From Municipal Solid Waste (MSW) to Energy

Sandro Buzzi, Buzzi Unicem S.p.A., Italien

Summary

"Waste derived solid fuel" based on 2/3 MSW + 1/3 CISW and processed through five steps:

Coarse MSW shredding to 250 to 300 mm size

Aerobic bio-stabilization with significant moisture reduction

Automatic proportioning of the two components controlled by "Calorific Value on-line analysis"

Chlorine reduction by NIR Spectroscopy (Near Infrared Spectroscopy)

Very fine grinding of the mix (0.2 to 5 mm)

The fuel obtained in this way can replace 80 to 90% of the pulverized coal or petcoke in Buzzis cement kilns, furthermore helping to reduce or eliminate the landfills.

Approx. 0.9 tonnes of Carbonverde (CBV) with a moisture content of approx. 14 % are obtained from 1 t Municipal Solid Waste (MSW) with a moisture content of approx. 50 % through moisture reduction and addition of Commercial & Industrial Solid Waste.

Standards

CBV belongs to the Solid Secondary Fuel (SSF) category (in Italy named CSS = Combustibile Solido Secondario) regulated by the DM n.22, Febr. 14, 2013 EoW (End of Waste) according to the European directive 2008/98/CE and CEN TC 15359- 2010. Recently - at the European Commission – Buzzi´s "Carbonext" fuel achieved the "REACH" classification (Registration, Evaluation, Authorisation and Restriction of Chemicals.

Introduction

MSW is the undifferentiated portion of “urban waste” always present downstream of community selections. MSW treatments are quite different in different countries and/or regions. In Northern Europe the incinerators depict the standard treatment of any undifferentiated waste source. In Italy MSW is either “non-treated” or “mechanically separated into two fractions” or submitted (rarely) to aerobic stabilization before dumping it at landfills. Incinerators are insufficient and anyway strongly opposed by public opinion and green politicians. The existing landfills are therefore still collecting the majority of MSW whereas new landfills are no longer permitted and subjected to UE fines.

In the Robilante cement plant of Buzzi (NW-Italy), the use of “solid” alternative fuels has been tried for a long time: tyres in the 80 s, commercial fluff (plastic and textiles) in the 90 s, whereas from 2002 on – the plant moved to “urban waste” processed in three different versions.

“Carbonverde” fuel or “Carbonext” (as new REACH denomination) is the result of Buzzi´s research on a high-quality “solid fuel” for their cement kilns and as the company believes for power plants as well aiming at a very high (80 % of SSF or more) replacement ratio of pulverized coal or petcoke. The interest in the Carbonverde process could then be higher in countries showing a similar situation to Italy i.e. trying to valorize large amounts of the “urban waste” by replacing coal or petroleum coke in cement kilns or power plants at a high substitution rate instead of taking it to the incinerators.

MSW energy content

Italy generates over 32 million t/a of MSW and approx. 50 million t/a of CISW that can be partially assimilated to MSW. Usually there is no perception of the huge amount of energy contained in MSW+CISW. Assuming that about 25 million t/a of MSW+CISW combined waste could be processed, the resulting 100 million Gcal or 35,000 GWh, are almost equivalent to 16 million tonnes of coal or in turn to the whole of the hydro-power produced in Italy, which amounts to 10 % of the total energy consumption of Italy. On a different scale, the MSW generated by six people, approximately 3,000 kg/a can in turn generate 3,000 kWh energy, which equals the an annual electrical consumption of one family. Furthermore MSW, as a renewable source of energy independent of oil and coal prices, is strongly alleviating the difficult problem of dumping in landfills, which understandably no one wants in their neighbourhood.

Production process

CBV fuel originates from a mix of 70 to 75 % household (urban) waste (pre-selected or not pre-selected) and 25 to 30 % industrial waste and is obtained by processing such a mix by an innovative process developed by Buzzi Unicem in an existing waste treatment plant located in Sommariva B (Turin area).

The main features are: stability of net heat value and high fineness achieved by the grinding CBV process consisting of five steps or phases:

coarse MSW shredding a particle size of 250 to 300 mm

bio-stabilizing the “whole MSW as it is” by the aerobic process

removing the organic chlorine with the help of infrared spectroscopy control

CISW addition, controlled by on-line NIR Spectroscopy analysis, for setting the desired caloric value

fine grinding and homogenising of the mix down to a fineness of 0.1 to 6 mm

The simplicity of the process layout is based on Buzzi´s chain mill being able to reduce the feed size of 300 mm to approx. 3 mm by just one passage (grinding ratio 100 to 1). This could turn into a significant advantage for cements plants wanting to perform the full process within their plant. They could look for a coarse and low value plastic mix, getting in this way a premium instead of paying for a final shredded fluff (Figures. 1 and 2).

