Biomass heating? It seems an attractive proposition – to produce heat using waste products. But just how viable is it?
Biomass can be used to heat and to generate electricity (or a combination of the two) but its main use is to provide heating
As we are becoming increasingly aware of the pollution and global warming, to a large extent caused by the burning of fossil fuels, so we have been looking for alternative energy sources.
Biomass is not a new concept. Our ancestors were burning wood for heat and light long before coal was used to generate electricity. But with world population rising at a somewhat alarming rate, together with the earth heating up, the renewed interest has something of a desperate feel about it.
What can we use for Biomass Heating?
- Wood in many forms – woodchips, waste wood, sawdust and pellets
- Animal and human waste – this has been used for generations to fertilise our fields. And the dung, dried, has been used in many places and times. People from the ancient Egyptians to Outer Mongolians have used it to heat their homes and cook their food. And you may have noticed how your cat likes to sit on top of the manure heap where your best melons grow – it’s warm.
- Agricultural waste – like wheat chaff
- Industrial waste and urban waste
- Crops especially are grown to provide biomass for heat production – this might include forests.
How We Use the Different Sources
- Burning Wood and agricultural waste can simply be burnt. This is the commonest method.
- Gasification This is a partial combustion which produces a synthetic gas from which we can obtain energy. This might suit wood products or dry agricultural residues.
- Anaerobic digestion the biomassdecomposes by bacterial action in the absence of oxygen. This is suitable for wet waste products.
- Pyrolysis The biomass is heated rapidly without oxygen. This process yields a bio-oil, which can be used in furnaces and boilers. A catch-the-others method for most other waste products.
However, There Are Problems
Agricultural land might be diverted from food production.
Nutrients are removed and not replaced in the soil. The ground is at risk of becoming infertile.
Forests are being destroyed and not replaced. We need them to store carbon safely. Without forests more carbon enters the atmosphere and the earth heats up.
Other Pollutants are Emitted. And Some are Dangerous.
It has been estimated that one in nine deaths globally is due to pollution, so we have to take it seriously. And it is not a new problem, but it is one that affects the poorer countries and the poorest people most. The ones that probably are most important from the point of view of wood burning are the particulates.
- Carbon monoxide – a highly poisonous gas is given off. It’s the gas our gas boiler men give us meters to measure any emissions from our boilers before the gas kills us!
- Nitrogen Oxides (NOx) – enough research has been done for us to be certain that these are a hazard to our health – especially to the more vulnerable, including the elderly and children. In fact, the level of NOx has been reduced by over 50% in the UK between 1990 and 2016, largely due to traffic regulations. But wood burning plants need to have effective filters, both for NOx and for particulate matter.
- Particulates – these have a seriously deleterious effect on our health as they waft their way into our lungs and set up inflammatory reactions. The smaller particles can go deep inside and may even pass into the bloodstream. There is evidence that these small particles may be a causative factor in strokes, heart disease, cancer and maybe dementia.
- Black Carbon -this comes from incomplete burning of biomass as well as biofuels and particularly from fossil fuels. This is probably the second most important factor causing global warming.
But There Are Clear Advantages in Using Biomass for Heating
By using waste products we can obtain heat with a less deleterious effect on our environment than if we used fossil fuels.
There is carbon emitted, but this carbon is a part of the natural cycle of renewal. When fossil fuels are burnt they release carbon in chunks which are hard to disperse and for the natural cycle to work well. A slower release of carbon, as in wood burning, gives time for natural recycling to be effective.
We have many choices as to the suitability and method of use for different projects. We might have a small wood burning stove our homes or a factory complex might have a large heating system using tons of biomass, together with the infrastructure need to maintain and service the system.
Where attention to these problems has been paid the results can be very good. Europe has a good record, but in other parts of the world, the pollution caused by bad systems and management as well as lack of forest regeneration has led to worsening of the environment.
Wood burning systems
Wood burning systems range from the very small to very large facilities. But except in certain very small home usage, the system works in the same -manner.
The wood (chips, logs or sawdust) are transported to the facility. They are then stored – in a shed or hopper and then fed into the combustion chamber or boiler at a controlled rate. Hot gas is generated and either
Where steam or hot water is produced, pumps circulate water from the hot water tank to the location desired, flow being controlled by a thermostat.
