Activated carbon

Activated carbon, also called activated charcoal or activated coal is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions. It has a high degree of microporosity, just one gram of activated carbon has a surface area in excess of 500 m2.

Suitable raw materials

Activated carbon can be produced from carbonaceous source materials such as agricultural by-products like nutshells, peat, wood, coir, almond shells, apricot stones, cherry stones and grape seeds and lignite, coal, petroleum pitch etc.

Uses

The carbon adsorbents can be applied for the removal of tri halomethanes and metal ions from water, purification of waste solutions, water treatment, separation and concentration of trace elements and radioactive isotopes, production and analysis of high purity substances, fuel gas treatment and hazardous waste remediation etc. It is also used for the removal of undesired colour, smell and other impurities in many types of industries including basic drugs, fine chemicals, glucose, sugar, electroplating plasticizers and so on.

Production processes

Activated carbon can be produced by one of the following processes:

Physical reactivation: The precursor is developed into activated carbons using gases. This is generally done by using one or a combination of the following processes:

Carbonization: Material with carbon content is pyrolyzed at temperatures in the range 600–900 °C, in absence of oxygen (usually in inert atmosphere with gases like argon or nitrogen). Activation/Oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (carbon monoxide, oxygen, or steam) at temperatures above 250 °C, usually in the temperature range of 600–1200 °C.

Chemical activation: Prior to carbonization, the raw material is impregnated with certain chemicals. The chemical is typically an acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, respectively). Then, the raw material is carbonized at lower temperatures (450–900 °C). It is believed that the carbonization / activation step proceeds simultaneously with the chemical activation. Chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating material.

Simple one-step method for agricultural by-products

Agricultural by-products have also proved to be promising raw materials for the production of activated carbons because of their abundant availability at a low price at the source of production. Examples are almond shells, nut shells, apricot stones, cherry stones and grape seeds. A simple one-step method for the production of activated carbons is a feasible alternative to the traditional two-stage process for the production of activated carbons by consecutive carbonization of the raw material and high temperature activation (900–1000°C) of the solid product from carbonization.

Agricultural by-products can be subjected to steam pyrolysis-activation at treatment at temperatures of 600 –700°C. The presence of water vapour during pyrolysis leads to a considerable increase in the liquid and gas product yields in addition to reducing the sulphur content in liquid and solid products. The properties of carbon adsorbents obtained by steam pyrolysis of depend on the treatment conditions such as the temperature and duration of treatment and the choice of raw materials.

Preparation of activated carbon from biomass

The activated carbons can be prepared by one-step pyrolysis of almond shells, nut shells, apricot stones, cherry stones and grape seeds, in the presence of steam.
The raw material is heated in an atmosphere of pure water vapour at a heating rate of 15°C per min to a final carbonization temperature of 800°C for duration of one hour. After the treatment it is left to cool. In order to obtain the carbons with steady content of surface oxides the steam flow is interrupted at a temperature of 300°C. The carbon particles preserve the shapes of the raw materials particles of regular shape.

Preparation of carbon adsorbents

Tar from steam pyrolysis and furfural are treated with 5% H2SO4 and the mixture is heated to 160°C with continuous stirring. The solid products obtained are heated to 600◦C in a covered container at a heating rate of 10°C per min under nitrogen. Oxidation of carbonizates with air at 400°C is used to obtain activated carbon with acidic surface properties. Steam activation with water vapour at 800◦C for one hour is used to obtain activated carbon with alkaline surface.

Electricity from waste water

Electricity can be produced from waste water using microbial Fuel Cell (MFC). It can turn the organic wastes into a source of electricity.
Fuel cell

A fuel cell is an electrochemical energy conversion device that converts the chemical energy from fuel (on the anode side) and oxidant (on the cathode side) directly into electricity. There are many types of fuel cells, depending on what kind of fuel, electrolyte and oxidant they employ. Fuel cells use hydrogen gas which is produced from fossil fuels.
Microbial Fuel Cell

Microbial Fuel Cell makes the treatment of organic pollutants a direct producer of electricity, not a consumer. Further it expands fuel-cell technology to use renewable organic materials as a fuel; the MFC can use organic fuels that are wet, the usual form for wastes and fuel crops. The MFC, by operating at ambient temperature, can double to triple the electricity-capture efficiency over combustion, while eliminating all the air pollution that comes from combustion.

