Biogas is a simple solution. Normally, simple solutions are undervalued and much attention is not paid to them.
We have been discussing biogas in the context of energy only, as cooking or transport fuel. There are, however, other benefits of biogas in terms of its byproducts.
As we will see in the case of biogas, which itself is a product of various waste streams but contains other subsidiary wastes within it; digestate and CO2.
Biogas, as it comes out of digestion tanks, contains methane, CO2, H2S and small fractions of miscellaneous parts. Mainly, methane content is approximately 55% and CO2 is 45%.
In simple cooking, it is burnt in stoves directly. For other uses such as piped gas, CNG, etc, methane is extracted and other products go waste. However, CO2 can be extracted and put to good use as we will discuss.
It may be noted that biogas is the cheapest and cleanest source of CO2, as it contains a limited amount of associated products as compared to exhaust gases from power plants, automotives, etc.
CO2 has many uses as a refrigerant such as dry ice in health care. Similarly, the liquid waste is used directly as fertiliser or is converted into packaged dry or liquid bio-fertilisers.
Biogas output contains approximately 20% digestate by weight of the biomass fed as initial feedstock. Some hydrogen sulfide is also produced in the ratio of 1-1.25%. All are useful materials as we will see.
In this space, we will take into account the possible use of digestate.
Biogas digestate is the residue left from the anaerobic digestion of a variety of wastes including animal dung, food waste, agricultural biomass, municipal solid waste, sewerage, etc. The digestate contains useful nutrients like nitrogen, phosphorous, potassium and calcium. It is a common practice to use biogas digestate as fertiliser, directly or after processing. Processing is required when there is the presence of metals, which are usually found if municipal solid waste and sewerage are there in the feedstock.
Otherwise, in rural areas, no processing is required and the digestate is applied directly.
Digestate is used in the following forms: 1. Solid digestate residue. 2. Liquid digestate. 3. Combined liquid digestate with mineral fertiliser. 4. Combined solid digestate and mineral fertiliser.
Mineral fertiliser can include urea, DAP, etc. It has been observed that liquid digestate with mineral fertiliser gives the highest yield and solid digestate gives the minimum yield, comparatively speaking.
Already in our rural areas, small self- biogas producers are utilising liquid residue as bio-fertiliser in their fields. In many jurisdictions, packaged bio-fertiliser is also produced on a large scale and marketed over wider geographical domains.
CO2 in cold chain
With the increase in global population, urbanisation and gradual rise in the standard of living, the demand for vegetables, meat, seafood and other related items is increasing by the day. Similarly, international heath sector requirements have given rise to the safe/ cold-temperature transport of vaccines and allied products.
As per FAO estimates, 40% of food produced in the world is lost due to it being rotten at various stages of production and marketing. It has been estimated that cold chain availability can reduce this wastage by half.
For the last 50 years, no major changes or innovations have been made in the vapour compression cycle-based refrigeration. The refrigerants were earlier Freon-based, which damaged the ozone layer. Now, HFCs (hydro-fluoro-carbons) are being used, which have several thousand times higher global warming potential (GWP) than CO2.
Thus, CO2 has emerged as a refrigerant of choice for the years to come. Following are the main properties and benefits of CO2. 1. CO2 is a natural refrigerant and is cost effective. 2. CO2 is energy efficient. 3. CO2 is not flammable. 4. CO2 is not toxic and it has the lowest GWP at 1.
Dry ice has been used in cold chain with great success but other variations of CO2, liquid and gas also provide viable cold chain applications.
The applications for CO2 across the food and beverage industry are numerous, particularly because the chemical compound can be used in its three forms – solid, gas or liquid. When it comes to cold chain, the flexible nature of CO2 makes it an ideal application for storage and transportation.
Biogas gaseous residue conversion to dry ice
Biogas is cleaned and upgraded to become a transport fuel as bio-methane (CH4). Bio-methane can be pumped into the gas network or into a CNG cylinder to be used as a motor fuel.
The gaseous residue of bio-methane production is CO2. CO2 share in biogas is 45% plus 1-2% H2S and other materials.
CO2 is then cooled to become liquid. Liquid CO2 is throttled through a valve to take a snow form. CO2 snow is pressed to solid CO2 blocks and pellets.
This process requires a separate plant, which can be co-located with a biogas plant or other liquid gas production plants like oxygen or ammonia.
Biogas route is cleaner and cheaper. It is much cleaner than other sources of CO2 like exhaust gases from vehicles or power plants, which require much more intensive and costly processing.
CO2 is being produced in Pakistan by a number of producers. CO2 market is increasing with the expansion and intensification of healthcare system where it is used in the transport of pharmaceutical products. Also, CO2 is being used in cold chain.
With the modernisation of agriculture and export campaigns, the demand for solid CO2 will increase. Biogas production in Pakistan will help in the production of cheaper CO2. Also CO2 itself can be exported.
Solution for smog
Every winter, we have smog in central Punjab and elsewhere. One of the reasons is rice stubble burning. Instead of burning, biogas can be produced from the bio-digestion of rice residue of various forms including stubble.
This would provide incentives to farmers to support and facilitate investment directly or indirectly in biogas production. Across the border, similar facilities have already been built under a recent campaign.
Concluding, biogas has a widespread geographical potential including and beyond smog areas. Wherever there is bio-waste, there is the scope of producing biogas. And where enough volume is available, byproducts like CO2 refrigerant and bio-fertiliser can be produced and marketed. Provincial governments and local bodies may collaborate with gas companies to develop several projects in this respect. While investment can be done by private parties, the facilitation by governments at various levels is a must.
It is hoped that this piece would generate more interest in biogas beyond energy requirements. It can boost both rural and urban economy.
The writer is former member energy, Planning Commission and the author of several books on energy
Published in The Express Tribune, September 25th2023.
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