Utilization of biomass for energy by the Sri Lankan industry has increased by 61% from 2001 to 2013. The single most influential factor for this trend was the ever increasing fossil fuel price, which recorded six-fold increase during the same period. Over 300 new industrial biomass energy conversion systems (thermal) have been installed over the past few years. Moreover, six private sector organizations have established themselves as competent suppliers of biomass energy conversion equipment in Sri Lanka. In addition, many biomass suppliers, mostly informal, have entered the fray and supply biomass in the thriving business environment being created. In the prevailing circumstances, sustainable supply of biomass to cater for this demand has become a challenge now. To face this challenge both the demand and supply side must be re-organised and re-oriented towards better efficiency and sustainability.
Introduction of efficient technology has been identified as a major intervention required to improve the efficiency of biomass energy conversion systems (BECS) and of the supply chain. Accordingly, the aim of this study was to establish the baseline on the infusion of technology to the BECS and to the supply chain, and identify potential areas for efficiency improvement. An inventory of some 320 modern thermal BECS installed in the country was prepared from information supplied by equipment suppliers in the country. In addition, 22 selected modern BECS installed in multiple of industries were studied and collected necessary data for the analysis of technology utilisation at present.
BECS are used to provide industrial services such as Steam, Hot water, Hot thermic oil, Hot air and to produce Electric power. There are approximately, 193 (60%) steam boilers, 93 (29%) hot water boilers and 32 (10%) thermic oil heaters using modern BECS. Of all BECS used in industry, nearly 53% use wood logs, the most inefficient way of using biomass for energy. However, of all the biomass consumed by the thermal BECS comprised of wood logs – 37%, multi-fuels – 32%, paddy husk – 14%, wood chips – 12%, saw dust – 3% and coconut shells – 2%. Actual consumption of wood logs should be higher than this because multi-fuels too include fair amount of wood logs.
The type and forms of biomass used and their quality vary widely, and there are no local standards for biomass to be used in BECS. What currently followed are different forms of biomass, such as wood logs, wood chips, saw dust, coconut shells, paddy husk, etc. without any quality aspects – size, moisture levels, calorific value, density, etc. However, the BECS’ are designed to use certain form/s and qualities of biomass for optimum performance. That means, if we are to operate the BECS with maximum efficiency our biomass supply must match (quality wise) that is recommended by the suppliers. It is, therefore, inevitable that our BECS underperform due to unavailability of quality biomass.
There are several combustion grate technologies being used in Sri Lanka. Fixed grate and moving grate are the leading technologies used at the moment. Dumping gates are also used in few cases with additional features. Fixed bed grates have been used mainly for firing wood logs, where as moving grate and dumping grate have been used for firing wood chips.
All most all the small capacity systems and many medium capacity systems were being operated manually. The only automatic features in these systems were the cut-in and cut-off control of FD and ID fans. In manually operated systems the operator skills play a major role in the performance of the system. It was observed that in certain cases the operator skills were inadequate to run the system efficiently. Some degree of automation has been introduced to medium and high capacity systems with added benefit of improved efficiency.
Accurate supply of combustion air (both quantity and method of supply) is one of the most important aspects that contribute to energy efficiency of BECS. It is most common among BECS that combustion air is supplied in stages – air staging – in order to promote good and efficient combustion of biomass. Most of the BECS visited had this feature where combustion air is supplied in two stages – primary air and secondary air.
There are several types and combinations of heat exchangers used in BECs installed in Sri Lanka depending on the services being provided. Most common type with steam and hot water systems included bundle of convective tubes, in multi-pass configuration, surrounded by water in an enclosure in which hot flue gases pass through the tubes. There were combination type of heat exchangers comprising a membrane section (radiation tubes) and a convective section coupled together used in some medium to large systems. Overall efficiency of this system is higher than the convective heat exchanger system. Small scale thermic oil heaters use multi-pass tubular spiral type heat exchanger which is placed vertically above the combustion chamber, and thermic oil pass through the tubes while hot combustion gases flow outside the tubes. There is combination type heat exchanger used mainly in high capacity systems. In this configuration the overall efficiency is higher than the convective type heat exchanger.
Flue gas emissions from BECS need special attention to mitigate their environmental impacts. Most problematic emission includes particulate matter and tar. Efficient combustion of biomass is the key to minimize emissions at source. In Sri Lanka, the most commonly used system is the cyclone separators for particles and we scrubber for the tar. In many cases visited have used multi-cyclones with smaller diameters, which is more efficient than single cyclone with larger diameter in removing particles. Tar in the flue gas is removed in a wet scrubber. Wet scrubbers were found only in very limited number of installations. Where wet scrubbers were installed, it was observed that many of them were malfunctioning due to multiple of reasons.
Energy recovery is one important option available to improve the overall efficiency of BECS. This option has been used by all BECS studied under this assignment. Most commonly applied methods were the air pre-heater (APH) and water pre-heater (WPH) to recover heat in flue gas. Though, there are other methods to recover heat from flue gases, such as drying biomass, it appears that no one uses this option in the country. There were cases where the pre-heated air temperature reaches 130˚C and flue gas temperature brought down to about 160˚C.
Though wood gasification technology has been experimented in the country since about 1980’s, adaptation of the technology to Sri Lankan soil has remained very low so far. Some creditable advances have been made in thermal energy applications, where a few such systems have been operating successfully for about 10 to 19 years. Application of wood gasifires for power generation has not been successful so far. A few pilot plants, including R&D initiatives, of small to medium size capacities set up at several locations in the country could not prove its proper adaptation for long term operations. And most of them have been either abandoned or closed down at the moment. According to experienced gained during R&D initiatives and as explained by one of the developers of a commercial plant, the reasons for failures of gasifire based power generation do not lie solely on the gasifire itself, but major contribution comes from such components as IC engine, management of the project, commitment of the main stakeholders of the project, economic scale of operation at initial stages, power purchase agreement (especially the tariff) and more importantly the availability of after sales services.
It is important for a country like Sri Lanka to know the stage of development and commercial readiness of BECS in order to take a valid purchasing decision. Diagram below shows some guidelines for selecting technologies based on the capacities as analysed by a leading organisation in the world.ange of applicable scales most suitable for different technologies when operated commercially.Range of applicable scales most suitable for different technologies when operated commercially.
Heat-only applications are the only applications relevant at the smallest scale. CHP applications span from small scale (with gasification and ICE) to very large scale (with boiler and steam turbine). To complement above, the diagram below looks at the Technology Readiness Level (TRL) and Commercial Readiness Index (CRI) of various BECS now in the market.
As indicated above, thermal only applications (boilers) and CHP applications (boiler + steam turbine) are fully matured technologies with high commercial rating, whereas CHP applications using gasifire – ICE configuration though has reached a fully developed technology status, are still running at commercial consolidation stage.