Proceedings

Digester gas is seen as a valuable resource because of its energy density, but it is frequently the case that most of this energy is wasted. A typical CHP will recover at best 40% of the available energy as electricity plus an amount used for digester heating. Old water industry assets suffered from low dry solids feeds leading to them needing all the heat, but newer advanced digestion assets and digesters fed on food waste only need a small amount of the recoverable heat from the CHP, leading to there being opportunities to use this heat. Furthermore, digester gas produces a CHP exhaust gas that is particularly rich in carbon dioxide and water vapour.

This paper explores the synergy that exists between using otherwise wasted resources derived from digester gas in greenhouses for tomato production and is largely based on a DECC funded project at NWL’s sludge centre at Bran Sands.

Overall Aim

This paper originated from a project, funded by Greenius, that sought to investigate the viability of utilising the waste heat and carbon dioxide (CO2) from the Combined Heat and Power (CHP) and Thermal Hydrolysis (TH) plants associated with the Advanced Anaerobic Digestion (AAD) plant at Northumbrian Water’s Bran Sands Regional Sludge Treatment Centre. The longer term goal of this project was to use these heat and CO2 streams to promote the growth of salad crops under glass for local delivery. This would be very much in line with a large proportion of Europe’s greenhouse crop production which is supplied with power, heat and CO2 from dedicated CHP plants burning natural gas, but offers the benefit of in effect replacing the fossil fuel with renewable digester gas. Other sources of renewable biogas such as food waste digestion are also considered in this paper.

Northumbrian Water Background

Northumbrian Water (NW), as the result of its wastewater treatment activities in the northeast of England, generates in the order of 80,000 tonnes dry solids (tDS) of sewage sludge per annum. All this sewage sludge is fed into two almost identical AAD plants, one at Howdon on Tyneside and the other at Bran Sands on Teesside. This latter site was the focus of the original project, primarily because it has been operated for longer and therefore has more operational data available. The AAD plant at Bran Sands pre-treats the sewage sludge using Thermal Hydrolysis (TH) prior to using anaerobic digestion (AD) to produce biogas and a stable sludge product. The TH plant is used to enhance the efficiency of AD, allowing more biogas to be produced and improving the recyclability of the sludge product. Thermal Hydrolysis involves heating the incoming sewage sludge using steam under pressure to 160oC before abruptly reducing that pressure to pasteurise the sludge and render it more digestible by the AD plant. The resulting sludge is cooled to an appropriate temperature for AD to take place (40 oC) producing a large quantity of low grade waste heat in the process. The biogas from the AAD plant is currently burnt in CHP gas engines to produce electricity and heat. Some of the heat is re-cycled to the TH plant. However, the majority of the low grade heat from gas engine lubrication oil / jacket cooling water is lost to the atmosphere.

From the plant’s original design (represented in the Sankey diagram below), it was known that approximately 40% of the 11.5 MW energy produced in the biogas is dissipated as waste heat from (i) the TH coolers and (ii) as un-utilised waste heat from the CHP. Note that the primary aim of the CHP plant is to produce renewable electricity, not renewable heat. Indeed, the current AAD installation at Bran Sands is producing in excess of 3.5MW of renewable electricity.

Figure 1: Sankey diagram to show the Energy balance (based upon 40,000 tDS sewage sludge Design Capacity)

Alongside the production of waste heat the AAD also produces CO2 from the CHP and THP steam generation plant exhaust stacks. These exhausts are particularly rich in CO2 because the biogas that is used as their fuel contains approximately 40% CO2 which is normally lost to atmosphere from the CHP exhaust. The site at Bran Sands also has a ready beneficial nutrient source in the form of sewage sludge digestate, which could be used as growing medium in horticulture because it is pasteurised and of consistent handling qualities Biogas 11.5 MW PTA Bio Gas

Keywords: Anaerobic Digestion, Greenhouses, CHP, Energy, Carbon Dioxide

Digestion and greenhouses – synergistic resource recovery