The dirty secret of biomass waste

Burning farm waste for energy is widespread in the developing world, causing carbon emissions and poor air quality even though clean tech exists. Kevin Kung finds the missing link.

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By Kevin Kung | Tata Fellow, PhD Candidate in MIT Department of Mechanical Engineering

Since 2011, my main interest has revolved around biomass waste. Different people have defined biomass variously. In the discussion below, I refer mainly to plant-based residues, such as post-harvest farm and agricultural waste, and/or forestry waste.

Initially, I saw tons and tons of rice husks, sugarcane bagasse, and other types of biomass being burned. The problem, I thought, was that there was no good technology to turn this waste into fuel. So I started a company in Kenya focused on providing the technical process for turning this hitherto unharnessed waste into a low-cost, safe, and high-quality cooking fuel. Households, by buying such fuel, would save 20% on their cooking expenditures. By replacing traditional charcoal, this biomass-derived fuel would also save forests and mitigate greenhouse emissions.

Through the years, as I progressed on my company, and interacted with many more people in the biomass energy conversion sector, something gradually dawned on me. There are plenty of biomass energy conversion technologies out there—for example, biomass boilers (heat or electricity production), pelleting machines (solid fuel production), gasification (electricity production), and pyrolysis (liquid fuel). The possibilities are endless, and so are the biomass energy companies. What I gradually realized is that what is missing, in this case, is not the crucial conversion technology. Rather, it is the challenge of moving biomass waste from point A to point B.

Biomass is available mostly in rural, dispersed locations in small batches. After harvest, the farm waste does not always present itself in the right form to be economically transported. If a batch of biomass is wet, then we are effectively paying freight to transport water. Likewise, if a batch of biomass is loose, then we cannot squeeze too much biomass mass (and thus energy content) into a truckload. Ironically, most biomass waste conversion facilities, on the other hand, are immensely centralized and capital-intensive installations. They often require at least tons and tons of biomass every hour to run themselves. Because collecting tons of biomass from rural areas is very expensive, most of these large conversion facilities can often only be co-located with existing agricultural processing mills.

The city of Muzaffarnagar, in Uttar Pradesh, India, for example, has the nearby fiber-rich agricultural residue go to paper-making and/or boilers. Likewise, a biomass pelleting mill in the United States only collects the biomass waste from about a 50 km radius, and even so, transportation accounts for 90% of their production cost. These conversion processes are therefore, by and large, economically uninteresting in remote areas, where most of the biomass waste from the small-holder farmers still do not have a significant economic value. If there is no significant value to the farmers, then farmers, anxious to clear their land for the next planting season, may simply choose to burn their biomass on-site in the open air.

In a satellite image taken by NASA in 2013 (above), we can see plumes of smoke rising from fields in Punjab. At certain times of year, this smoke covers much of northern India, including Delhi — a severe public health concern. A 2014 Stanford study (Jacobson, 2014) shows that burning biomass may contribute to up to 18% of global anthropogenic CO2 emissions. What’s more, by burning biomass in their field rural farmers are burning an equivalent of US$120 billion/year in cash that they could have potentially earned, if they had the chance to economically convert and sell their biomass waste as a valuable product.

This is one reason why I decided to dedicate my PhD research at the Tata Center to looking at a thermochemical process called torrefaction for densifying biomass waste on-site before transportation. Imagine a mobile, low-cost, and easy-to-operate unit that can travel from farm to farm after a harvest season, converting the biomass waste into a form with a much longer shelf life and less moisture. This then becomes a product that can be much more easily transported and processed. The biomass energy producers can now source their input feedstock at a lower cost. This value chain may even expand to rural areas whose biomass waste was previously burned in open air, creating an additional source of income for millions of remote farmers.