perforated pipe washers & then Tar & Ash is cleaned by passing through the specially designed Tar & Dust separators & also dehumidified to be ultra clean for injecting directly into the I.C. Engines modified for Syn-Gas mode of operation by us.

The in-line ESP’s (Electro static precipitators) help in total removal of tar developed in the process of Pyrolysis.

WE modify & Convert HFO / DIESEL Engines into PRODUCER GAS / NATURAL GAS MODE of OPERATION

To operate the Power Plant, the ULTRA-CLEAN SYN-GAS IS DIRECTLY FED INTO THE I.C. ENGINE MODIFIED & CONVERTED BY US from HFO / DIESEL operation mode  to Syn-Gas / Producer Gas / Nat.Gas mode of operation with EXHAUST EMISSIONS WITHIN THE POLLUTION  CONTROL NORMS.

Ultra clean gas is fed into the engines with the help of a Microprocessor based, Furnace Oil to gas conversion system developed by us. When a Variable / Surge / sudden load viz.- Furnace in a Steel Plant, or a similar load is activated, dual-fuel mode of operation of the engine starts automatically at 85% gas and 15% Furnace Oil /L.D.O. for a short duration only & reverts back to 100% Gas mode when the load becomes constant.

The exhaust emissions of the engine, can be used for a Waste Heat Recovery Boiler & 800 Kg Steam at 7kg. pressure per MW per hr. can be obtained & also it can be put to use for other thermal applications.

As such, it is an extremely viable source of non-polluting Alternative renewable energy

FEATURES & ADVANTAGES

q       The producer gas generated in the Pyrolyser / Gasifier has higher Calorific Value in the range of 1000-1400 K.Cal/NM³ suitable for getting high flame temperature in the range of 1200-1400°C as required by most of the thermal processes.

q       Tar Content in the gas by pyro-gasification is nil. The gas produced  being at high temp. has higher calorific value & as such enhances gasification efficiency and lowers consumption of Coal/ MSW /Biomass.

q       There is no effluent output in the process. The washing  of gas is done with re-circulated  water which separates out the solid impurities & Tar to some extent in liquid form.

q       The in-line ESP’s or Electro Static Precipitators innovated by us, help in total removal of Tar & Ash from the gas produced during the process of pyrolysis. Tar, available as a by product, can be sold in the market  or can be used for heating/burning in a Furnace.

q       Due to ultra clean gas, the occurrence of choking of the burner filter and duct pipes is negligible, resulting in increase in productivity and minimal cleaning interruptions in the process.

q       Extremely Low Particulates and smoke emission, even lesser than Pollution Control board norms, due to the complete direct firing of Producer Gas.

q       Worldwide accepted, subsidized, environment friendly Renewable Energy technology.

q       Very early payback period.

Advantages of Gasifier over STG:

The Gasifier based Power Plants have a distinct advantage over the coal fired conventional Steam/Gas Turbine Power Plants.

1. The conventional power plants with steam Turbine are highly polluting as Coal / Bio-mass is burnt to fire the boiler and toxic gases emitted in the form of smoke are highly polluting whereas in the Gasifier based Power Plant, the Gas produced by the cracking of coal by heat in the absence of Oxygen is fed directly into the engine after cleaning it thoroughly & whole of the process is ecofriendly. The exhaust of the engine or the flue gases are well within the pollution control norms.
2. The whole setup with the conventional Turbines is much more expensive as compared to the Gasifier based Power Plant.
3. In the conventional Turbine & Boiler type Power Plant the fuel to be used is generally Coal ONLY. Whereas in the Gasifier type Power Plant, any type of bio-mass can be used as fuel.
4. The cost per unit of the Gasifier based Power Plant after accounting for Maintenance, Operation & other over heads is close to half the rates of the State Electricity Board rates.

For more information visit www.refuel.eu