Emission factors and chemical characterisation of fine particulate emissions from modern and old residential biomass heating systems determined for typical load cycles

Abstract It is already well known that there are significant differences regarding the emissions, especially particulate matter (PM) emissions, of old and modern as well as automatically and not automatically controlled biomass based residential heating systems. This concerns their magnitude as well as their chemical composition. In order to investigate emission factors for particulate emissions and the chemical compositions of the PM emissions over typical whole day operation cycles, a project on the determination and characterisation of PM emissions from the most relevant small-scale biomass combustion systems was performed at the BIOENERGY 2020+ GmbH, Graz, Austria, in cooperation with the Institute for Process and Particle Engineering, Graz University of Technology. The project was based on test stand measurements, during which relevant operation parameters (gaseous emissions, boiler load, flue gas temperature, combustion chamber temperature etc.) as well as PM emissions have been measured and PM samples have been taken and forwarded to chemical analyses. Firstly, typical whole day operation cycles for residential biomass combustion systems were specified for the test runs. Thereby automatically fed and automatically controlled boilers, manually fed and automatically controlled boilers as well as manually fed stoves were distinguished. The results show a clear correlation between the gaseous emissions (CO and OGC) and the PM1 emissions. It is indicated that modern biomass combustion systems emit significantly less gaseous and PM emissions than older technologies (up to a factor of 100). Moreover, automatically fed systems emit much less gaseous and PM emissions than manually fed batch-combustion systems. PM emissions from modern and automatically controlled systems mainly consist of alkaline metal salts, while organic aerosols and soot dominate the composition of aerosols from old and not automatically controlled systems. As an important result comprehensive data concerning gaseous and PM emissions of different old and modern biomass combustion systems over whole day operation cycles are now available. Derived from these data, correlations between burnout quality, particulate emissions as well as particle composition of the PM emissions can be deduced.</jats:p

Green on-site power generation : environmental considerations on small-scale biomass gasifier fuel-cell CHP systems for the residential sector

Contemporary combined heat and power (CHP) systems are often based on fossil fuels, such as natural gas or heating oil. Thereby, small-scale cogeneration systems are intended to replace or complement traditional heating equipment in residential buildings. In addition to space heating or domestic hot water supply, electricity is generated for the own consumption of the building or to be sold to the electric power grid. The adaptation of CHP-systems to renewable energy sources, such as solid biomass applications is challenging, because of feedstock composition and heat integration. Nevertheless, in particular smallscale CHP technologies based on biomass gasification and solid oxide fuel cells (SOFCs) offer significant potentials, also regarding important co-benefits, such as security of energy supply as well as emission reductions in terms of greenhouse gases or air pollutants. Besides emission or air quality regulations, the development of CHP technologies for clean on-site small-scale power generation is also strongly incentivised by energy efficiency policies for residential appliances, such as e.g. Ecodesign and Energy Labelling in the European Union (EU). Furthermore, solid residual biomass as renewable local energy source is best suited for decentralised operations such as micro-grids, also to reduce long-haul fuel transports. By this means such distributed energy resource technology can become an essential part of a forward-looking strategy for net zero energy or even smart plus energy buildings. In this context, this paper presents preliminary impact assessment results and most recent environmental considerations from the EU Horizon 2020 project "FlexiFuel-SOFC" (Grant Agreement no. 641229), which aims at the development of a novel CHP system, consisting of a fuel flexible smallscale fixed-bed updraft gasifier technology, a compact gas cleaning concept and an SOFC for electricity generation. Besides sole system efficiencies, in particular resource and emission aspects of solid fuel combustion and net electricity effects need to be considered. The latter means that vastly less emission intensive gasifier-fuel cell CHP technologies cause significant less fuel related emissions than traditional heating systems, an effect which is further strengthened by avoided emissions from more emission intensive traditional grid electricity generation. As promising result, operation "net" emissions of such on-site generation installations may be virtually zero or even negative. Additionally, this paper scopes central regulatory instruments for small-scale CHP systems in the EU to discuss ways to improve the framework for system deployment

Modeling of aerosol formation during biomass combustion in grate furnaces and comparison with measurements

Results from mathematical modeling of aerosol formation during combustion of woody biomass fuels were compared with results from particle size distribution (PSD) measurements at a pilot-scale biomass combustion unit with moving grate and flame tube boiler. The mathematical model is a plug flow model considering coagulation, nucleation, condensation, and particle deposition mechanisms (thermophoresis, particle diffusion, turbophoresis, and gravitational settling) of spherical particles as well as condensation of vapors on cooled boiler walls and a changing flue gas composition determined by equilibrium calculations. Additionally, the influence of kinetically limited homogeneous and heterogeneous reactions was taken into account in the case of sulfation reactions. To check the modeling results, investigations regarding PSDs and total aerosol mass loadings of the flue gas resulting from biomass combustion (beech chips, waste wood) were performed by measurements at a fixed bed biomass combustion unit (nominal boiler capacity: 440 kW). Furthermore, the composition of the aerosols sampled was determined. The comparison of calculated and measured particle size distributions and aerosol compositions agreed well, which permits the conclusion to be drawn that the model is applicable to the estimation of amount and chemical composition of aerosol emissions from fixed-bed biomass combustio

Concentrations of inorganic elements in biomass fuels and recovery in the different ash fractions

Inorganic elements and compounds in biomass fuels influence the combustion process and the composition of the ashes produced. Consequently, knowledge about the material fluxes of inorganic elements and compounds during biomass combustion for different kinds of biofuels and their influencing variables is of great importance. The results gained will especially influence the future design and control of biomass furnaces and boilers in order to prevent slagging, fouling and corrosion and to assist in the definition of quality requirements for biofuels as well as the possibilities of a sustainable ash utilization. For this reason, comprehensive test runs were carried out in several biomass combustion plants equipped with different combustion technologies and using various biomass fuels (wood chips, bark, straw and cereals). During continuous observation periods of at least two days, samples of the biomass and the different ash fractions were taken and analysed. Furthermore, the most important operating data of the plants were recorded. The results of the material balances for inorganic elements showed that the concentrations of environmentally relevant heavy metals (especially Cd and Zn) in biomass ashes increase with decreasing precipitation temperature and particle size. This effect is independent of the biofuel used. Consequently, a major requirement for a sustainable ash utilization is a fractionated heavy metal separation, distinguishing between different fly-ash fractions and taking the temperature of fly-ash precipitation into consideration for new furnace technologies. Research has also shown that straw and cereals, as well as their ashes, contain significantly lower amounts of heavy metals than woody biofuels and wood ashes. The same principles pointed out for environmentally relevant heavy metals are also valid for K, Na, Cl and S. The high concentrations of these elements in the filter fly-ash as well as in the boiler fly-ash are of great relevance for reactions that can take place in the boiler section where the flue gas is subjected to a considerable temperature gradient which is accompanied by chemical reactions, phase transitions and precipitation processes that can support or initiate fouling and corrosion. These effects are of special importance for biofuels that are rich in alkali metals and Cl such as straw and cereals