Mechanism

MFCs function on different carbohydrates but also on complex substrates present in wastewaters. As yet there is limited information available about the energy metabolism and nature of the bacteria using the anode as electron acceptor; few electron transfer mechanisms have been established unequivocally. Nonetheless, the efficient electron transfer between the microorganism and the anode (e.g., microorganisms forming a biofilm on graphite fibers) seems to play a major role in the performance of the fuel cell.

Designing considerations for 1MW DOWNDRAFT GASIFIER WITH TAR CRACKING

This write up gives complete considerations for designing and setting up a biomass gasifier based power plant of 1 MW to meet the energy needs of an industry.
1. Need for fixed bed gasifiers
As gasifiers up to 1MW capacity may be of the fixed bed type. Due to its simplicity, this type is recommended. Fluidized bed gasifiers can be built and operated only at higher capacities. Two down draft gasifiers of Imbert type are recommended. Two gasifiers each of 500 kW capacities will give the following advantages.
i. If one unit is shut down for maintenance, the other will deliver power without interrupting the operation of the FC power plant.
ii.Both 500 kW units can work near full load taking advantage of the higher efficiency of the gasifiers at full load.
iii. Flexibility in meeting the fuel wood inventory even in lean periods of supply.
2. Components of the gasifier plant
The gasifier plant is proposed to have the following components in order: a) Fixed bed gasifier (down/up draft), b) Cyclone, c) Hot gas cleaning unit, d) Tar cracking unit , e) High temp. tar trap (filter), f) Secondary gas cleaning unit, g) Shift converter, h) CO2 Scrubber (if needed), I) Boiler, j) Combustor and k) Heat exchangers.
The fuel handling, communition and drying equipments will include: a) Chain conveyors for handling fuel logs to crushers, b) Two stage toothed roll crushers to produce chips of required sizes, c) Chain conveyor for taking chips form the roll crushers to the storage bin, which is assisted by a hot air blower for drying chips, d) Storage bin, e) Screw conveyors with lock hoppers for handling the chips and feeding to the gasifiers, f) Gas sampling ports, g) Temperature sensors, h) CO alarm and oxygen masks and I) Flame arrestors.
3. The gasifier
3.1 Dimensions
The key dimensions of a typical down draft gasifier can be worked out based on a hearth load of 1 Nm3 /cm2. h as follows: Throat diameter: 450mm; No of nozzles: 10; Air nozzle diameter: 18mm ; Fuel container diameter: 1.5 m, Height: 2.5m; Overall dimensions: 2.5 m; Height : 3.0m.
3.2 Insulation of the gasifier
The thickness of insulation with kaolin is 500 mm for the assumed heat load.
3.3 Biomass storage
The requirement of biomass is about 18 tonnes per day. The storage space is required for an inventory of 600 tonnes to meet the demand for one month.
The wood logs can be stored as a stack facilitating natural drying. An area of 40 m by 30m is needed for the installation of two stage roll crushers and chain conveyors to feed the logs into the crushers. An additional area of a minimum of 2m by 7m is needed for the trucks to unload the logs into the storage space.
3.4 Chips handling
In order to feed the gasifiers with chips of say 30 by 40 by 70 mm size, roll crushers are needed. In the first stage logs are broken down to small pieces. In the second stage chips of desired size are prepared. The power consumption of the crushers and chain conveyors will be 30 kW with 225 mm serrated rolls to handle logs of up to 150 mm diameter.
The chips shall be dried by hot air generated by the waste heat available from the combustor exhaust gas through a heat exchanger or by dilution with air.
