A green alternative in gasification

NUS Chemical and Biomolecular Engineering (ChBE) Professor Wang Chi-Hwa has a keen interest in exploring the large potential of gasification — a thermal process which utilises raised temperatures of above 500 degrees and an oxygen-deficient environment — as a low-cost green alternative to common waste disposal methods like incineration, which generate greenhouse gases and produce toxins that could adversely affect human health if not properly controlled. This is due to the ability of gasification to harness energy from biomass in the form of synthetic gas (syngas) without causing severe environmental impacts, yet producing valuable solid residues that can be used in other applications.

In a recent project, Prof Wang and his team disposed of food waste through co-gasification with woodchips, producing syngas. Following this, the team investigated the effect of the resulting biochar residue produced on the nutrient-poor acidic soil of adinandra belukar — a tropical secondary forest found in the region. The alkaline biochar was mixed with samples of the soil collected along Kent Ridge Road and then used in a cultivation study involving water spinach, or kangkong as it more commonly known locally. 

In addition to improving the pH value of the soil and reducing aluminum ions, the large specific surface area of the biochar enabled it to absorb and retain minerals. Its numerous hollow pores also helped the plant’s roots access the nutrients and moisture within the deeper layers of the soil, a process typically restricted in the compacted adinandra belukar soil.

After eight weeks of cultivation, the water spinach grown in a soil-biochar mixture of 2:1 exhibited a 4-fold increase in height, 10-fold increase in weight and 5-fold increase in leaf surface area.  

Prof Wang is currently in talks with several government agencies to explore the application of these findings in local greenery areas.

Samples used in the studies (clockwise from top right): water spinach, adinandra belukar soil, woodchips, biochar and horse manure with wood bedding

In another project, Prof Wang found that gasification could be used as an environmentally-friendly way to recover ammonia from manure waste while producing syngas for electricity generation.

More than 100 tons of chicken and horse manure waste is produced daily in Singapore. It is a dangerous source of pathogens and nitrates with the possibility of contaminating surface and ground water and threatening human health if not properly treated. Of greatest concern is the emission of ammonia, which could contribute to the formation of aerosols and cannot be controlled by traditional disposal methods.

Prof Wang’s team collected horse manure waste from the Singapore Turf Club. Around 70 per cent of the ammonia in the waste was captured and converted to struvite to be used as fertiliser. The leached waste was then co-gasified with wood bedding and straw. The syngas produced from just 10kg of waste had the ability to generate 10kW of electricity, enough to power a standard lightbulb for 160 hours. A financial feasibility analysis conducted showed an almost 85 per cent profitability in adoption of the concept in Singapore.

“This work will be very valuable in undeveloped areas where there is no connection to the power grid. You need only add wood to the waste and you can have electricity. You could use horse manure and wood waste to run a refrigerator to keep medical supplies in optimal condition, helping to save a lot of people. This is the proof of concept of how simple it can be,” said Prof Wang.

Providing further evidence of gasification as a sustainable green technology, another research paper by the team estimated that in Indonesia, the world’s leading palm oil producing country, using gasification in 100 villages to dispose of oil palm waste could reduce around 179 tons of greenhouse gas emissions each year, compared to current diesel-based electricity generation. Electricity produced through gasification could also be fed into the national grid, saving around 60,000 tons of annual greenhouse gas emissions, compared to combustion-based practices in palm oil mills.

The next steps for Prof Wang include examining ways to achieve higher energy efficiency with less carbon dioxide emissions and to minimise tar formation during gasification in line with his zero-waste goals.

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