Asia's cement makers are cutting their fossil fuel consumption and turning to the rice paddy across the street for a greener, cheaper and potentially more abundant fuel to heat their kilns.
Cement plants in the world's largest rice exporters, Thailand and India, have started to replace coal with rice husks that would have otherwise been incinerated and dumped in landfills.
Offering savings of millions of dollars a year, other rice producers such as China -- the world's largest cement producer -- Vietnam, Sri Lanka and the Philippines could soon be making the switch from coal to compost.
One of the pioneers, Thailand's Siam City Cement, says spending 15 million baht (US$360,400) three years ago on equipment to handle agricultural waste is now saving as much as 240 million baht a year, about 2 percent of last year's operational costs.
"It's a win-win solution, but you need to invest and take the effort," said Siam City Cement's energy development manager Choompon Lertchuwongsa.
"But I can see a big opportunity in all Asian countries, especially China, Vietnam and India," he said.
Cement makers have long used industrial waste as a cheap way of adding raw materials iron, calcium and silicon to lime-stone to make clinker, which is then ground with ash to make cement.
But modern plants have little room to improve energy efficiency, so producers wanting to cut costs need cheaper fuels in their clinker kilns, which reach 1,500oC -- a third of the temperature of the sun's surface.
Australian companies, such as Boral Ltd's cement unit Blue Circle Southern, burn old tires.
In developing Asian countries, agricultural waste is the most readily available fuel.
Cement companies' reliance on coal and lignite as fuel has ignited concern among environmentalists who claim the resulting carbon dioxide emissions are damaging the earth's atmosphere and contributing to global warming.
While rice husks start out greener than coal, they do release carbon dioxide into the atmosphere, but with around one-fifth of the sulphur emissions of coal and lignite.
Supporters of the husk fuel alternative point out that rice mills were burning the husks anyway, so net carbon dioxide emissions should fall.
And apart from generating heat in the kiln, the ash from the rice husks is mixed into the cement, leaving no waste.
Rice husks and palm kernel residues, a by-product of palm oil extraction, now account for around 5 percent of Siam City's energy
needs.
It buys 200 tonnes of rice husks a day from nearby mills at about 150 baht per tonne, but wants 10 times that amount.
"We can take 2,000, 3,000 or 4,000 tonnes a day, but the mills can't supply that much and it's bulky, so transportation is diffi-cult," Choompon said.
"The mills are also starting to put up the price because they know we want it. A couple of years ago it was only 50 baht a tonne," Choompon said.
Rice husks release about 16 joules of energy per kilogram, about the same as lignite but less than bituminous coal's 25 joules. Palm kernel residue releases 19 joules per kg while rubber tires yield around 30.
Siam City says executives from companies owned by Switzerland's Holcim Ltd in Vietnam, the Philippines, Indonesia and Sri Lanka have visited its plant, 130km north of Bangkok, to consider adopting rice husks as a fuel.
Industry officials from the world's second-largest cement producer, India, have also started using rice husks and bamboo shavings from the furniture industry.
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