It is proposed that the dry chips shall be fed by screw conveyors to the gasifiers from the top. The chip storage bunker may contain a scraper for feeding the horizontal screws fitted at the end of the storage space.
The screw conveyor may consist of a horizontal section to collect chips from the bunker, an upright section to the top of the gasifier and a short section to feed the gasifiers. Lock hoppers may be provided at appropriate locations to prevent gas leakage during feeding. The diameter of the screw is 225 mm in this case.
3.5 Ash handling
A short screw section can be used followed by pneumatic handling after collecting from the bottom of the gasifiers. The grate needs to be kept cool in order to avoid warping. For collecting ash from the bottom a rotating grate with a scraper can be used.
3.6 Air handling
A continuous supply of air at about 30 % of the stoichiometric requirement is needed. This shall be supplied by a blower of 600 Nm3 per hour capacity. The hot air blower for chip drying shall have a capacity of 200 Nm3 per hour.
4. Tar cracking
Particulates, volatiles and tar have to be removed from the producer gas prior to the reformer in case power is to be produced by internal combustion engine. Particulates are removed in a cyclone and hot gas filters. For tar cracking a temperature of 800 to 900 deg C is necessary. Several studies indicate that dolomite is an efficient catalyst. Glanshammar dolomite has been shown to yield 2 g tar per kg DS. The hydrogen content of the gas is increased at the same time. It is possible to have the tar cracking unit either integrated with the gasifier taking advantage of the heat loss at high temperature zone or to set up the unit outside the gasifier. The dust load and fluctuation in temperature coupled with gas leakage problems favour a separate tar cracker .
4.1 T.C. Unit specifications
The amount of tar contained in the producer gas ranges as follows. Downdraft: 0.001 to 0.01 kg per kg dry feed.
Studies conducted on tar cracking using pyrolysis gas indicate that a temperature of 800 to 900 deg.C is required to minimize the tar content in the exit gas. A catalyst load of 0.25 to 0.30 kg per kg DS may be adopted. The detailed dimensions is to be worked out based on the component fractions, amount of tar, regeneration possibilities of catalyst, catalyst granulometry, temperature profile, etc.
4.2 Combustor
A combustor is needed to produce hot gas to heat the reforming catalyst bed to around 900 deg. C. A swirl gas combustor of 300 mm diameter and 280 mm shroud length can be tentatively suggested. The fuel gas can be taken from the producer gas plant. The requirement shall be calculated based on the differential temperature, catalyst bed temperature, catalyst bed temperature and heat capacity of the bed.
4.3 Drying of chips
It is wise to dry chips rather than logs. Hot air at 60 to 70 deg C is recommended. Hot air can be obtained by mixing about 10 times air with the combustor exhaust gas.
5. Steam boiler
Steam has to be supplied to the reformer and the shift converter to get hydrogen rich gas in case of running a fuel cell. The hot water from the fuel cell generator and waste heat from the gasifier can be used for steam generation. The steam capacity of the boiler is estimated to be 800 kg per hour at a pressure of 20 bar giving super heated steam at 850 deg.C.
6. Housing and instrumentation
The housing requirement is estimated as follows. Biomass storage, crushers and dryer: 50 m by 40 m. Gasifiers, tar cracking units and reformer: 10 m by 4.5m, Steam boiler, combustor and heat exchanger: 2.5 m by 3 m. Safety instruments such as CO monitor & alarm (150ppm), oxygen masks, fire protection machineries will also be housed. Flame arresters are to be provided in the gasifiers and combustors. Data monitoring and acquisition systems for chips level in bunker and gasifier, temperature and gas flow need to be incorporated.

