Rice-husk-fired boiler for parboiling rice mills
Combining high-efficiency with low energy
Some eat to live while others live to eat. No matter which school one belongs to, nothing quite matches the pleasure of eating a plateful of what one relishes. Yet, sometimes our peculiar food habits could be unhealthy for the environment. In Bangladesh, for example, 90% of all harvested rice is parboiled, and in conventional furnaces. (This populous country harvests over 30 million tonnes of rice each year from its rich alluvial soil in the Gangetic delta.) And though Bangladeshis love their rice, it certainly has left a bad taste in the mouths of environmentalists and rice mill owners.
Food for thought
Bangladesh is one of the largest producers of rice in the world–it processes about 28 million tonnes a year. Since a majority of this was sent to furnaces for parboiling purposes, the exercise became a losing game for rice mill owners, farmers, and even the green lobby in environmentalists waged a war against the pollution that these furnaces led to.
In January 2001, for a project conducted in collaboration with the BRRI (Bangladesh Rice Research Institute), Dhaka and the NRI (Natural Resource Institute), Greenwich University, UK, TERI decided to help by developing a furnace that would replace the conventional furnace technology with an improved ricehusk- fired furnace that not only satisfied the rice mill owners but put the environmentalists at peace as well.
The traditional rice parboiling furnaces in Bangladesh are known to operate at an extremely low efficiency of 20%. Moreover, they even use large quantities of rice bran as fuel for the furnace that could easily be used for other more meaningful purposes like fodder for animals. Worse, these furnaces take five times longer than an energy-efficient one to parboil a limited quantity of rice. Pollution levels in these traditional furnaces are enough to asphyxiate workers as carbon monoxide levels in flue gases soar to over 10 000 PPM (parts per million).
A total solution
2001 was a watershed year for the rice mills of Bangladesh. Scientists at TERI came up with an interesting answer for one of the largest small-to-medium scale industries in Bangladesh. A demonstration unit was set up on the outskirts of Dhaka, the capital of Bangladesh. A rice-husk-fired furnace was designed to produce steam for the process heating of paddy in rice mills.
The results from the demonstration unit were highly encouraging. The improved furnace operated at 44% efficiency, thereby saving substantial quantities of rice husk. With the rice husk supply being constant, the saved raw material could be diverted to produce briquettes that easily replaced conventional, polluting fuels used for steam generation in the furnaces of other industries as well. Of all the advantages that resulted after the shift to the new furnace, saving rice bran from being used as fuel was particularly significant.
Emissions of carbon monoxide in flue gases came down to 3300 PPM, well within the permissible limits of 5000 PPM. The steam output rate of the furnace increased from 1 to 2 tonnes per hour at 2 kg/cm2 pressure.
The then-applied technologies in brick manufacture such as clamps, downdraught kilns, and BTKs require huge proportions of highly polluting coal (with an ash content as high as 40%), firewood, and biomass as fuel. In fact, the coal consumed by this industry was an astronomical 24 million tonnes.
A lot of large- and small-scale rice mills benefitted from the improved furnace. Operations became profitable and sustainable. Moreover, conventional steam generating industries also gained from the fuel that was made available to them and it no longer remained a technology exclusively used by the rice mills. The new furnace also ensured a healthy working environment and air quality, since the flue gas would exit only through the chimney, thus improving the working conditions for workers.
All rice-producing nations, especially those that parboil their rice before consumption, stand to gain from TERI's energy-efficient rice husk-fired furnace.
The improved furnaces are twice as efficient as the conventional units. The cost of operation too has come down by over 30%. In fact, the new furnaces cost even less than the conventional ones: the cost has dropped from 95 000 to 64 000 Bangladesh takas. In addition to efficiency improvement, factors of human safety and comfort too have been taken into consideration while designing the furnace. This is reflected in the new building material used–a combination of local clay and husk for the construction of the sidewall. The husk saved could be briquetted since it has good market value and therefore boosts the income of rice mill operators.
Flue gas heat could be used in place of diesel and electricity for the briquetting process, in the case of decentralized briquetting plants. Operators were saved from any direct radiation, as in the case of conventional furnaces. A real win-win situation.