Issue Date: May 9, 2016
Uphill climb for biogas in Asia
For centuries, people have burned waste and wood for energy, using those fires to heat their homes and cook their food. All across rural Asia, many residents are still using waste to create energy for their homes—the technology’s just a little more sophisticated these days.
In places like China and India, people collect the waste from their kitchen, toilets, and livestock and dump it into a large, air-tight machine called a digester. Blades inside the device churn the material, mixing it with microorganisms living inside. These microbes digest the waste to produce useful substances: fertilizer, which can be applied to crop fields, and biogas, which can be burned as fuel.
In the past few decades, the governments of China, India, and Thailand, among others in Asia, have been installing digesters throughout their countries. The devices are widely viewed as a single technology that can address multiple issues at once, from producing renewable energy to providing some form of waste management in rural areas without sanitation infrastructure.
As of 2013, China had spurred the installation of over 40 million household digesters through aggressive policies and generous subsidies. SNV Netherlands Development Organisation, an international rural development nonprofit group, has also been pushing the use of small-scale digesters in countries like Nepal, Vietnam, and Laos. By 2014, SNV had helped install more than 600,000 household digesters in the region. And with sustained commitment from governments, the number is expected to grow.
Thailand, which has the most advanced biogas sector in Southeast Asia, is aiming for a six-fold increase in biogas production by 2021. And China is expected to double or triple its biogas capacity in the next five years, says Guanyi Chen, who leads the School of Environmental Science & Engineering at Tianjin University.
“There is an obvious need for energy in China,” says Yongzhong Feng, a professor of ecology and engineering at Northwest A&F University. An increasingly affluent Chinese population is consuming more and more electricity, heat, and meat. Many think biogas—mainly composed of methane—could help meet those needs in a sustainable way.
Biogas could offer other benefits, advocates say. Rural areas in Asia that are struggling to manage vast quantities of manure from pig, dairy, and chicken farms would especially benefit: Generating biogas from livestock waste would prevent these animals’ feces from being dumped directly into rivers and lakes, a common practice in parts of Asia that lack waste management infrastructure. Also in the plus column for biogas: It burns more efficiently and cleanly than traditional biomass such as dung or firewood, releasing less carbon monoxide and fewer harmful particulates such as black carbon (Environ. Sci. Technol. 2013, DOI: 10.1021/es304942e).
Still, biogas hasn’t taken off. At the moment, biogas contributes less than 1% of the energy consumed by most countries across the globe. And, by some estimates, China has accessed only about 5% of its biogas potential (Renew. Sust. Energ. Rev. 2016, DOI: 10.1016/j.rser.2015.09.097).
Meanwhile, the biogas landscape is changing: Livestock farms in Asia are consolidating in response to increased demand for meat. At the same time, the agricultural workforce is shrinking as job-seeking citizens migrate en masse from rural areas to more lucrative urban ones. Nations such as China are responding by investing in the development of medium- to large-scale digester technologies capable of handling waste from industrial farms and big cities at reasonable costs.
Such a scale-up of biodigester technology and biogas production, however, means that scientists and government officials now face several challenges, including developing ways to digest larger, more complicated waste streams in a way that’s cost-competitive with cheaper fossil fuel energy sources. And in countries where the electric grids are already operating at full capacity, they need to find uses other than electricity generation for the resulting biogas.
Perfecting an ancient technology
Anaerobic digestion—the oxygen-free process used by microorganisms inside digesters to break down waste—occurs in four steps, with each facilitated by a different type of microbe. First, hydrolytic bacteria chow down on the complex carbohydrates, proteins, and lipids in waste, breaking up glycosidic, peptide, and ester bonds to convert them into smaller and more soluble sugars, amino acids, and fatty acids. Fermentative bacteria then feed on these soluble compounds, oxidizing them to form simple organic acids, which are passed along to acetogenic bacteria. These microbes split the carbon-carbon and carbon-hydrogen bonds within the acids to turn them into acetate, producing carbon dioxide and hydrogen as by-products. Finally, the acetate and some of these gaseous compounds are consumed by methanogenic bacteria, which use the building blocks to produce methane. Throughout the process, some carbon dioxide and hydrogen sulfide are released with the methane, making up the components of raw biogas.
“These steps are sequential,” says Yen Wah Tong, an associate professor of chemical engineering at the National University of Singapore, “and the bacteria require different conditions.” With small-scale digesters, all the different bacteria comingle in a single reactor and carry out their reactions side by side. But at a large scale, the reactions start to interfere with each other, lowering the efficiency of biogas production. For instance, large amounts of acids generated in the first two steps can inhibit the activity of the methanogenic bacteria in the last step and cause a drop in methane production.
To improve the technology for large-scale digestion, scientists are leaning toward machines that separate the bacteria in two or three chambers, Tianjin’s Chen says.
The most common strategy is to separate the acidic steps from the methane-forming step. In a two-stage reactor, for example, the waste material goes into the first reactor, and simple organic acids produced are transferred to the second reactor, where methane formation takes place. With separate reactors, scientists can optimize conditions such as pH and temperature to suit the tastes of different bacteria, which can also be added separately into the appropriate reactor. A digester system can involve more than two reactors, but adding too many results in a complex system that can become costly to install, Tong says.
