Monitoring biomass combustion

The world’s most common form of renewable energy production is not solar energy or wind power, but energy generated by burning biomass or biogas. Like coal-fired power generation, the efficiency of the process needs to be monitored, as well as the range of toxic materials that can be released when burning organic materials and other waste.

In a general sense, bioenergy is a form of renewable energy derived from biomass, which is used to generate electricity and heat or to produce liquid fuels. Biomass is any organic matter of recently living plant or animal origin, and is available in many forms such as agricultural products, forestry products, and municipal and other waste.

Bioenergy technologies make up a significant proportion of renewable energy production around the world. For example, the US Institute for Energy Research¹ reports that approximately 5.2% of energy is generated from biomass, accounting for 55% of renewable sources. In Australia this is considerably less, with the Australian Renewable Energy Agency (ARENA)² reporting that bioenergy sources account for approximately 1% of electricity generation and about 7% of renewable sources. According to ARENA:

“Australia’s bioenergy industry currently uses a range of biomass resources including:

• bagasse, which remains after sugar has been extracted from sugarcane
• landfill gas
• wood waste and black liquor
• energy crops
• agricultural products
• municipal solid waste.

The majority of Australia’s installed bioenergy capacity is derived from bagasse cogeneration.”

Although the burning of biomass releases carbon into the atmosphere in the same way that coal-fired power generation does, the difference is that the fuel is captured from the Earth’s biosphere. That is, the material being burned releases carbon that was only recently captured from the atmosphere, and so is a carbon-neutral cycle – in contrast with coal and natural gas, in which the carbon being released was trapped underground for many millions of years and is no longer a natural part of the Earth’s biosphere.

As carbon-neutral as biomass generation may be, however, there are still challenges associated with other toxic compounds that are released during the burning of the fuel, requiring the same type of careful measurement and monitoring as other processes.

Challenges

There are number of challenges associated with the transformation of biomass into electricity. Firstly, the efficient use of fuel and the protection of assets are of utmost importance to ensure optimum profitability; secondly, emission monitoring and pollution control is a requirement in nearly every country. It is imperative for every industrial plant to monitor the production process from material flow to pollution control and to maximise energy efficiency – but with minimal danger to the plant staff or damage to the environment. To achieve these goals, there are important monitoring requirements for:

• fuel flow to the burner for custody transfer and boiler efficiency monitoring
• the efficiency of the pollution removal system – for pollutants such as dust or gas (SO₂, NO_x, etc)
• bulk material transport and storage monitoring – measuring the level and volume flow of the fuel.

Biogas

Biogas typically refers to a mixture of gases resulting from the breakdown of organic matter in the absence of oxygen, and can be produced from raw biomass materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste.

Biogas can be produced as landfill gas, by anaerobic digestion (in which anaerobic bacteria are used to digest material inside a closed system) or by fermentation of biodegradable materials. Biogas is primarily methane (CH₄) and carbon dioxide (CO₂) and may have small amounts of hydrogen sulfide (H₂S), moisture and siloxanes. The gases methane, hydrogen and carbon monoxide (CO) can be combusted or oxidised with oxygen, allowing biogas to be used as a fuel.

Process monitoring

Fuel storage and delivery

Solid biomass is generally stored in silos until it is ready to be used as fuel. Such silos require some form of level monitoring for quantifying the fuel supply and for overfill prevention.

Conveyors are used to transfer solid biomass to shredders, and from the shredders to the incinerators. In order to monitor fuel use, and to measure the efficiency of the plant, the volume of material being moved needs to be measured. Conveyor mass flow is often measured by continuous weighing, but this can result in significant inaccuracies in this application as on rainy days, the biomass absorbs a significant amount of water and becomes heavier. A better method is to use a laser scanner to measure the material volume.

In the case of biogas, an ultrasonic flow meter can be used to measure the volumetric flow rate.

The raw fuel storage also needs protection from combustion in the silos and shredders. For this purpose, a gas analyser that simultaneously measures O₂ and CO can be used to detect smouldering fires in the material.

Incinerator optimisation

Incineration requires O₂, and measurement of the O₂ concentration at the outlet of the combustion chamber allows the oxygen concentration to be optimised to maximise the burner’s efficiency.

Denitrification systems

The reduction of NO_x emissions is performed by two methods: non-catalytic and catalytic reduction.

In non-catalytic reduction, an ammonia (NH₃) or urea solution is injected into the combustion process at 900-1100°C. The compound reacts with the nitrogen oxides to produce nitrogen and water. It is therefore necessary to measure the NO concentration at the combustion chamber outlet, as well as unreacted NH₃.

Catalytic reduction involves the removal of nitrogen oxides from flue gases with the injection of ammonia along with a catalyst at 200-400°C. As for non-catalytic reduction, the NO concentration after reduction, as well as unreacted NH₃, need to be measured.

Measurement of NO and NH₃ are performed with a gas analyser suitable for measurement of nitrogen compounds. For compliance with emission regulations, these compounds also need to be detected and measured in the final flue gas output.

Flue gas scrubbers

Flue gas scrubbers are used to remove further toxic compounds: hydrochloric acid (HCl), sulfur dioxide (SO₂) and mercury (Hg and HgCl₂).

Wet scrubbers spray a cleaning solution into the output gas, while dry scrubbers use lime powder or milk of lime. To remove heavy metals and organic pollutants, activated carbon is also added. To optimise the consumption of the reagents and to monitor the scrubbing effectiveness, a gas analyser is needed that can simultaneously measure SO₂, HCl, water and, optionally, O₂.

Mercury can be released when incinerating waste. If the Hg concentration is very high (greater than 3000 μg/m³), action needs to be taken to ensure that the emission thresholds are respected.

Dedusting

The flue gas is dedusted to remove particulates using electrostatic precipitators and fabric filters. The filters also separate bicarbonate and activated carbon left over from the scrubbing process.

Dust concentration is most effectively monitored using a laser scattering instrument. The particulates also need to be measured in this way in the final exhaust stack for compliance purposes.

The dust particles that are filtered out are collected in an ash hopper. To determine when the hopper is full, a vibrating fork level switch can be the most effective method for detecting when the hopper is full.

Emission measurement

For environmental compliance, the final output from the stack needs to be monitored. Depending on the type of fuel, the following pollutants will need to be detected:

• HCl, HF, CO, NO_x, SO₂ and NH₃
• Total organic carbon
• Dust
• Gas velocity, pressure, temperature, O₂ and H₂O

In some countries, continuous measurement of mercury content is also required.

The gaseous components can be measured using direct in-situ or sample extraction methods. The pressure and temperature parameters are measured for normalising the gas sample measurements.

The dust component can be measured using an instrument that uses a laser light scattering method.

If HF or Hg detection is required, instruments specially designed for the purpose are readily available.

Other measurements

As consumables, the supply of reagents and activated carbon also need to be monitored. For these purposes, overfill protection can be afforded using a vibrating fork level switch, both for solid and liquid reagents.

As for solid fuel silos, activated carbon filter beds need to be monitored to be sure that fires do not occur - usually with a twin-component multigas analyser that can measure changes in CO concentration.

Conclusion

Similar to coal-fired power generation, biomass and biogas power generation requires significant monitoring and offers a good example of what can be measured and what benefits can be derived through investment in the right probes and sensors. Through direct real-time measurement, renewable power generation plants can be accurately monitored for plant efficiency, fuel and reagent consumption and emission monitoring.

References:

1. http://instituteforenergyresearch.org/topics/encyclopedia/renewable-energy/
2. http://arena.gov.au/about-renewable-energy/bioenergy/

Image credit: ©iStockphoto.com/George Clerk