You can see the system working in this 30-second video
Compared with other renewable energy systems, like wind or solar, biomass heating does need more maintenance. Fuel has to be transported and stored. Generally, at least three days’ supply is recommended to cover the weekends. The ash has to be removed and the machinery has to be maintained. But it is very flexible and very reliable. The maintenance usually is minimal – just a few hours per year.
It is also very easy to control the output required at different times.
However, the pollutants have to be monitored, and they need to comply with local regulations. There is a raft of filtering devices to catch the particles emitted.
How to Assess the Power Produced
This can be confusing! Different unit systems specify capacity in different ways. Usually, this will be in Btu per hour. Or a million Btu per hour (MBtu), or even MMBtu per hour.
Watts (W), kW or MW might be used and then there is boiler horsepower (1 horsepower is roughly equal to 9,810 W or 33,479 Btu/hr). And if that isn’t enough EDR might be quoted – (equivalent radiation). At least most sellers specify in terms of boiler output capacity although a few use fuel input as their measurement. Quite a challenge to calculate the efficiency of a biomass unit.
A Back-up Helps to Reduce the Cost
While the setting up of an automated system is expensive, there will be savings in maintenance and operative costs. It is cheaper to build a smaller system than a larger one.
Many Biomass systems include a back up fossil-fuel heating system. This reduces the initial cost as smaller units can be built, it reduces uncertainty if the bio-system fails and there is more flexibility.
Types and Costs of Biomass Systems
An average of $500 to $1500 per thermal kW capacity is the present cost – and is not likely to change anytime soon.
The Efficiency of the System Depends on Various Factors:
- Moisture content (a typical moisture content of 40% gives an efficiency of 60-65%). Pre-drying would cost more in capital outlay.
- Air distribution and amounts (is it excessive)
- Temperature and pressure needed to operate the system
- Exhaust temperatures
Which Type of System Suits?
Factors include such things as:
- Location – transport distance and type
- Local cost and availability – chips or sawdust or waste logs. Eg wood chips for large buildings and wood pellets for smaller ones. Their prices are usually lower than fossil fuels – and considerably less polluting.
- Competition from other fuels – natural gas, fuel oils
- Space available for the building
- Local rules and regulations especially regarding emissions
- Staff availability
Costs vary greatly, but averages out at around $10-$20 per million Btu.
Good candidates for biomass heating include such places as prisons, hospitals, large schools etc.
But the time allowed to use the incentive may not be enough to build the facility. Regional experts should be consulted when considering whether to use biomass to heat a facility. The subject is complex and varies from place to place. It might p[rove hard to find suppliers of wood chip for example.
The Clean Air Act for Emissions
The Clean Air Act has been protecting American citizens for over 45 years. It seeks to control levels ofparticles, ozone, lead, carbon monoxide, nitrogen dioxide and sulfur dioxide — as well as numerous toxic pollutants.
And it has achieved considerable success. From 1970 to 2017 these emissions dropped by 73% while the American gross domestic product grew by 324 per cent. Standards for boilers and incinerators that burn solid waste and sewage sludge were established.
What About Anaerobic Digestion of Waste Products
So far, we have talked about wood-burning combustion systems -the commonest form of waste product use. But interest in using anaerobic digestion for biomass heatiomg is growing for biomass heating
The process is designed to produce methane gas which can be used as a fuel.
The waste (often wet waste or sludge) enters the feed, for here it is macerated before being heated in the digester where anaerobic bacteria digest it from which the gas is collected.
Some of the Problems
- It smells!
- It’s a slow process
- It needs a qualified operator
- The process for biomass heating can get “stuck”
- It’s not yet very efficient
- Economically, it’s still rather borderline
But There Are Advantages
Interest is growing as there is great potential
The technology is well established in Asia and Europe
The gas it produces is already commonly used in America
There are incentives
It can be an off-the-shelf technology
Burning waste has been with us for as long as we have had fire, and animals have been digesting waste and giving off methane for much longer. But now we are able to contain and utilise our waste products to produce controlled heat, directly by combustion or by anaerobic metabolism.
Fortunately, we have come some way from the cartoon of the Faraday, father of electricity giving his business card to the filthy Father Thames! But there is much still to do and using waste products to provide heat is an attractive concept with huge potential.