Another challenge to optimizing large-scale reactors is balancing the nutrients being fed to the bacteria. Too much nitrogen causes ammonia to accumulate during fermentation, which is toxic to methanogenic bacteria. On the other hand, too much carbon slows overall bacterial growth because the microbes need nitrogen to build cell biomass. An optimal ratio of carbon to nitrogen is about 25:1, but that can vary with the waste material being used as microbe food.
To strike the right balance, many research groups are now experimenting with codigestion, in which nitrogen-rich materials such as chicken and goat manure are digested together with carbon-rich materials such as corn stalks and crop straw. Northwest A&F University’s Feng tested various combinations of goat manure with residues such as corn stalks and rice straw and found that all of the combinations produced more biogas than if they were fed to the microbes alone (PLOS One 2013, DOI: 10.1371/journal.pone.0066845).
The problem with that, though, is that manure is much easier to digest than crop residues, which typically contain hard, cellulosic materials that are difficult to break down. To speed up digestion of these tough bits, some researchers are testing pretreatment methods. For example, using strong acids or alkalis helps to reduce cellulosic materials to saccharides, which are easier for bacteria to digest. Engineers typically favor the acids over the alkalis, however, because hydrolytic bacteria in the first digestion step can easily acclimatize to acidic conditions.
In megacities like Singapore, Beijing, and Shanghai, where government officials would like to see digesters handle municipal solid waste, an additional complication is that different bacteria thrive on different types of waste. Municipal waste is often a complex mixture of paper, food waste, and sewage sludge.
“It’s very different for different types of waste, even different types of food,” says Tong, who is working to identify the various strains of bacteria that best digest specific wastes. Tong’s research group collects food and sewage waste, and even peat soil from Indonesia, and lets the material sit undisturbed for months. After a while, certain naturally occurring bacterial strains living in the material begin to dominate.
Tong and his team culture and sequence those dominant strains. By collecting this information, Tong hopes to identify the types of bacteria that do well in different environments and create formulations of bacterial communities that can enhance biogas production in digesters handling various types of waste.
Opening the market
Residents and businesses in Asia can burn raw biogas for cooking or generating electricity. But in countries like India and Thailand, electrical grids are at full capacity, making it impractical to add another electricity source. To make use of biogas in those areas, researchers are working on isolating methane from it. Liquefied or compressed methane could then serve as fuel for the many vehicles in those countries (Appl. Biochem. Biotechnol. 2014, DOI: 10.1007/s12010-013-0652-x).
In India, more than 1 million vehicles already run on compressed natural gas, and in Thailand, there are some 400,000 such vehicles. “In place of compressed natural gas, vehicles can move on biogas,” says Virendra Vijay, who leads the Center for Rural Development & Technology at the Indian Institute of Technology, in Delhi. “This opens up the market.”
It’s not easy to purify raw biogas, though. Scientists have to remove carbon dioxide, which is present at about 20 to 30% by volume, and traces of hydrogen sulfide and ammonia, which can corrode engines. In the early 2000s, Vijay developed a simple purification method called water scrubbing. It works by forcing raw biogas at high pressures through water, which absorbs the hydrogen sulfide and carbon dioxide. Although water scrubbing has been proven to work on a large scale, it requires a lot of water and can be wasteful in instances where the water is not recycled.
Other researchers are using chemicals or membranes to separate methane from biogas. Similar to how carbon capture technologies work, scrubbing agents such as amines can absorb carbon dioxide and hydrogen sulfide from raw biogas. And membranes can be designed to retain pure methane while allowing everything else to pass through.
Although membranes do not consume water or generate chemical waste, they can be expensive to install, which is a major drawback. In general, the up-front cost for investing in a separation plant to upgrade biogas is about 20% higher than a biogas-based generator for electricity, according to Pruk Aggarangsi, an environmental engineer at Chiang Mai University.
Looking to the future
Some researchers want to repurpose the carbon dioxide emissions that are part of biogas production to maximize biogas’s potential as a renewable source of energy. One strategy under investigation uses carbon dioxide to feed and grow algae and then harvests the algae’s biomass for breakdown in digesters—a sustainable cycle (Biotechnol. Adv. 2013, DOI: 10.1016/j.biotechadv.2013.06.005).
Advocates will need to overcome other, nontechnological challenges for biogas to become a significant energy source. In China, because low-quality digesters built in the 1950s did not work well, some residents have become skeptical of the technology and have stopped using the devices, Northwest A&F University’s Feng says. The government-backed Biogas Institute of the Ministry of Agriculture has since set up the Biogas Research & Training Center to provide personnel with technical expertise so that they can help rural households use their digesters successfully.
But the biggest challenge facing biogas development may be low oil prices, says Vincent Choy, director of the Biogas Asia Pacific Forum. Cheap oil and shale gas make the initial cost of setting up a biogas production system unattractive. In the U.S., several large-scale biogas plants have recently been forced out of the energy market because of ultralow oil prices. Similarly, some biogas programs in Malaysia and Indonesia have been delayed.
For countries without existing sanitation infrastructure, waste management remains the main driver for biogas technology development, which is still heavily dependent on government support. Once the subsidies run out, though, the biogas industry will have to be able to turn a profit in order to survive, a struggle that will play out over the next decade.
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