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Renewable Energy P2 – Biomass Energy

 

There is a significant waste problem around the world and here in Sri Lanka that is not being addressed properly (P1)

Introduction

Sri Lanka is currently going through an energy crisis with no new power plants been added to the grid in the last 4 years. In a previous article we looked more closely at the current generation challenges to our grid as well as explored the potential of other Non Conventional Renewable Energy Sources including; mini-hydro, solar, and wind.

In this article we will discuss the potential of biomass energy generation in Sri Lanka. Biomass refers to “organic material on Earth that has stored sunlight in the form of chemical energy”.

Biomass power plants in Sri Lanka can be broken down into 2 categories; 1. Dendro Plants, and 2. Agriculture and Industrial Waste Plants. *Note that there is one 80kW grid connected Biogas plant in Sri Lanka but it no power has been purchased from it by the CEB in the years 2017 and 2016 (2018 Sales and Generation statistics have yet to be released).

Dendro plants are biomass plants that use wood as their fuel source to generate energy while agriculture and Industrial waste plants combust waste products or use them to create bio liquids and gases which can then be combusted for the generation of electricity and heat. More details to these processes can be found below but first let’s look at the different sources of feedstock that can be used in modern biomass power plants. A significant benefit of biomass power plants is that they are able to produce a base load output; ie a consistent, reliable power output that is not intermittent, unlike the output of other renewable energy plants.

 
 
conveyor belt waste sorting

Many countries have implemented advanced waste collection and segregation factories to turn this problem into an opportunity. (P2)

Sources of biomass

Wood and Agriculture refuse – This includes wood, leaves and even agricultural waste derived from fruit farming or paper manufacturing.

Ethanol (Ethyl Alcohol) – Obtained by fermenting and then distilling the sugars and other starches found in plants. Any organic material containing cellulose, starch, or sugar can be made into ethanol.

Biodiesel - Made by reacting alcohol with vegetable oils, animal fats, and greases, even recycled restaurant grease. It is important to note that Biodiesel contains virtually no sulfur.

Solid Waste - One ton of mixed household garbage contains about as much energy as 250 kilograms of coal. However, garbage is not all biomass; perhaps half of its energy content comes from plastics, which are made from petroleum and natural gas.

Biogas –Bacteria and fungi that eat biodegradable matter produce methane gas as the waste decays. Landfills and Biogas Digesters facilitate the growth of these bacteria by fermenting waste in airtight containers or steel-lined pits and harvesting the produced methane gas. The remaining solids, once segregated properly, is a rich fertilizer that can be used by the agriculture industry. (1)

 
 
biogas digestor

Biogas digesters promote the growth of bacteria to harvest methane gas which can be burnt to produce energy (P3)

Gasification and Pyrolysis

Gasification is a process in which solid materials containing carbon, such as kitchen waste are converted into a gas. It is a thermochemical process, meaning that the feedstock is heated to high temperatures, producing gases which can undergo chemical reactions to form a synthesis gas (syn gas). This gas is a fuel mixture consisting of primarily hydrogen, carbon monoxide, and carbon dioxide which can be used to produce heat or energy or even further refined into a range of chemicals, including liquid and gaseous transport fuels.

There is considerable interest in methods to convert syn gas to liquid biofuels involving gasification. Such methods are attractive due as they accept a wider, more variable range of feedstock, have a lower cost and a higher efficiency, while also producing fuels with improved characteristics. (2)

Pyrolysis is one of the main stages of the gasification process and prepares the waste to be used as an energy or fuel source. It is the thermal decomposition of biomass at high temperatures in a low oxygen environment. Depending on the temperature as well as the speed at which the pyrolysis process occurs, the process yields different products. For example; at a temperature below 230°C, with slow heat transfer the process yields primarily a biochar, which can be used as a partial replacement for coal or coke. Between temperatures of 320°C to 430°C with relatively high heating rates the process mainly yields bio-oil. The main advantages of bio-oil include its; ease of handling and storage, along with the its ability to be combusted in existing gas power stations. Alternatively, the oil can be upgraded to either a special engine fuel or through the rest of the gasification processes to a syngas (synthesis gas) and then a bio-diesel. At temperatures greater than 430°C the main products of pyrolysis are dense biogases. (3)

After the process of pyrolysis, the gasification process continues to break down some of the tars and hydro carbons in the vapours produced to create carbon monoxide and hydrogen. The final output of the gasification process is a mixture of gases that can be used to generate heat and energy.

 
 
Gliricidia Sepium

Gliricidia sepium, used rurally for fences, commonly known as Ginisiria, Ladappa, Wetama, Albisia is also used in local Dendro power plants (P4)

Combustion

Dendro power plants use combust biomass to produce energy, with special tree planting projects specifically for this purpose.

The trees used as fuel in Dendro Power Plants in Sri Lanka can be classified into 2 types:

Long Rotational Species (LRS); those that take a year or more to mature and harvest, and

Short Rotation Coppicing Species (SRCS); which take less than a year to harvest.

Certain exotic LRS species can take up to 10 years to harvest but they aren’t used to generate power and instead grown mainly as fuelwood in tea production.

SRCS are grown not just to be combusted in power plants but are also used AS support, fencing, and shade around the country. Their increased demand as fuel wood has also increases its value. (4)

Direct combustion, during which the biomass fuel is burnt in excess, forced air to produce heat, is the most commonly used method for converting biomass to energy. The excess air is because the efficiency of the process depends on a good contact between the oxygen in the air and the biomass solids which also reduces the pollutant that would be produced including; carbon dioxide, water vapor, tars, smoke, and alkaline ash particles. The further minimization of these emissions are important concerns in the design of an environmentally acceptable biomass combustion system.

Biomass combustion systems, based on a range of furnace designs, can be very efficient at producing hot gases, hot air, hot water or steam, typically recovering 65-90% of the energy contained in the fuel. Lower efficiencies are generally associated with wetter fuels. (5)

 
 
biogas plant

Biogas power plants hold a lot of potential for both utilizing our waste and addressing our energy challenges (P5)

Conclusion

The challenge in implementing Biomass, Biogas and Biodiesel plants in Sri Lanka lies with obtaining high quality biomass feedstock with a low moisture content, be it fuel crops or solid waste. Additionally, a lot of fine tuning is needed to optimize the process for the production of these upgraded fuel sources. It’s potential is huge as the energy cost from Agri-waste and Industrial waste plants is just Rs 9.43/kWh (on average according to 2017 CEB purchasing statistics). This makes it the only price comparable renewable alternative to coal power generation which costed the CB on average Rs. 7.13/kWh in fuel costs in 2017. (6) This combined by its ability to provide consistent base load power makes it a highly suitable solution for the power crisis on the island.

   
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