Biomass for Heat

The process of burning biomass to generate heat is well understood and has been utilized for many decades. Woody biomass provided approximately 90% of the world’s energy requirements some 125 years ago. Today, the technologies that exist to produce heat from biomass are safe, reliable, clean burning and are used extensively around the world, including Canada. Biomass for heat is applicable for the residential, commercial, and industrial sectors. Examples include homes, hospitals, schools, municipal buildings, campuses, hotels, community district energy centres, greenhouses, industrial/manufacturing sites, forest products industry facilities, etc.

Biomass heating systems convert biomass fuels to thermal energy through the process of combustion, pyrolysis and gasification. Thermal energy can then be used in a number of ways:

  • Space and domestic hot water heating
  • Process heating
  • Process cooling
  • Thermal portion of combined heat and power (CHP)

 

Here, we focus on the most common heating application – combustion

An Introduction to Biomass Heating

Residential Stoves

Residential stoves are used to generate space heating for one or more rooms. A wood stove burns cordwood, and regular tending is required to keep an even temperature as the airflow and fuel supply must be manually handled. A wood stove must be refueled multiple times a day, as well as having the ash pan emptied and the glass panes cleaned daily. A pellet stove burns pellets (i.e., compressed woody or agricultural material). These systems are more automatic as fuel is fed from the storage hopper into the combustion chamber via an auger. A typical pellet stove will generally require the user to top up the storage hopper on a daily basis. Although they demand attention and maintenance, wood and pellet stoves make for an aesthetically pleasing and cozy option for space heating.

Components of a wood pellet stove.
(Image source: adapted from www.wdpellet.com/what_are_pellet_fuel_appliances.php)

Wood Stoves vs. Pellet Stoves
A Guide to Residential Wood Heating
Biomass Heating: A Guide to Small Log and Wood Pellet Systems
Residential Heating
Pellet Stove Factsheet

Residential Boilers

A pellet boiler (furnace) is a central heating system that takes the place of natural gas or oil boilers. They provide warm water or warm air that is used to heat the entire house. For an indoor wood boiler, water surrounding the combustion chamber absorbs heat. The heated water is then circulated throughout the building, either through a system of pipes for radiant baseboard or radiant floor heating, or through a heat exchanger to allow for forced air heating. Temperature is controlled with a thermostat. Pellet systems have the simplest controls and are closest to the fossil fuelled boilers in terms of maintenance and operation. An advantage to residential boiler systems is that they can also be used to heat water, as well as providing heat for the house.


Inside of a wood pellet boiler.
(Image source: www.building.co.uk)

An outdoor wood boiler works much the same, except that it is located outside the home, typically 30 – 200 ft away. The heated water is delivered to the main building(s) through insulated underground pipes, where a water-to-air or water-to-water heat exchanger sends the heat into the building’s heating system. An advantage to an outdoor system is that no fuels enter the building, eliminating fire hazards, the mess or smoke, and removing the risk of carbon monoxide buildup. The feedstock for outdoor boilers can be pellets, chips or cordwood. In the case of cordwood boilers, they do not have automatic feed systems so they require a certain amount of physical labour and must be fed on a daily basis.

Fuel can be stored in a range of ways, including silos, hoppers, containerized stores, flexible fabric silos and in sheds. The time in between refills is dependent on the size of the storage container, but generally averages about a week or two.


Layout of an outdoor cordwood boiler and distribution system.
(Image source: http://mapawatt.com/2010/01/07/outdoor-wood-furnace/)

Biomass Heating: A Guide to Medium Scale Wood Chip and Wood Pellet Systems
Wood Boiler Systems Overview
Small Heating Units

Commercial/Industrial Boilers

The main technical components found in residential appliances are also found in commercial and industrial systems, just at a larger scale.

However, they are different from residential systems in two main ways:
1. They incorporate automated feed and control systems.
2. They require emissions control equipment to ensure strict emissions regulations are met.

The main components of commercial biomass heating systems are:

Fuel Storage - Generally a large silo or in ground storage container. Fuel storage systems need to be completely waterproof and kept at a temperature above freezing to keep large chunks from damaging the system.

Feed System - A conveyor system (e.g., screw conveyor, moving floor) that moves fuel from the storage container to the combustion chamber. Combustion temperatures are monitored by thermostats, which automatically adjust the speed at which the fuel is fed into the combustion chamber.

Combustor – The enclosed chamber where fuel is burned by heating it and adding oxygen in the right amount and proportion to ensure complete combustion. There are a number of different types of combustors, but are generally categorized into direct combustors and gasifier combustors.

Heat Exchanger – Removes the heat from the combustion gases, and places it in the air or water that is used for heating or other needs.

Ash Handling System – Combustion ash is collected and deposited automatically into a bin for disposal. Ash can be landfilled or used as a fertilizer or soil amendment.

Air Filters – A series of pollution control devices are installed that eliminate harmful emissions into the atmosphere. Flue Stack – With the aid of an electric fan, flue gases are carried to the atmosphere.

Major components of a commercial/industrial scale biomass combustor system.
(Image source: http://pubs.cas.psu.edu/FreePubs/PDFs/ub046.pdf)

Large Scale Heating With Biomass
Commercial-Scale Biomass Combustion Equipment

District Energy Systems

District energy provides a medium that allows for the transfer of energy. A district energy system may be designed in a number of ways:
1. Central heating plant,
2. Series of mini plants (combination of several smaller systems), or
3. Multiple plants.

All systems are connected by pipes that provide space heating, hot water, steam and chilled water to individual customers. This approach allows residential, municipal, commercial and institutional buildings to cover their necessary heating or cooling requirements without each building having its own heating or cooling appliance.

District energy systems generally consist of three sub-systems:
1. Production facility & energy source,
2. Distribution (piping) system, and
3. Energy transfer station & customer building.

Click here to see a video that shows how district energy works.



General layout of a district heating system for a small community.

Is District Energy Right for Your Community?
The Concept
Sizing the System
Supporting Resources

Biomass Community District Energy Systems