Building on rice

The Torftech Group has developed a novel application of its Torbed technology for the precise control of the combustion of rice husks to produce energy for industrial processes and a valuable ash by-product - Rice Husk Ash (RHA).

During the last three years, this novel application of the TORBED Technology has been developed on behalf of Phoenix Agri-Silica Corporation and the Canadian International Development Agency (CIDA) of Canada.

The Torbed Technology, owned and developed by Reading-based Torftech Group, was invented in 1981. It combines the action of a cyclone and a fluidised-bed reactor, with particles held in a cyclonic motion inside the reactor by a high-velocity air stream flowing up through the reactor.

Centrifugal forces inside the reactor throw the conveyed particles out to the reactor wall, where they meet a spinning downward air flow that carries them to the base of the reactor. Here, they meet the upward airflow again, and the cycle repeats. At any given time, there is a small solids load in the reactor; this allows the temperature conditions to be controlled precisely.

Particles are retained for longer than in a circulating fluidised bed reactor, and are therefore processed more thoroughly. Torbed reactors typically work at gas velocities of 3-12m/sec, compared with 0.1-0.6m/sec for circulating fluidised bed, so a Torbed reactor can have a cross-section 20-30 times smaller than a circulating fluidised bed for the same gas flow. Moreover, there is no need for external cyclones for separating and reinjecting particles.

Torbed technology is now used for processes as diverse as breakfast cereal production, pasteurisation of spices, treating hazardous wastes, industrial mineral production, recycling of waste materials and air cleaning. Several of these processes involve the application of heat to solids, but rice husks are a new form of feed for Torbed.

Why is this application so interesting when it is common practice to combust rice husks? The answer lies in the form of the RHA that is produced by this new combustion process. Most combustion processes produce crystalline ash, which is carcinogenic. But the Torbed system, through its very close temperature control, produces ash that is non-crystalline or amorphous. This gives it unique properties: for example, it can be blended into concrete as a cement replacement.

Amorphous RHA has been extensively tested as a cement substitute in concrete to levels of 10-12% by weight. At this level, the concrete has the same ultimate strength but is less porous than concrete made with only cement. This slows down the rate of 'concrete rot', and is extremely beneficial for permanent structures such as dams, bridges and large buildings. Silica fume, a by-product of the metallurgical industry, is already used for exactly this purpose, but it is becoming limited in supply and is too expensive in the developing economies.

Torftech subjected the RHA combustion process to extensive pilot testing at its main test facilities in Ontario, Canada. These tests showed that a high-quality amorphous RHA could be produced. As a result of this, the first full scale plant has been installed and tested at a rice mill owned by L T Overseas, the largest basmati rice mill in India, situated north of New Delhi. This plant can combust more than a tonne of rice husks per hour under highly controlled conditions, producing more than 1400tpa of high quality RHA as well as generating steam.

How large is the potential for this new process? Global rice production has the potential to generate up to 22million tpa of RHA, which could be used as a substitute for silica fume. Assigning a price of US$300/tonne would make the potential value of this ash product worldwide some US$5-6.5billion per year. Also, when combusted, the global rice husk production has an energy content equivalent to over 1 billion barrels of oil per year.

Another important feature of this new market is the beneficial global environmental impact. Cement manufacturers are among the most prolific producers of carbon dioxide, suspected to be a prime contributor to the threatened global warming. By substituting 10-12% of cement with RHA, less cement is used in the concrete in the first place and by making concrete last longer, the amount of cement that must be produced in the longer term is also reduced. The combustion of rice hulls is a 'neutral' process in this regard since the husks will decay naturally to produce the same amount of carbon dioxide.

Thus, this new process allows cheaper and longer lasting infrastructure projects, less environmental pollution, renewable energy production, local employment and wealth creation where it is needed most.

Chris Dodson is chairman of the Torftech Group and inventor of the Torbed process.

<b>Hydrogen from waste</b>

Treating organic wastes at high temperature is not unusual, but plasma torch treatment takes the concept a stage further than most, Stuart Nathan writes. What's more, it could help provide the fuel for the new generation of fuel-cell vehicles.

Plasma, often thought of as the fourth state of matter, is a high-temperature ionised gas. Made by exposing air to an extremely high voltage discharge, it's generally at around 20,000K.

'At that temperature, you're going to destroy anything you expose it to,' comments Dan Lazzara, chief executive of Westinghouse Plasma Corp (WPC), which specialises in plasma torch applications. 'But it's extremely expensive energy, because of the high voltages you need to generate it. The trick is to use it judiciously.'

WPC is a spin-off from Westinghouse Electric, which originally developed its plasma technology over 50 years ago. Early applications including simulating atmospheric re-entry conditions for NASA's Apollo programme. The technology was incorporated into a plasma gasification reactor (PGR) during the 1990s, for Japanese firm Hitachi. The aim was to replace existing waste-to-energy incineration plants with a new technology that would produce less dioxin and ash, and improve the energy recovery efficiency.

Almost any kind of material can be fed into a PGR, including coal, plant materials, municipal solid waste and industrial sludge. The fierce temperatures perform two functions - the feed material is reduced to carbon monoxide and hydrogen, known as synthesis gas.

The process is so efficient that 100% of the carbon is converted to CO; lower-temperature gasification processes often leave a residue of unconverted carbon in the form of tar. Meanwhile, any inorganic material in the feed, which would become ash in a conventional incinerator, is melted into an inert glass similar to the natural mineral obsidian, which can be used in the contruction and road-building industries.

Synthesis gas is a valuable industrial commodity, which can be used for a variety of purposes. Currently, it is mainly of interest as a source of hydrogen, both for gas-to-liquids technology and as a feedstock for fuel cells. 'We produce a very clean syngas,' says Lazzara. 'Many of the companies who use our technology apply further processes to it to clean it up further.'

Flexibility of feedstock isn't the only advantage of plasma gasifiers. The feed needs no pre-treatment or processing; a mixture of organic and inorganic feeds can be used; and the feed can be fed into the reactor by gravity, rather than through locks and hoppers. Moreover, the plasma can be generated from air, so there's no need for pure oxygen to react with the carbon from the feed.