Adsorbed platinum Catalyst improves fuel cell performance

Chemists in the US have developed a new catalyst that could help in a key reaction used to generate hydrogen for fuel cells.
The water-gas shift (WGS) reaction is used in industry to help purify the hydrogen that is generated as a by product in the reforming of natural gas. In the reaction, residual carbon monoxide in the hydrogen is combined with steam in the presence of a catalyst to produce hydrogen and carbon dioxide. The process purifies the hydrogen gas to a level where it can be used in fuel cells.
However, none of the catalysts currently used in the WGS reaction are ideal. Copper based catalysts essentially do the job, but copper can spontaneously ignite in air, a dangerous property given that air is often used in fuel-cell operation. One alternative has been platinum, yet on its own this element is prohibitively expensive. To stand any chance of commercial viability it must be prepared in tiny particles supported by the rare earth oxide ceria. Unfortunately, ceria is only found in a few places and its supply is restricted. But platinum could be used as a catalyst for the water gas shift reaction.
Researchers at Tufts University and Harvard University, Massachusetts, have been investigating other ways to make platinum viable as a WGS reaction catalyst. By chance, they discovered that alkali metal ions like sodium or potassium can activate fine platinum particles when they are adsorbed on alumina or silica. When they examined the structure, they found extra active oxygen species that seemed to help the platinum complete the reaction cycle. The researchers claim that this finding will surely be a stepping-stone to new and more efficient catalytic formulation in particular with a view at reducing the quantity of platinum needed to obtain an active catalyst.

Microbial fuel cell for power and treat waste water

Penn State environmental engineers have shown that a microbial fuel cell (MFC) can generate electricity while simultaneously cleaning the wastewater that is flushed down the drain or toilet. Between 10 and 50 milli Watts of power per square meter of electrode surface can be produced, while removing up to 78 percent of organic matter as measured by biochemical oxygen demand (BOD). Thus MFCs represents a completely new approach to wastewater treatment in addition to power generation. The MFC technology can provide a new method to offset operating costs of wastewater treatment plant, making advanced wastewater treatment more affordable for both developing and industrialized nations as claimed by the researchers.

Other researchers have shown that MFCs can be used to produce electricity from water containing pure chemicals including glucose, acetate or lactate.

Principle

Microbial fuel cells work by the action of bacteria which can pass electrons to an anode, the negative electrode of a fuel cell. The electrons flow from the anode through a wire, producing a current, to a cathode, the positive electrode of a fuel cell, where they combine with hydrogen ions (protons) and oxygen to form water in accordance with the conventional fuel cell principles. The naturally occurring bacteria in wastewater drive power production via a reaction that allows them to transport electrons from the cell surface to the anode. In addition, a reaction (oxidation) that occurs in the interior of the bacterial cell lowers the biochemical oxygen demand thus cleaning the water. MFC used has an overall dimension of about six inches in length and 2.5 inches in diameter and contains eight anodes, composed of graphite. The cathode is a carbon/platinum catalyst/proton exchange membrane fused to a plastic support tube. The unit has about 36 square inches of surface area to which the bacteria can adhere and pass electrons.

See for photos at: http://live.psu.edu/still_life/2004_02_23_research/index.html and for animation : http://www.nsf.gov/od/lpa/newsroom/pr.cfm?ni