Regarding bio-stabilization, Buzzi like the Biocubi® process which cuts down MSW moisture from 50 to 20 % and the bacterial flora to one tenth, so achieving with 14 days of aerobic digestion at 55 °C a fairly dry hygienic material (named Amabilis®) with a calorific value in the range of 3 to 4 Mcal/kg or 12.5 to 16.5 MJ/kg (Figure 3).

Regarding grinding, Buzzi´s first development – at the beginning of 2010 was to install and optimize, as a flow process a pilot plant consisting of a 2 t/h rocket, a fine grinding mill derived from a machine used, as a batch process, to demolish household appliances (Figure 4). The grinding system consists of chains connected to a hub rotating at 7 to 800 rpm. The chains can find different setups at the hub connection. The grinding plant is completely automated and equipped with a supervisory system (Figure 5).

By optimizing the mill Buzzi managed to keep its energy consumption below 90 kWh/t. Which is indeed a high consumption, yet acceptable for the desired fineness. The end result is a fine product measuring between 0.1 and 6 mm. with high percentage below 1 mm, which is very much appreciated in view of an efficient combustion

One year ago Buzzi installed a much larger mill with 5 to 6 t/h CBV capacity based on two grinding bodies. Its present output amounts to 12,000 t/a for one working shift or 24,000 t/a for two shifts, with the aim of increasing it to 40,000 t/a within the next two years, installing a second mill in parallel with the first one. This way Buzzi will be capable of processing the total amount of waste received by the Sommariva plant. At the same time the landfill where the waste is currently dumped will be exhausted and going out of service. (Figures. 6 to 8).

Fineness and combustion

One could wonder why such a high fineness is deemed necessary. Solid SF – at high percentage is more difficult to burn as opposed to other SF such as solvents, oils and animal meals. Fine grinding although increasing energy consumption is a basic feature – as in the case of petcoke – in order to achieve perfect combustion. Regarding the main burner combustion: flame pattern, temperature along the kiln axe axis, kiln inlet temperature can be kept in line only using a fine product when the required substitution rate – just by ‘solid fuel’ increases to 70 to 80 % at the main burner. Up to today, according to Buzzi´s knowledge such high SSF rates are achieved only in presence of liquid waste, animal meal and other easier alternative fuels but not yet using just SSF.

Regarding precalciner combustion: the CO level is dramatically reduced by using high fineness SSF like CBV. Therefore CBV is particularly suitable for precalciners with low retention time (3 to 4 s, see Figures 10 and 11).

Investment Cost

A CBV medium-sized plant is designed to process 50,000 t/a MSW achieving 45,000 t/a of final product. It can operate with different layouts and purposes according to the planned CBV using NB. Treatment plants for 100,000 t/a MSW and more are feasible as well as at lower unity cost. The investment costs for the two basic cases are:

case a) without bio-stabilization: Investment is in the range of 8 to 9 million € for only a grinding plant processing a previously dried “light fraction of MSW”, excluding therefore bio-stabilizing, however including chlorine separation and CISW on-line controlled addition

case b) including bio-stabilization: Investment is in the range of 20 million € for a 50,000 t/a MSW plant or € 400/t MSW.

This is the necessary process when the target consists also in a complete elimination of the landfill.

Cost of Operation

In case a) the yearly cost for 8,000 hours of operation, lies at approx. 3,000 k€/a, broken down in labour k€ 600 + maintenance k€ 700 + electrical energy k€ 500 + freight k€ 500 + royalties k€ 500 + contingency k€ 200 = approx. 60 €/t MSW or approx. 67 €/t CBV, considering the CISW addition at breakeven. Depreciation at 500 k€/a over ten years accounts for approx. 10 €/t MSW.

In case b) Introducing bio-stabilization, the additional costs to case “a” are 100 k€ labour + 500 k€ maintenance + 200 k€ electrical energy therefore reaching cost of approx. 4,000 €/a equivalent to 80 €/t MSW or 90 €/t CBV. Depreciation of 1 million €/a over 20 years amounts to 20 €/tMSW.

Usually in Italy CISW is supplied with a premium of 60 to 70 €/t which alleviate the CBV final cost in the range of 20 €/t CBV (an addition of 1/3), basically covering depreciation.

CBV production cost of 80 €/t MSW including depreciation, shows a saving of ca. 30 to 40 €/t MSW as opposed to the usual costs covered in Italy by the communities for MSW treatments.