Carbonizing urban biowaste for low-cost char production in developing countries - a review of knowledge, practices and technologies
Solid waste in the cities of low- and middle-income countries is characterized by a high organic fraction and management of this municipal solid waste is known for low collection and recycling rates, and inade-quate disposal methods. Most of these countries also face the major challenge of supplying an affordable, reliable and sustainable cooking fuel to urban households. The majority of city residents use wood-based charcoal and the number of charcoal users is expected to rise over the years with increasing urbanization. Wood-based charcoal has several advantages compared to other cooking fuels, including continuous availability in local markets (even in small quantities), ease of storage, higher heating value, lightweight, cleaner burning compared to wood, cheap stoves, and in many countries it is sold at lower price com-pared to liquefied petroleum gas (LPG) and electricity. However, wood-based charcoal is derived from a predominately informally organized supply chain, which is linked to unsustainable forest logging, low efficiency production methods and long transportation routes, which are all factors that contribute to environmental degradation. Since improving the sustainability of charcoal production is considered the most effective and realistic measure to enhance the sustainability of household cooking, several develop-ing countries have started to initiate biomass energy strategies, which include efforts to promote access to appropriate, alternative sources of bioenergy. The current state of the environment in cities of developing countries and their increasing charcoal de-mand raise the question if organic solid waste could be a suitable feedstock for low-cost char production. The char generated through slow pyrolysis of organic solid waste could be further processed into charcoal briquettes, a product of economic value with stable market demand and advantageous distribution prop-erties. Use of municipal biowaste could stimulate their collection and, thereby, diminish the amount des-tined for disposal at dump sites. This approach could lead to reduced emissions linked to the uncontrolled decomposition of inappropriately disposed waste and reduced waste transportation, as well as the partial substitution of an unsustainably produced cooking fuel (wood-derived charcoal) without causing signifi-cant changes in traditional cooking appliances and behaviour. This report reviews existing knowledge on char-making to help stakeholders understand under which circumstances carbonization of municipal biowaste may be feasible. The report starts with a general over-view of common municipal solid waste management challenges in low-and middle-income countries. It then summarizes the current situation regarding conventional charcoal production and consumption as cooking fuel, reviewing some of the trends and theories behind the concept of ‘household fuel switching’. It also describes biomass carbonization in details, i.e., input requirements, chemical conversion processes and output properties, and reviews information on existing biomass-to-char technologies: e.g., process and reactor types, capacity, construction materials, conversion efficiency, energy source, residence time, emissions, fixed carbon yield, auxiliary requirements, working life and capital cost. This part of the report draws heavily from literature on wood pyrolysis because there is limited information on slow pyrolysis of biowaste. The last chapter draws an analysis of the feasibility of biowaste carbonization in cities of devel-oping countries and highlights challenges, opportunities and areas for further research. This review concludes that the high demand for carbonized fuel in cities of low-and middle-income coun-tries has created the market for waste-derived char briquettes. Yet, a major challenge to their production is having continuous access to dry, unmixed, homogeneous, uncontaminated substrates, which are avail-able at no or low costs. In other words, a good supply of source-separated wastes that can be obtained near the point of their production is needed. Furthermore, most existing carbonization systems are either inefficient and polluting or relatively expensive. For a sustainable and financially viable waste-to-char business an appropriate, locally manufactured and operated, cost-effective system is required, which is non-polluting and energy-efficient with controlled use of all combustible by-products and waste heat. On the policy level, there is a need to address the broader regulatory and tax framework in the charcoal sector. The current cost of charcoal in most developing countries does not reflect its true value because of lack of clear policies and enforcements, hence efforts to promote sustainably produced alternative cook-ing fuels will always be undercut by illegal charcoal, which is unregulated, bypasses many costs and reaches consumers at lower price at the expense of diminishing forests, adverse consequences on the environment and lost governments revenue.