Carbonization of wood

Charcoal is produced by controlled combustion by supplying very limited available oxygen by which the volatile vapours and gases are driven off from biomass particularly wood.
Charcoal
Charcoal has a higher energy density than wood and is smokeless. Traditional charcoal kilns are used to make Charcoal by simply heaping mounds of wood covered with earth, or pits in the ground. But the process of carbonisation is very slow and inefficient in these kilns. Traditional earthen kilns have yields closer to 10% as there is less control. In general during the process of carbonization, considerable amount of carbon is emitted to the air and approximately 50% of the carbon in the biomass can be obtained in the form of charcoal. Carbon yield obtained using various types of kilns is less than 50 % on the average, which means that half of the total carbon contained in the raw material will be emitted to the air or stored in the form of half-carbonized matter which can easily get decomposed, but there is a need to increase the carbon yield. Field research has improved the efficiency of charcoal furnaces.Improved charcoal furnaces operating at about 600°C produce 25-35% of the dry wood as charcoal, and even the gases produced can be used for kiln drying. The charcoal produced is 75 - 85% carbon and is useful as a compact, controllable fuel. It can be burnt to provide heat on a large and small scale.
Composition of charcoal
Charcoal is composed of (1) moisture, (2) ash content, (3) volatile matter and (4)fixed carbon. These parameters are calculated as given below using standard methods.
Methods for calculation
(1) Moisture content: Moisture content of charcoal immediately after finishing carbonization in the kiln will be nil, but increases subsequently due to absorption from the air. Moisture content of charcoal is calculated as :
Moisture content (%) = (Weight of charcoal – Oven dried weight of charcoal)/Weight of charcoal x 100
(2) Ash content: It comes from few minerals contained in material (CaO, K2O,MgO, etc.)Ash content of charcoal is calculated as:
Ash content (%) = Weight of ash in charcoal/Oven dried weight of charcoal x 100
(3) Volatile matter: It contains all liquid and tarry matter not fully driven off in the process of carbonization. If the carbonization is prolonged and if the temperature is high , then the volatiles content of is low. Volatile matter content of charcoal is calculated as:
Volatile matter content (%) = Weight of volatile matter in charcoal/Oven dried weight of charcoal x 100
(4) Fixed carbon:Fixed carbon content of charcoal is calculated as:
Fixed carbon content (%) = 100 – Moisture content - Ash content – Volatile matter content.
Pure carbon in charcoal is difficult to be decomposed as it shows no chemical affinity to the oxygen of the air and it does not rot, neither in an aerobic nor in an anaerobic way. This component mainly contributes to carbon storage.
(5) Charcoal yield and carbon yield: Usually, efficiency of charcoal production is indicated by charcoal yield. Charcoal yield is calculated using formula as:
Charcoal yield (%) = Weight of charcoal (kg)/Oven dry weight of wood material (kg) x 100
It expresses the percentage of pure carbon derived from wood material which is preserved in the produced charcoal. This indicator, however, cannot explain the efficiency of carbon fixation potential.

Co-firing of biomass and wastes

Co-firing means firing materials along with coal or other fuels in the existing or modified boiler firing systems. Materials such as biomass and waste are solid fuels in their own right, but of low calorific value, of variable composition, and requiring special low-intensity combustion conditions. If mixed with coal, the average calorific value is increased, enabling it to be burned in conventional combustors and to have more average fuel properties. If coal is mixed with material such as a typical biomass, then the combustion process can be considered to be increasingly CO 2 neutral. If coal is mixed with refuse or other waste materials, its combustion provides a route to the disposal of unwanted materials with the advantageous production of energy.

Biomass co-firing

There have been extensive research to increase the use of biomass, and one of the easiest ways of introducing this technology is by co-firing with coal in existing coal installations. A similar approach can also be taken with the disposal of waste, especially domestic refuse. The combustion of biomass therefore also parallels that of coal in many respects. The first step involves devolatilization similar to that of coal.

Namely: Biomass volatiles + char

In this reaction volatiles have a composition controlled by the biomass composition, the heating rate, and the final temperature. A typical biomass will have cellulose linked via oxygen bands; hence the high oxygen content in biomass fuels. Many biomass materials contain mainly carbon and some hydrocarbons; consequently, the composition of the volatiles is dominated by the production of CO, some hydrocarbons, and a small amount of tar. The char itself has reactivity very similar to coal char and has enhanced catalytic reactivity at low temperatures due to the metallic content of Na and K. For the mixtures of biomass and coal, the initial components devolailize and their chars burn independently. The volatiles mix, however, and the calorific value of this mixture and resultant flame temperature is very much a function of the composition of the components; the combined flame temperature is fed back to the initial chemical steps and consequently there is a very significant synergistic interaction. The subsequent rates of burn out of the chars are determined by their individual reactivity (and combined temperature) and generally biomass chars burn out more rapidly than coal chars.

Waste co-firing

The types of wastes suitable for co-firing include biomass waste, municipal solid waste, and automotive tires. The comparison is variable but has a calorific value between 10-20 MJ/kg. When the waste is taken from a controlled source, e.g., waste paper, with a constant calorific value, then the process of co-firing is simplified. In the other extreme, relatively small amounts of certain toxic wastes can be co-fired with coal in cement manufacture. The cement contains the trace elements in a generally satisfactory way although great care has to be used to ensure that it meets the required environmental emission standards.